debug-api

<i>This interface is partly implemented in Firefox Nightly builds. Some parts are not implemented, some APIs may yet change, and anything marked "(future plan)" is a long way off. You can [https://github.com/jimblandy/DebuggerDocs fork this specification on github] to draft and discuss revisions.</i>

The <code>Debugger</code> constructor makes objects with methods for debugging code running in global objects in other compartments. Given a <code>Debugger</code> instance, you can:
<ul>
<li>add debuggee global objects to its purview using its <code>addDebuggee</code> method;
<li>request notification of basic debugging events like stack frame entry and <code>debugger</code> statement execution by providing appropriate handler functions;
<li>set breakpoints in scripts;
<li>set watchpoints on objects and their properties;
<li>examine the debuggee's stack frames and lexical enviroments;
<li>inspect and manipulate its objects;
</ul>
and so on.

Each global object may be debugged by any number of <code>Debugger</code> instances simultaneously.

Your event handling methods run in the same thread as the debuggee, on the same JavaScript stack: when the event occurs, the debuggee pauses while your handler methods run, and resumes (unless you say otherwise) when your methods return. Your event handling methods run in the compartment to which they belong, typically the debugger's compartment. The compartment system mediates the debugger's and debuggee's access to each other's objects.

The <code>Debugger</code> object provides various sorts of objects representing the debuggee's state, which the debugger code can examine and manipulate:
<ul>
<li><code>Debugger.Object</code> instances represent objects in the debuggee.
<li><code>Debugger.Frame</code> instances represent debuggee stack frames.
<li><code>Debugger.Script</code> instances represent the debuggee's code, whether it is a function body, code passed to <code>eval</code>, or a top-level script.
<li><code>Debugger.Environment</code> instances represent the variable environments in effect at different points in the debuggee's code.
</ul>

A <code>Debugger</code> instance can only debug code running in other compartments. You may not create cycles of debugger/debuggee compartments.

The <code>Debugger</code> interface does not itself support cross-thread or multi-threaded debugging. As a general rule, only one thread may use a compartment at a time. When a debugger in one compartment is debugging globals in another, many kinds of events in the debuggees may cause the debugger to call its handler methods. Conversely, a call to almost any method in this interface could cause the debugger to try to interact with a debuggee in some way. Thus, as a general rule, you should never permit different threads to run in the debugger and debuggee compartments simultaneously. Tools should instead use the [[Remote Debugging Protocol]] for all inter-thread communication.

== General Conventions ==

=== Properties ===

Properties of objects that comprise the <code>Debugger</code> interface, and those that the interface creates, follow some general conventions:
<ul>
<li>Instances and prototypes are extensible; you can add your own properties and methods to them.
<li>Properties are configurable. This applies to both "own" and prototype properties, and to both methods and data properties. (Leaving these properties open to redefinition will hopefully make it easier for JavaScript debugger code to cope with bugs, bug fixes, and changes in the interface over time.)
<li>Method properties are writable.
<li>We prefer inherited accessor properties to own data properties. Both are read using the same syntax, but inherited accessors seem like a more accurate reflection of what's going on. Unless otherwise noted, these properties have getters but no setters, as they cannot meaningfully be assigned to.
</ul>

=== Debuggee Values ===

The <code>Debugger</code> interface follows some conventions to help debuggers safely inspect and modify the debuggee's objects and values. Primitive values are passed freely between debugger and debuggee; copying or wrapping is handled transparently. Objects received from the debuggee (including host objects like DOM elements) are fronted in the debugger by <code>Debugger.Object</code> instances, which provide reflection-oriented methods for inspecting their referents; see <code>Debugger.Object</code>, below.

Of the debugger's objects, only <code>Debugger.Object</code> instances may be passed to the debuggee: when this occurs, the debuggee receives the <code>Debugger.Object</code>'s referent, not the <code>Debugger.Object</code> instance itself.

In the descriptions below, the term "debuggee value" means either a primitive value or a <code>Debugger.Object</code> instance; it is a value that might be received from the debuggee, or that could be passed to the debuggee.

=== Debuggee Code ===

Each <code>Debugger</code> instance maintains a set of global objects that, taken together, comprise the debuggee. Code evaluated in the scope of a debuggee global object, directly or indirectly, is considered <i>debuggee code</i>. Similarly:
<ul>
<li>a <i>debuggee frame</i> is a frame running debuggee code;
<li>a <i>debuggee function</i> is a function that closes over a debuggee global object (and thus the function's code is debuggee code); and
<li>a <i>debuggee script</i> is a script containing debuggee code.
</ul>

=== Completion Values ===

When a debuggee stack frame completes its execution, or when some sort of debuggee call initiated by the debugger finishes, the <code>Debugger</code> interface provides a value describing how the code completed; these are called <i>completion values</i>. A completion value has one of the following forms:
<dl>
<dt>{ return: <i>value</i> }
<dd>The code completed normally, returning <i>value</i>. <i>Value</i> is a debuggee value.
<dt>{ yield: <i>value</i> }
<dd>The running code is a generator frame which has yielded <i>value</i>. <i>Value</i> is a debuggee value.
<dt>{ throw: <i>value</i> }
<dd>The code threw <i>value</i> as an exception. <i>Value</i> is a debuggee value.
<dt>null
<dd>The code was terminated, as if by the "slow script" dialog box.
</dl>

If control reaches the end of a generator frame, the completion value is <code>{throw: <i>stop</i>}</code> where ''stop'' is a <code>Debugger.Object</code> object representing the <code>StopIteration</code> object being thrown.

=== Resumption Values ===

As the debuggee runs, the <code>Debugger</code> interface calls various debugger-provided handler functions to report the debuggee's behavior. Some of these calls can return a value indicating how the debuggee's execution should continue; these are called <i>resumption values</i>. A resumption value has one of the following forms:
<dl>
<dt>undefined
<dd>The debuggee should continue execution normally.
<dt>{ return: <i>value</i> }
<dd>Return <i>value</i> immediately as the current value of the function. <i>Value</i> must be a debuggee value. (Most handler functions support this, except those whose descriptions say otherwise.) If the function was called as a constructor (that is, via a <code>new</code> expression), then <i>value</i> serves as the value returned by the function's body, not that produced by the <code>new</code> expression: if the value is not an object, the <code>new</code> expression returns the frame's <code>this</code> value.
<dt>{ yield: <i>value</i> }
<dd>Yield <i>value</i> immediately as the next value of the current frame, which must be a generator frame. <i>Value</i> is a debuggee value. The current frame must be a generator frame that has not yet completed in some other way. You may use <code>yield</code> resumption values to substitute a new value or one already yielded by a generator, or to make a generator yield additional values.
<dt>{ throw: <i>value</i> }
<dd>Throw <i>value</i> as an execption from the current bytecode instruction. <i>Value</i> must be a debuggee value.
<dt>null
<dd>Terminate the debuggee, as if it had been cancelled by the "slow script" dialog box.
</dl>

If a function that would normally return a resumption value to indicate how the debuggee should continue instead throws an exception, we never propagate such an exception to the debuggee; instead, we call the associated <code>Debugger</code> instance's <code>uncaughtExceptionHook</code> property, as described below.

=== The Debugger.DebuggeeWouldRun Exception ===

Some debugger operations that appear to simply inspect the debuggee's state may actually cause debuggee code to run. For example, reading a variable might run a getter function on the global or on a <code>with</code> expression's operand; and getting an object's property descriptor will run a handler trap if the object is a proxy. To protect the debugger's integrity, only methods whose stated purpose is to run debuggee code can do so. These methods are called [[#Invocation_Functions_and_.22debugger.22_Frames|invocation functions]], and they follow certain common conventions to report the debuggee's behavior safely. For other methods, if their normal operation would cause debuggee code to run, they throw an instance of the <code>Debugger.DebuggeeWouldRun</code> exception.

A <code>Debugger.DebuggeeWouldRun</code> exception may have a <code>cause</code> property, providing more detailed information on why the debuggee would have run. The <code>cause</code> property's value is one of the following strings:

{| frame="box" rules="all" cellpadding="8"
! <i>cause</i> value
! meaning
|-
| "proxy"
| Carrying out the operation would have caused a proxy handler to run.
|-
| "getter"
| Carrying out the operation would have caused an object property getter to run.
|-
| "setter"
| Carrying out the operation would have caused an object property setter to run.
|}

If the system can't determine why control attempted to enter the debuggee, it will leave the exception's <code>cause</code> property undefined.

== The Debugger Object ==

When called as a constructor, the <code>Debugger</code> object creates a new <code>Debugger</code> instance.

<dl>
<dt>new Debugger([<i>global</i>, ...])
<dd>Create a debugger object, and apply its <code>addDebuggee</code> method to each of the given <i>global</i> objects to add them as the initial debuggees.
</dl>

=== Accessor Properties of the Debugger Prototype Object ===

A <code>Debugger</code> instance inherits the following accessor properties from its prototype:

<dl>
<dt>enabled
<dd>A boolean value indicating whether this <code>Debugger</code> instance's handlers, breakpoints, watchpoints, and the like are currently enabled. It is an accessor property with a getter and setter: assigning to it enables or disables this <code>Debugger</code> instance; reading it produces true if the instance is enabled, or false otherwise. This property is initially <code>true</code> in a freshly created <code>Debugger</code> instance.

This property gives debugger code a single point of control for disentangling itself from the debuggee, regardless of what sort of events or handlers or "points" we add to the interface. 

<dt>uncaughtExceptionHook
<dd>Either <code>null</code> or a function that SpiderMonkey calls when a call to a debug event handler, breakpoint handler, watchpoint handler, or similar function throws some exception, <i>debugger-exception</i>. Exceptions thrown in the debugger are not propagated to debuggee code; instead, SpiderMonkey calls this function, passing <i>debugger-exception</i> as its sole argument and the <code>Debugger</code> instance as the <code>this</code> value. This function should return a [[#Resumption_Values|resumption value]], which determines how the debuggee should continue.

If the uncaught exception hook itself throws an exception, <i>uncaught-hook-exception</i>, SpiderMonkey throws a new error object, <i>confess-to-debuggee-exception</i>, to the debuggee whose message blames the debugger, and includes textual descriptions of <i>uncaught-hook-exception</i> and the original <i>debugger-exception</i>.

If <code>uncaughtExceptionHook</code>'s value is <code>null</code>, SpiderMonkey throws an exception to the debuggee whose message blames the debugger, and includes a textual description of <i>debugger-exception</i>.

Assigning anything other than a callable value or <code>null</code> to this property throws a <code>TypeError</code> exception.

(This is not an ideal way to handle debugger bugs, but the hope here is that some sort of backstop, even if imperfect, will make life easier for debugger developers. For example, an uncaught exception hook may have access to browser-level features like the <code>alert</code> function, which this API's implementation does not, making it possible to present debugger errors to the developer in a way suited to the context.)
</dl>

=== Debugger Handler Functions ===

Each <code>Debugger</code> instance inherits accessor properties with which you can store handler functions for SpiderMonkey to call when given events occur in debuggee code.

When one of the events described below occurs in debuggee code, the engine pauses the debuggee and calls the corresponding debugging handler on each <code>Debugger</code> instance that is observing the debuggee. The handler functions receive the <code>Debugger</code> instance as their <code>this</code> value. Most handler functions can return a [[#Resumption_Values|resumption value]] indicating how the debuggee's execution should proceed.

On a new <code>Debugger</code> instance, each of these properties is initially <code>undefined</code>. Any value assigned to a debugging handler must be either a function or undefined; otherwise a <code>TypeError</code> is thrown.

Handler functions run in the same thread in which the event occurred. They run in the compartment to which they belong, not in a debuggee compartment.

<dl>
<dt>onNewScript(<i>script</i>, <i>global</i>)
<dd>New code, represented by the <code>Debugger.Script</code> instance <i>script</i>, has been loaded in the scope of the debuggee global object <i>global</i>. <i>global</i> is a <code>Debugger.Object</code> instance whose referent is the global object.

This method's return value is ignored.

<dt>onDebuggerStatement(<i>frame</i>)
<dd>Debuggee code has executed a <i>debugger</i> statement in <i>frame</i>. This method should return a [[#Resumption_Values|resumption value]] specifying how the debuggee's execution should proceed.

<dt>onEnterFrame(<i>frame</i>)
<dd>The stack frame <i>frame</i> is about to begin executing code. (Naturally, <i>frame</i> is currently the youngest [[#Visible_Frames|visible frame]].) This method should return a [[#Resumption_Values|resumption value]] specifying how the debuggee's execution should proceed.

SpiderMonkey only calls <code>onEnterFrame</code> to report [[#Visible_Frames|visible]], non-<code>"debugger"</code> frames.

<dt>onThrow(<i>frame</i>, <i>value</i>) <i>(future plan)</i>
<dd>The exception <i>value</i> is being thrown by <i>frame</i>, which is running debuggee code. This method should return a [[#Resumption_Values|resumption value]] specifying how the debuggee's execution should proceed. If it returns <code>undefined</code>, the exception is thrown as normal.

A call to the <code>onThrow</code> handler is typically followed by one or more calls to the <code>onExceptionUnwind</code> handler.

''(pending discussion)'' If the debuggee executes <code>try { throw 0; } finally { f(); }</code> and <code>f()</code> executes without error, the <code>onThrow</code> handler is called only once. The debugger is not notified when the exception is set aside in order to execute the <code>finally</code> block, nor when it is restored after the <code>finally</code> block completes normally.

''(An alternative design here would be:  onException(status, frame, value) where status is one of the strings "throw", "unwind", "catch", "finally", "rethrow". JS_SaveExceptionState would trigger a "finally" event, JS_RestoreExceptionState would trigger a "rethrow", JS_ClearPendingException would trigger a "catch"; not sure what JS_DropExceptionState or a return/throw from a finally block should do.)''

<dt>onExceptionUnwind(<i>frame</i>, <i>value</i>)
<dd>The exception <i>value</i> has been thrown, and has propagated to <i>frame</i>; <i>frame</i> is the youngest remaining stack frame, and is a debuggee frame. This method should return a [[#Resumption_Values|resumption value]] specifying how the debuggee's execution should proceed. If it returns <code>undefined</code>, the exception continues to propagate as normal: if control in <code>frame</code> is in a <code>try</code> block, control jumps to the corresponding <code>catch</code> or <code>finally</code> block; otherwise, <i>frame</i> is popped, and the exception propagates to <i>frame</i>'s caller.

When an exception's propagation causes control to enter a <code>finally</code> block, the exception is temporarily set aside. If the <code>finally</code> block finishes normally, the exception resumes propagation, and the debugger's <code>onExceptionUnwind</code> handler is called again, in the same frame. (The other possibility is for the <code>finally</code> block to exit due to a <code>return</code>, <code>continue</code>, or <code>break</code> statement, or a new exception. In those cases the old exception does not continue to propagate; it is discarded.)

<dt>sourceHandler(<i>ASuffusionOfYellow</i>)
<dd>This method is never called. If it is ever called, a contradiction has been proven, and the debugger is free to assume that everything is true.

<dt>onError(<i>frame</i>, <i>report</i>)
<dd>SpiderMonkey is about to report an error in <i>frame</i>. <i>Report</i> is an object describing the error, with the following properties:

<dl>
<dt>message
<dd>The fully formatted error message.
<dt>file
<dd>If present, the source file name, URL, etc. (If this property is present, the <i>line</i> property will be too, and vice versa.)
<dt>line
<dd>If present, the source line number at which the error occurred.
<dt>lineText
<dd>If present, this is the source code of the offending line.
<dt>offset
<dd>The index of the character within lineText at which the error occurred.
<dt>warning
<dd>Present and true if this is a warning; absent otherwise.
<dt>strict
<dd>Present and true if this error or warning is due to the strict option (not to be confused with ES strict mode)
<dt>exception
<dd>Present and true if an exception will be thrown; absent otherwise.
<dt>arguments
<dd>An array of strings, representing the arguments substituted into the error message.
</dl>

This method's return value is ignored.
</dl>

=== Function Properties of the Debugger Prototype Object ===

The functions described below may only be called with a <code>this</code> value referring to a <code>Debugger</code> instance; they may not be used as methods of other kinds of objects.

<dl>
<dt>addDebuggee(<i>global</i>)
<dd>Add the global object designated by <i>global</i> to the set of global objects this <code>Debugger</code> instance is debugging. Return this <code>Debugger</code>'s <code>Debugger.Object</code> instance referring to <i>global</i>'s referent.

One can use any of the following values to designate the global object to add as a debuggee:
<ul>
<li>a cross-compartment wrapper of a global object;
<li>a <code>Debugger.Object</code> instance belonging to this <code>Debugger</code> instance whose referent is a global object;
<li>a cross-compartment wrapper for any object, in which case the global in whose scope that object was allocated is used; or
<li>a <code>Debugger.Object</code> instance belonging to this <code>Debugger</code> instance whose referent is any object, in which case the global in whose scope that object was allocated is used.
</ul>

The global designated by <i>global</i> must be in a different compartment than this <code>Debugger</code> instance itself. If adding the designated global's compartment would create a cycle of debugger and debuggee compartments, this method throws an error.

If <i>global</i> is a cross-compartment wrapper, this method returns a <code>Debugger.Object</code> instance referring to the wrapper's referent. If <i>global</i> is a <code>Debugger.Object</code> instance belonging to this <code>Debugger</code> instance, this method returns <i>global</i> itself.

The <code>Debugger</code> instance does not hold a strong reference to its debuggee globals: if a debuggee global is not otherwise reachable, then it is dropped from the <code>Debugger</code>'s set of debuggees.

<dt>removeDebuggee(<i>global</i>)
<dd>Remove the global object designated by <i>global</i> from this <code>Debugger</code> instance's set of debuggees. Return <code>undefined</code>.

This method interprets <i>global</i> using the same rules that <code>addDebuggee</code> does.

<dt>hasDebuggee(<i>global</i>)
<dd>Return <code>true</code> if the global object designated by <i>global</i> is a debuggee of this <code>Debugger</code> instance.

This method interprets <i>global</i> using the same rules that <code>addDebuggee</code> does.

<dt>getDebuggees()
<dd>Return an array of distinct <code>Debugger.Object</code> instances whose referents are all the global objects this <code>Debugger</code> instance is debugging.

Since <code>Debugger</code> instances don't hold strong references to their debuggee globals, if a debuggee global is otherwise unreachable, it may be dropped at any moment from the array this method returns.

<dt>getNewestFrame()
<dd>Return a <code>Debugger.Frame</code> instance referring to the youngest [[#Visible_Frames|visible frame]] currently on the calling thread's stack, or <code>null</code> if there are no visible frames on the stack.

<dt>findScriptURLs([<i>query</i>])
<dd>Return an array containing the values of the <code>url</code> properties of all debuggee scripts matching <i>query</i>. Each distinct value appears only once in the array. <i>Query</i> is an object whose properties restrict which scripts are returned; a script must meet all the criteria given by <i>query</i> to be returned. If <i>query</i> is omitted, we return the <code>url</code> values of all debuggee scripts.

<i>Query</i> may have the following property:
<dl>
<dt>global
<dd>The script must be in the scope of the given global object. If this property's value is a <code>Debugger.Object</code> instance belonging to this <code>Debugger</code> instance, then its referent is used. If the object is not a global object, then the global in whose scope it was allocated is used.
</dl>

Note that the result may include values for scripts that can no longer ever be used by the debuggee, say, those for eval code that has finished running, or unreachable functions. Whether such script's <code>url</code> values appear can be affected by the garbage collector's behavior, so this function's behavior is not entirely deterministic.

<dt>findScripts([<i>query</i>])
<dd>Return an array of <code>Debugger.Script</code> instances for all debuggee scripts matching <i>query</i>. Each instance appears only once in the array. <i>Query</i> is an object whose properties restrict which scripts are returned; a script must meet all the criteria given by <i>query</i> to be returned. If <i>query</i> is omitted, we return the <code>Debugger.Script</code> instances for all debuggee scripts.

<i>Query</i> may have the following properties:
<dl>
<dt>url
<dd>The script's <code>url</code> property must be equal to this value.
<dt>line
<dd>The script must at least partially cover the given source line. If this property is present, the <code>url</code> property must be present as well.
<dt>column
<dd>The script must include  given column on the line given by the <code>line</code> property. If this property is present, the <code>url</code> and <code>line</code> properties must both be present as well.
<dt>innermost
<dd>If this property is present and true, the script must be the innermost script covering the given source location; scripts of enclosing code are omitted.
<dt>global
<dd>The script must be in the scope of the given global object. If this property's value is a <code>Debugger.Object</code> instance belonging to this <code>Debugger</code> instance, then its referent is used. If the object is not a global object, then the global in whose scope it was allocated is used.
</dl>
All properties of <i>query</i> are optional. Passing an empty object returns all debuggee code scripts.

Note that the result may include <code>Debugger.Script</code> instances for scripts that can no longer ever be used by the debuggee, say, those for eval code that has finished running, or unreachable functions. Whether such scripts appear can be affected by the garbage collector's behavior, so this function's behavior is not entirely deterministic.

<dt>clearBreakpoint(<i>handler</i>)
<dd>Remove all breakpoints set in this <code>Debugger</code> instance that use <i>handler</i> as their handler. Note that, if breakpoints using other handler objects are set at the same location(s) as <i>handler</i>, they remain in place.

<dt>clearAllBreakpoints()
<dd>Remove all breakpoints set using this <code>Debugger</code> instance.

<dt>clearAllWatchpoints() <i>(future plan)</i>
<dd>Clear all watchpoints owned by this <code>Debugger</code> instance.

</dl>

== Debugger.Frame ==

A <code>Debugger.Frame</code> instance represents a [[#Visible_Frames|visible stack frame]]. Given a <code>Debugger.Frame</code> instance, you can find the script the frame is executing, walk the stack to older frames, find the lexical environment in which the execution is taking place, and so on.

For a given <code>Debugger</code> instance, SpiderMonkey creates only one <code>Debugger.Frame</code> instance for a given visible frame. Every handler method called while the debuggee is running in a given frame is given the same frame object. Similarly, walking the stack back to a previously accessed frame yields the same frame object as before. Debugger code can add its own properties to a frame object and expect to find them later, use <code>==</code> to decide whether two expressions refer to the same frame, and so on.

(If more than one <code>Debugger</code> instance is debugging the same code, each <code>Debugger</code> gets a separate <code>Debugger.Frame</code> instance for a given frame. This allows the code using each <code>Debugger</code> instance to place whatever properties it likes on its <code>Debugger.Frame</code> instances, without worrying about interfering with other debuggers.)

When the debuggee pops a stack frame (say, because a function call has returned or an exception has been thrown from it), the <code>Debugger.Frame</code> instance referring to that frame becomes inactive: its <code>live</code> property becomes <code>false</code>, and accessing its other properties or calling its methods throws an exception. Note that frames only become inactive at times that are predictable for the debugger: when the debuggee runs, or when the debugger removes frames from the stack itself.

Stack frames that represent the control state of generator-iterator objects behave in a special way, described in [[#Generator_Frames|Generator Frames]] below.

=== Visible Frames ===

When inspecting the call stack, <code>Debugger</code> does not reveal all the frames that are actually present on the stack: while it does reveal all frames running debuggee code, it omits frames running the debugger's own code, and omits most frames running non-debuggee code. We call those stack frames a <code>Debugger</code> does reveal <i>visible frames</i>.

A frame is a visible frame if any of the following are true:
<ul>
<li>it is running [[#debuggee_code|debuggee code]];
<li>its immediate caller is a frame running debuggee code; or
<li>it is a [[#Invocation_Functions_and_.22debugger.22_Frames|<code>"debugger"</code> frame]], representing the continuation of debuggee code invoked by the debugger.
</ul>

The "immediate caller" rule means that, when debuggee code calls a non-debuggee function, it looks like a call to a primitive: you see a frame for the non-debuggee function that was accessible to the debuggee, but any further calls that function makes are treated as internal details, and omitted from the stack trace. If the non-debuggee function eventually calls back into debuggee code, then those frames are visible.

(Note that the debuggee is not considered an "immediate caller" of handler methods it triggers. Even though the debuggee and debugger share the same JavaScript stack, frames pushed for SpiderMonkey's calls to handler methods to report events in the debuggee are never considered visible frames.)

=== Invocation Functions and "debugger" Frames ===

An <i>invocation function</i> is any function in this interface that allows the debugger to invoke code in the debuggee: <code>Debugger.Object.prototype.call</code>, <code>Debugger.Frame.prototype.eval</code>, and so on.

While invocation functions differ in the code to be run and how to pass values to it, they all follow this general procedure:

<ol>
<li>Let <i>older</i> be the youngest visible frame on the stack, or <code>null</code> if there is no such frame. (This is never one of the the debugger's own frames; those never appear as <code>Debugger.Frame</code> instances.)
<li>Push a <code>"debugger"</code> frame on the stack, with <i>older</i> as its <code>older</code> property.
<li>Invoke the debuggee code as appropriate for the given invocation function, with the <code>"debugger"</code> frame as its continuation. For example, <code>Debugger.Frame.prototype.eval</code> pushes an <code>"eval"</code> frame for code it runs, whereas <code>Debugger.Object.prototype.call</code> pushes a <code>"call"</code> frame.
<li>When the debuggee code completes, whether by returning, throwing an exception or being terminated, pop the <code>"debugger"</code> frame, and return an appropriate [[#Completion_Values|completion value]] from the invocation function to the debugger.
</ol>

When a debugger calls an invocation function to run debuggee code, that code's continuation is the debugger, not the next debuggee code frame. Pushing a <code>"debugger"</code> frame makes this continuation explicit, and makes it easier to find the extent of the stack created for the invocation.

=== Accessor Properties of the Debugger.Frame Prototype Object ===

A <code>Debugger.Frame</code> instance inherits the following accessor properties from its prototype:

<dl>
<dt>type
<dd>A string describing what sort of frame this is:
<ul>
<li><code>"call"</code>: a frame running a function call. (We may not be able to obtain frames for calls to host functions.)
<li><code>"eval"</code>: a frame running code passed to <code>eval</code>.
<li><code>"global"</code>: a frame running global code (JavaScript that is neither of the above).
<li><code>"debugger"</code>: a frame for a call to user code invoked by the debugger (see the <code>eval</code> method below).
</ul>

<dt>this
<dd>The value of <code>this</code> for this frame (a debuggee value).

<dt>older
<dd>The next-older visible frame, in which control will resume when this frame completes. If there is no older frame, this is <code>null</code>. (On a suspended generator frame, the value of this property is <code>null</code>; see [[#Generator_Frames|Generator Frames]].)

<dt>depth
<dd>The depth of this frame, counting from oldest to youngest; the oldest frame has a depth of zero.

<dt>live
<dd>True if the frame this <code>Debugger.Frame</code> instance refers to is still on the stack (or, in the case of generator-iterator objects, has not yet finished its iteration); false if it has completed execution or been popped in some other way.

<dt>script
<dd>The script being executed in this frame (a <code>Debugger.Script</code> instance), or <code>null</code> on frames that do not represent calls to debuggee code. On frames whose <code>callee</code> property is not null, this is equal to <code>callee.script</code>.

<dt>offset
<dd>The offset of the bytecode instruction currently being executed in <code>script</code>, or <code>undefined</code> if the frame's <code>script</code> property is <code>null</code>.

<dt>environment
<dd>The lexical environment within which evaluation is taking place (a <code>Debugger.Environment</code> instance), or <code>null</code> on frames that do not represent the evaluation of debuggee code, like calls non-debuggee functions, host functions or <code>"debugger"</code> frames.

<dt>callee
<dd>The function whose application created this frame, as a debuggee value, or <code>null</code> if this is not a <code>"call"</code> frame.

<dt>generator
<dd>True if this frame is a generator frame, false otherwise.

<dt>constructing
<dd>True if this frame is for a function called as a constructor, false otherwise.

<dt>arguments
<dd>The arguments passed to the current frame, or <code>null</code> if this is not a <code>"call"</code> frame. When non-<code>null</code>, this is an object, allocated in the same global as the debugger, with <code>Array.prototype</code> on its prototype chain, a non-writable <code>length</code> property, and properties whose names are array indices. Each property is a read-only accessor property whose getter returns the current value of the corresponding parameter. When the referent frame is popped, the argument value's properties' getters throw an error.
</dl>

=== Handler Methods of Debugger.Frame Instances ===

Each <code>Debugger.Frame</code> instance inherits accessor properties holding handler functions for SpiderMonkey to call when given events occur in the frame.

Calls to frames' handler methods are cross-compartment, intra-thread calls: the call takes place in the thread to which the frame belongs, and runs in the compartment to which the handler method belongs.

<code>Debugger.Frame</code> instances inherit the following handler method properties:

<dl>
<dt>onStep
<dd>This property must be either <code>undefined</code> or a function. If it is a function, SpiderMonkey calls it when execution in this frame makes a small amount of progress, passing no arguments and providing this <code>Debugger.Frame</code> instance as the <code>this</code>value. The function should return a [[#Resumption_Values|resumption value]] specifying how the debuggee's execution should proceed.

What constitutes "a small amount of progress" varies depending on the implementation, but it is fine-grained enough to implement useful "step" and "next" behavior.

If multiple <code>Debugger</code> instances each have <code>Debugger.Frame</code> instances for a given stack frame with <code>onStep</code> handlers set, their handlers are run in an unspecified order. If any <code>onStep</code> handler forces the frame to return early (by returning a resumption value other than <code>undefined</code>), any remaining debuggers' <code>onStep</code> handlers do not run.

This property is ignored on frames that are not executing debuggee code, like <code>"call"</code> frames for calls to host functions and <code>"debugger"</code> frames.

<dt>onPop
<dd>This property must be either <code>undefined</code> or a function. If it is a function, SpiderMonkey calls it just before this frame is popped, passing a [[#Completion_Values|completion value]] indicating how this frame's execution completed, and providing this <code>Debugger.Frame</code> instance as the <code>this</code> value. The function should return a [[#Resumption_Values|resumption value]] indicating how execution should proceed. On newly created frames, this property's value is <code>undefined</code>.

When an <code>onPop</code> call reports the completion of a construction call (that is, a function called via the <code>new</code> operator), the completion value passed to the handler describes the value returned by the function body. If this value is not an object, it may be different from the value produced by the <code>new</code> expression, which will be the value of the frame's <code>this</code> property. (In ECMAScript terms, the <code>onPop</code> handler receives the value returned by the <code><nowiki>[[Call]]</nowiki></code> method, not the value returned by the <code><nowiki>[[Construct]]</nowiki></code> method.)

When a debugger handler function forces a frame to complete early, by returning a <code>{ return:... }</code>, <code>{ throw:... }</code>, or <code>null</code> resumption value, SpiderMonkey calls the frame's <code>onPop</code> handler, if any. The completion value passed in this case reflects the resumption value that caused the frame to complete.

When SpiderMonkey calls an <code>onPop</code> handler for a frame that is throwing an exception or being terminated, and the handler returns <code>undefined</code>, then SpiderMonkey proceeds with the exception or termination. That is, an <code>undefined</code> resumption value leaves the frame's throwing and termination process undisturbed.

When a generator frame yields a value, SpiderMonkey calls its <code>Debugger.Frame</code> instance's <code>onPop</code> handler method, if present, passing a <code>yield</code> resumption value; however, the <code>Debugger.Frame</code> instance remains live.

If multiple <code>Debugger</code> instances each have <code>Debugger.Frame</code> instances for a given stack frame with <code>onPop</code> handlers set, their handlers are run in an unspecified order. The resumption value each handler returns establishes the completion value reported to the next handler.

This property is ignored on <code>"debugger"</code> frames.

<dt>onResume(<i>value</i>)
<dd>This property must be either <code>undefined</code> or a function. If it is a function, SpiderMonkey calls it if the current frame is a generator frame whose execution has just been resumed. The function should return a [[#Resumption_Values|resumption value]] indicating how execution should proceed. On newly created frames, this property's value is <code>undefined</code>.

If the program resumed the generator by calling its <code>send</code> method and passing a value, then <i>value</i> is that value. Otherwise, <i>value</i> is <code>undefined</code>.
</dl>

=== Function Properties of the Debugger.Frame Prototype Object ===

The functions described below may only be called with a <code>this</code> value referring to a <code>Debugger.Frame</code> instance; they may not be used as methods of other kinds of objects.

<dl>
<dt>eval(<i>code</i>)
<dd>Evaluate <i>code</i> in the execution context of this frame, and return a [[#Completion_Values|completion value]] describing how it completed. <i>Code</i> is a string. If this frame's <code>environment</code> property is <code>null</code>, throw a <code>TypeError</code>. All extant handler methods, breakpoints, watchpoints, and so on remain active during the call. This function follows the [[#Invocation_Functions_and_.22debugger.22_Frames|invocation function conventions]].

<i>Code</i> is interpreted as strict mode code when it contains a Use Strict Directive, or the code executing in this frame is strict mode code.

If <i>code</i> is not strict mode code, then variable declarations in <i>code</i> affect the environment of this frame. (In the terms used by the ECMAScript specification, the <code>VariableEnvironment</code> of the execution context for the eval code is the <code>VariableEnvironment</code> of the execution context that this frame represents.) If implementation restrictions prevent SpiderMonkey from extending this frame's environment as requested, this call throws an Error exception.

<dt>evalWithBindings(<i>code</i>, <i>bindings</i>)
<dd>Like <code>eval</code>, but evaluate <i>code</i> in the environment of this frame, extended with bindings from the object <i>bindings</i>. For each own enumerable property of <i>bindings</i> named <i>name</i> whose value is <i>value</i>, include a variable in the environment in which <i>code</i> is evaluated named <i>name</i>, whose value is <i>value</i>. Each <i>value</i> must be a debuggee value. (This is not like a <code>with</code> statement: <i>code</i> may access, assign to, and delete the introduced bindings without having any effect on the <i>bindings</i> object.)

This method allows debugger code to introduce temporary bindings that are visible to the given debuggee code and which refer to debugger-held debuggee values, and do so without mutating any existing debuggee environment.

Note that, like <code>eval</code>, declarations in the <i>code</i> passed to <code>evalWithBindings</code> affect the environment of this frame, even as that environment is extended by bindings visible within <i>code</i>. (In the terms used by the ECMAScript specification, the <code>VariableEnvironment</code> of the execution context for the eval code is the <code>VariableEnvironment</code> of the execution context that this frame represents, and the <i>bindings</i> appear in a new declarative environment, which is the eval code's <code>LexicalEnvironment</code>.) If implementation restrictions prevent SpiderMonkey from extending this frame's environment as requested, this call throws an <code>Error</code> exception.

<dt>pop(<i>completion</i>) <i>(future plan)</i>
<dd>Pop this frame (and any younger frames) from the stack as if this frame had completed as specified by the completion value <i>completion</i>.

Note that this does <i>not</i> resume the debuggee's execution; it merely adjusts the debuggee's state to what it would be if this frame's execution had completed. The debuggee will only resume execution when you return from the handler method that brought control to the debugger originally.

This cannot remove any <code>"call"</code> frames for calls to host functions from the stack. (We might be able to make this work eventually, but it will take some cleverness.)

<dt>replaceCall(<i>function</i>, <i>this</i>, <i>arguments</i>) <i>(future plan)</i>
<dd>Pop any younger frames from the stack, and then change this frame into a frame for a call to <i>function</i>, with the given <i>this</i> value and <i>arguments</i>. <i>This</i> should be a debuggee value, or <code>{ asConstructor: true }</code> to invoke <i>function</i> as a constructor, in which case SpiderMonkey provides an appropriate <code>this</code> value itself. <i>Arguments</i> should be an array of debuggee values. This frame must be a <code>"call"</code> frame.

This can be used as a primitive in implementing some forms of a "patch and continue" debugger feature.

Note that this does <i>not</i> resume the debuggee's execution; it merely adjusts the debuggee's state to what it would be if this frame were about to make this call. The debuggee will only resume execution when you return from the handler method that brought control to the debugger originally.

Like <code>pop</code>, this cannot remove <code>"call"</code> frames for calls to host functions from the stack.

</dl>

=== Generator Frames ===

SpiderMonkey supports generator-iterator objects, which produce a series of values by repeatedly suspending the execution of a function or expression. For example, calling a function that uses <code>yield</code> produces a generator-iterator object, as does evaluating a generator expression like <code>(i*i for each (i in [1,2,3]))</code>.

A generator-iterator object refers to a stack frame with no fixed continuation frame. While the generator's code is running, its continuation is whatever frame called its <code>next</code> method; while the generator is suspended, it has no particular continuation frame; and when it resumes again, the continuation frame for that resumption could be different from that of the previous resumption.

A <code>Debugger.Frame</code> instance representing a generator frame differs from an ordinary stack frame as follows:
<ul>
<li>A generator frame's <code>generator</code> property is true.
<li>A generator frame disappears from the stack each time the generator yields a value and is suspended, and reappears atop the stack when it is resumed to produce the generator's next value. The same <code>Debugger.Frame</code> instance refers to the generator frame until it returns, throws an exception, or is terminated.
<li>A generator frame's <code>older</code> property refers to the frame's continuation frame while the generator is running, and is <code>null</code> while the generator is suspended.
<li>A generator frame's <code>depth</code> property reflects the frame's position on the stack when the generator is resumed, and is <code>null</code> while the generator is suspended.
<li>A generator frame's <code>live</code> property remains true until the frame returns, throws an exception, or is terminated. Thus, generator frames can be live while not present on the stack.
</ul>

The other <code>Debugger.Frame</code> methods and accessor properties work as described on generator frames, even when the generator frame is suspended. You may examine a suspended generator frame's variables, and use its <code>script</code> and <code>offset</code> members to see which <code>yield</code> it is suspended at.

A <code>Debugger.Frame</code> instance referring to a generator-iterator frame has a strong reference to the generator-iterator object; the frame (and its object) will live as long as the <code>Debugger.Frame</code> instance does. However, when the generator function returns, throws an exception, or is terminated, thus ending the iteration, the <code>Debugger.Frame</code> instance becomes inactive and its <code>live</code> property becomes <code>false</code>, just as would occur for any other sort of frame that is popped. A non-live <code>Debugger.Frame</code> instance no longer holds a strong reference to the generator-iterator object.

== Debugger.Script ==

A <code>Debugger.Script</code> instance refers to a sequence of bytecode in the debuggee; it is the JavaScript-level presentation of a JSAPI <code>JSScript</code> object. Each of the following is represented by single JSScript object:
<ul>
<li>The body of a function&mdash;that is, all the code in the function that is not contained within some nested function.
<li>The code passed to a single call to <code>eval</code>, excluding the bodies of any functions that code defines.
<li>The contents of a <code>&lt;script&gt;</code> element.
<li>A DOM event handler, whether embedded in HTML or attached to the element by other JavaScript code.
<li>Code appearing in a <code>javascript:</code> URL.
</ul>

The <code>Debugger</code> interface constructs <code>Debugger.Script</code> objects as scripts of debuggee code are uncovered by the debugger: via the <code>onNewScript</code> handler method; via <code>Debugger.Frame</code>'s <code>script</code> properties; via the <code>functionScript</code> method of <code>Debugger.Object</code> instances; and so on. For a given <code>Debugger</code> instance, SpiderMonkey constructs exactly one <code>Debugger.Script</code> instance for each underlying script object; debugger code can add its own properties to a script object and expect to find them later, use <code>==</code> to decide whether two expressions refer to the same script, and so on.

(If more than one <code>Debugger</code> instance is debugging the same code, each <code>Debugger</code> gets a separate <code>Debugger.Script</code> instance for a given script. This allows the code using each <code>Debugger</code> instance to place whatever properties it likes on its <code>Debugger.Script</code> instances, without worrying about interfering with other debuggers.)

A <code>Debugger.Script</code> instance is a strong reference to a JSScript object; it protects the script it refers to from being garbage collected.

Note that SpiderMonkey may use the same <code>Debugger.Script</code> instances for equivalent functions or evaluated code&mdash;that is, scripts representing the same source code, at the same position in the same source file, evaluated in the same lexical environment.

=== Accessor Properties of the Debugger.Script Prototype Object ===

A <code>Debugger.Script</code> instance inherits the following accessor properties from its prototype:

<dl>
<dt>url
<dd>The filename or URL from which this script's code was loaded.

<dt>startLine
<dd>The number of the line at which this script's code starts, within the file or document named by <code>url</code>.

<dt>lineCount
<dd>The number of lines this script's code occupies, within the file or document named by <code>url</code>.

<dt>staticLevel
<dd>The number of function bodies enclosing this script's code.

Global code is at level zero; bodies of functions defined at the top level in global code are at level one; bodies of functions nested within those are at level two; and so on.

A script for code passed to direct <code>eval</code> is at a static level one greater than that of the script containing the call to <code>eval</code>, because direct eval code runs within the caller's scope. However, a script for code passed to an indirect <code>eval</code> call is at static level zero, since it is evaluated in the global scope.

Note that a generator's expressions are considered to be part of the body of a synthetic function, produced by the compiler.

Scripts' static level be useful in deciding where to set breakpoints. For example, a breakpoint set on line 3 in this code:

  function f() {
    x = function g() {  // line 2
                        // line 3; no code here
      ...;
    }
  }

should be set in <code>g</code>'s script, not in <code>f</code>'s, even though neither script contains code at that line. In such a case, the most deeply nested script&mdash;the one with the highest static level&mdash;should receive the breakpoint.

<dt>strictMode
<dd>This is <code>true</code> if this script's code is ECMAScript strict mode code, and <code>false</code> otherwise.
</dl>

=== Function Properties of the Debugger.Script Prototype Object ===

The functions described below may only be called with a <code>this</code> value referring to a <code>Debugger.Script</code> instance; they may not be used as methods of other kinds of objects.

<dl>
<dt>decompile([<i>pretty</i>])
<dd>Return a string containing JavaScript source code equivalent to this script in its effect and result. If <i>pretty</i> is present and true, produce indented code with line breaks.

(Note that <code>Debugger.Object</code> instances referring to functions also have a <code>decompile</code> method, whose result includes the function header and parameter names, so it is probably better to write <code><i>f</i>.decompile()</code> than to write <code><i>f</i>.getFunctionScript().decompile()</code>.)

<dt>getAllOffsets()
<dd>Return an array <i>L</i> describing the relationship between bytecode instruction offsets and source code positions in this script. <i>L</i> is sparse, and indexed by source line number. If a source line number <i>line</i> has no code, then <i>L</i> has no <i>line</i> property. If there is code for <i>line</i>, then <code><i>L</i>[<i>line</i>]</code> is an array of offsets of byte code instructions that are entry points to that line.

For example, suppose we have a script for the following source code:
  a=[]
  for (i=1; i < 10; i++)
    // It's hip to be square.
    a[i] = i*i;

Calling <code>getAllOffsets()</code> on that code might yield an array like this:
  [[0], [5, 20], , [10]]

This array indicates that:
<ul>
<li>the first line's code starts at offset 0 in the script;
<li>the <code>for</code> statement head has two entry points at offsets 5 and 20 (for the initialization, which is performed only once, and the loop test, which is performed at the start of each iteration);
<li>the third line has no code;
<li>and the fourth line begins at offset 10.
</ul>

<dt>getLineOffsets(<i>line</i>)
<dd>Return an array of bytecode instruction offsets representing the entry points to source line <i>line</i>. If the script contains no executable code at that line, the array returned is empty.

<dt>getOffsetLine(<i>offset</i>)
<dd>Return the source code line responsible for the bytecode at <i>offset</i> in this script.

<dt>getChildScripts()
<dd>Return a new array whose elements are Debugger.Script objects for each function and each generator expression in this script. Only direct children are included; nested children can be reached by walking the tree.

<dt>setBreakpoint(<i>offset</i>, <i>handler</i>)
<dd>Set a breakpoint at the bytecode instruction at <i>offset</i> in this script, reporting hits to the <code>hit</code> method of <i>handler</i>. If <i>offset</i> is not a valid offset in this script, throw an error.

When execution reaches the given instruction, SpiderMonkey calls the <code>hit</code> method of <i>handler</i>, passing a <code>Debugger.Frame</code> instance representing the currently executing stack frame. The <code>hit</code> method's return value should be a [[#Resumption_Values|resumption value]], determining how execution should continue.

Any number of breakpoints may be set at a single location; when control reaches that point, SpiderMonkey calls their handlers in an unspecified order.

Any number of breakpoints may use the same <i>handler</i> object.

Breakpoint handler method calls are cross-compartment, intra-thread calls: the call takes place in the same thread that hit the breakpoint, and in the compartment containing the handler function (typically the debugger's compartment).

The new breakpoint belongs to the <code>Debugger</code> instance to which this script belongs; disabling the <code>Debugger</code> instance disables this breakpoint.

<dt>getBreakpoints([<i>offset</i>])
<dd>Return an array containing the handler objects for all the breakpoints set at <i>offset</i> in this script. If <i>offset</i> is omitted, return the handlers of all breakpoints set anywhere in this script. If <i>offset</i> is present, but not a valid offset in this script, throw an error.

<dt>clearBreakpoints(handler, [<i>offset</i>])
<dd>Remove all breakpoints set in this <code>Debugger</code> instance that use <i>handler</i> as their handler. If <i>offset</i> is given, remove only those breakpoints set at <i>offset</i> that use <i>handler</i>; if <i>offset</i> is not a valid offset in this script, throw an error.

Note that, if breakpoints using other handler objects are set at the same location(s) as <i>handler</i>, they remain in place.

<dt>clearAllBreakpoints([<i>offset</i>])
<dd>Remove all breakpoints set in this script. If <i>offset</i> is present, remove all breakpoints set at that offset in this script; if <i>offset</i> is not a valid bytecode offset in this script, throw an error.

</dl>

== Debugger.Object ==

A <code>Debugger.Object</code> instance represents an object in the debuggee, providing reflection-oriented methods to inspect and modify its referent. The referent's properties do not appear directly as properties of the <code>Debugger.Object</code> instance; the debugger can access them only through methods like <code>Debugger.Object.prototype.getOwnPropertyDescriptor</code> and <code>Debugger.Object.prototype.defineProperty</code>, ensuring that the debugger will not inadvertently invoke the referent's getters and setters.

SpiderMonkey creates exactly one <code>Debugger.Object</code> instance for each debuggee object it presents to a given <code>Debugger</code> instance: if the debugger encounters the same object through two different routes (perhaps two functions are called on the same object), SpiderMonkey presents the same <code>Debugger.Object</code> instance to the debugger each time. This means that the debugger can use the <code>==</code> operator to recognize when two <code>Debugger.Object</code> instances refer to the same debuggee object, and place its own properties on a <code>Debugger.Object</code> instance to store metadata about particular debuggee objects.

(If more than one <code>Debugger</code> instance is debugging the same code, each <code>Debugger</code> gets a separate <code>Debugger.Object</code> instance for a given object. This allows the code using each <code>Debugger</code> instance to place whatever properties it likes on its own <code>Debugger.Object</code> instances, without worrying about interfering with other debuggers.)

While most <code>Debugger.Object</code> instances are created by SpiderMonkey in the process of exposing debuggee's behavior and state to the debugger, the debugger can use  <code>Debugger.Object.prototype.makeDebuggeeValue</code> to create <code>Debugger.Object</code> instances for given debuggee objects, or use <code>Debugger.Object.prototype.copy</code> and <code>Debugger.Object.prototype.create</code> to create new objects in debuggee compartments, allocated as if by particular debuggee globals.

<code>Debugger.Object</code> instances protect their referents from the garbage collector; as long as the <code>Debugger.Object</code> instance is live, the referent remains live. This means that garbage collection has no visible effect on <code>Debugger.Object</code> instances.

=== Accessor Properties of the Debugger.Object prototype ===

A <code>Debugger.Object</code> instance inherits the following accessor properties from its prototype:

<dl>
<dt>proto
<dd>The referent's prototype (as a new <code>Debugger.Object</code> instance), or <code>null</code> if it has no prototype.

<dt>class
<dd>A string naming the ECMAScript <code><nowiki>[[Class]]</nowiki></code> of the referent.

<dt>callable
<dd><code>true</code> if the referent is a callable object (such as a function or a function proxy); false otherwise.

<dt>name
<dd>If the referent is a function, its function name. If the referent is an anonymous function, or not a function at all, this is <code>undefined</code>.

<dt>parameterNames
<dd>If the referent is a function, the names of the its parameters, as an array of strings. If the referent is not a function at all, this is <code>undefined</code>.

If the referent is a host function for which parameter names are not available, return an array with one element per parameter, each of which is <code>undefined</code>.

If the referent is a function proxy, return an empty array.

If the referent uses destructuring parameters, then the array's elements reflect the structure of the parameters. For example, if the referent is a function declared in this way:

  function f(a, [b, c], {d, e:f}) { ... }

then this <code>Debugger.Object</code> instance's <code>parameterNames</code> property would have the value:

  ["a", ["b", "c"], {d:"d", e:"f"}]

<dt>script
<dd>If the referent is a function that is debuggee code, this is that function's script, as a <code>Debugger.Script</code> instance. If the referent is a function proxy or not debuggee code, this is <code>undefined</code>.

<dt>environment
<dd>If the referent is a function that is debuggee code, a <code>Debugger.Environment</code> instance representing the lexical environment enclosing the function when it was created. If the referent is a function proxy or not debuggee code, this is <code>undefined</code>.

<dt>proxyHandler
<dd>If the referent is a proxy whose handler object was allocated by debuggee code, this is its handler object&mdash;the object whose methods are invoked to implement accesses of the proxy's properties. If the referent is not a proxy whose handler object was allocated by debuggee code, this is <code>null</code>.

<dt>proxyCallTrap
<dd>If the referent is a function proxy whose handler object was allocated by debuggee code, this is its call trap function&mdash;the function called when the function proxy is called. If the referent is not a function proxy whose handler object was allocated by debuggee code, this is <code>null</code>.

<dt>proxyConstructTrap
<dd>If the referent is a function proxy whose handler object was allocated by debuggee code, its construction trap function&mdash;the function called when the function proxy is called via a <code>new</code> expression. If the referent is not a function proxy whose handler object was allocated by debuggee code, this is <code>null</code>.

</dl>

=== Function Properties of the Debugger.Object prototype ===

The functions described below may only be called with a <code>this</code> value referring to a <code>Debugger.Object</code> instance; they may not be used as methods of other kinds of objects. The descriptions use "referent" to mean "the referent of this <code>Debugger.Object</code> instance".

Unless otherwise specified, these methods are not [[#Invocation_Functions_and_.22debugger.22_Frames|invocation functions]]; if a call would cause debuggee code to run (say, because it gets or sets an accessor property whose handler is debuggee code, or because the referent is a proxy whose traps are debuggee code), the call throws a [[#The_Debugger.DebuggeeWouldRun_Exception|<code>Debugger.DebuggeeWouldRun</code>]] exception.

<dl>
<dt>getProperty(<i>name</i>)
<dd>Return the value of the referent's property named <i>name</i>, or <code>undefined</code> if it has no such property. <i>Name</i> must be a string. The result is a debuggee value.

<dt>setProperty(<i>name</i>, <i>value</i>)
<dd>Store <i>value</i> as the value of the referent's property named <i>name</i>, creating the property if it does not exist. <i>Name</i> must be a string; <i>value</i> must be a debuggee value.

<dt>getOwnPropertyDescriptor(<i>name</i>)
<dd>Return a property descriptor for the property named <i>name</i> of the referent. If the referent has no such property, return <code>undefined</code>. (This function behaves like the standard <code>Object.getOwnPropertyDescriptor</code> function, except that the object being inspected is implicit; the property descriptor returned is allocated as if by code scoped to the debugger's global object (and is thus in the debugger's compartment); and its <code>value</code>, <code>get</code>, and <code>set</code> properties, if present, are debuggee values.)

<dt>getOwnPropertyNames()
<dd>Return an array of strings naming all the referent's own properties, as if <code>Object.getOwnPropertyNames(<i>referent</i>)</code> had been called in the debuggee, and the result copied in the scope of the debugger's global object.

<dt>defineProperty(<i>name</i>, <i>attributes</i>)
<dd>Define a property on the referent named <i>name</i>, as described by the property descriptor <i>descriptor</i>. Any <code>value</code>, <code>get</code>, and <code>set</code> properties of <i>attributes</i> must be debuggee values. (This function behaves like <code>Object.defineProperty</code>, except that the target object is implicit, and in a different compartment from the function and descriptor.)

<dt>defineProperties(<i>properties</i>)
<dd>Add the properties given by <i>properties</i> to the referent. (This function behaves like <code>Object.defineProperties</code>, except that the target object is implicit, and in a different compartment from the <i>properties</i> argument.)

<dt>deleteProperty(<i>name</i>)
<dd>Remove the referent's property named <i>name</i>. Return true if the property was successfully removed, or if the referent has no such property. Return false if the property is non-configurable.

<dt>seal()
<dd>Prevent properties from being added to or deleted from the referent. Return this <code>Debugger.Object</code> instance. (This function behaves like the standard <code>Object.seal</code> function, except that the object to be sealed is implicit and in a different compartment from the caller.)

<dt>freeze()
<dd>Prevent properties from being added to or deleted from the referent, and mark each property as non-writable. Return this <code>Debugger.Object</code> instance. (This function behaves like the standard <code>Object.freeze</code> function, except that the object to be sealed is implicit and in a different compartment from the caller.)

<dt>preventExtensions()
<dd>Prevent properties from being added to the referent. (This function behaves like the standard <code>Object.preventExtensions</code> function, except that the object to operate on is implicit and in a different compartment from the caller.)

<dt>isSealed()
<dd>Return true if the referent is sealed&mdash;that is, if it is not extensible, and all its properties have been marked as non-configurable. (This function behaves like the standard <code>Object.isSealed</code> function, except that the object inspected is implicit and in a different compartment from the caller.)

<dt>isFrozen()
<dd>Return true if the referent is frozen&mdash;that is, if it is not extensible, and all its properties have been marked as non-configurable and read-only. (This function behaves like the standard <code>Object.isFrozen</code> function, except that the object inspected is implicit and in a different compartment from the caller.)

<dt>isExtensible()
<dd>Return true if the referent is extensible&mdash;that is, if it can have new properties defined on it. (This function behaves like the standard <code>Object.isExtensible</code> function, except that the object inspected is implicit and in a different compartment from the caller.)

<dt>copy(<i>value</i>)
<dd>Apply the HTML5 "structured cloning" algorithm to create a copy of <i>value</i> in the referent's global object (and thus in the referent's compartment), and return a <code>Debugger.Object</code> instance referring to the copy.

Note that this returns primitive values unchanged. This means you can use <code>Debugger.Object.prototype.copy</code> as a generic "debugger value to debuggee value" conversion function&mdash;within the limitations of the "structured cloning" algorithm.

<dt>create(<i>prototype</i>, [<i>properties</i>])
<dd>Create a new object in the referent's global (and thus in the referent's compartment), and return a <code>Debugger.Object</code> referring to it. The new object's prototype is <i>prototype</i>, which must be an <code>Debugger.Object</code> instance. The new object's properties are as given by <i>properties</i>, as if <i>properties</i> were passed to <code>Debugger.Object.prototype.defineProperties</code>, with the new <code>Debugger.Object</code> instance as the <code>this</code> value.

<dt>makeDebuggeeValue(<i>value</i>)
<dd>Return the debuggee value that represents <i>value</i> in the debuggee. If <i>value</i> is a primitive, we return it unchanged; if <i>value</i> is an object, we return the <code>Debugger.Object</code> instance representing that object, wrapped appropriately for use in this <code>Debugger.Object</code>'s referent's compartment.

Note that, if <i>value</i> is an object, it need not be one allocated in a debuggee global, nor even a debuggee compartment; it can be any object the debugger wishes to use as a debuggee value. 

In Firefox, all cross-compartment object references go through wrappers, meaning that code in different compartments may see different views of the same object. For example, code in the same compartment as the object can access it directly, but code in another compartment can only refer to the object through a wrapper, which may restrict the code's access to the object. This API takes care to ensure that <code>Debugger.Object</code> instances present the same view of the object the debuggee itself would see. Given a <code>Debugger.Object</code> instance <i>d</i> and an object <i>o</i>, the call <code><i>d</i>.makeDebuggeeValue(<i>o</i>)</code> returns a <code>Debugger.Object</code> instance that presents <i>o</i> as it would be seen by code in <i>d</i>'s compartment.

<dt>decompile([<i>pretty</i>])
<dd>If the referent is a function that is debuggee code, return the JavaScript source code for a function definition equivalent to the referent function in its effect and result, as a string. If <i>pretty</i> is present and true, produce indented code with line breaks. If the referent is not a function that is debuggee code, return <code>undefined</code>.

<dt>call(<i>this</i>, <i>argument</i>, ...)
<dd>If the referent is callable, call it with the given <i>this</i> value and <i>argument</i> values, and return a [[#Completion_Values|completion value]] describing how the call completed. <i>This</i> should be a debuggee value, or <code>{ asConstructor: true }</code> to invoke the referent as a constructor, in which case SpiderMonkey provides an appropriate <code>this</code> value itself. Each <i>argument</i> must be a debuggee value. All extant handler methods, breakpoints, watchpoints, and so on remain active during the call. Details of how the call is carried out are given in the description of [[#Debugger.Frame.Debugger|Debugger.Frame.Debugger frames]]. If the referent is not callable, throw a <code>TypeError</code>. This function follows the [[#Invocation_Functions_and_.22debugger.22_Frames|invocation function conventions]].

<dt>apply(<i>this</i>, <i>arguments</i>)
<dd>If the referent is callable, call it with the given <i>this</i> value and the argument values in <i>arguments</i>, and return a [[#Completion_Values|completion value]] describing how the call completed. <i>This</i> should be a debuggee value, or <code>{ asConstructor: true }</code> to invoke <i>function</i> as a constructor, in which case SpiderMonkey provides an appropriate <code>this</code> value itself. <i>Arguments</i> must either be an array (in the debugger) of debuggee values, or <code>null</code> or <code>undefined</code>, which are treated as an empty array. All extant handler methods, breakpoints, watchpoints, and so on remain active during the call. Details of how the call is carried out are given in the description of [[#Debugger.Frame.Debugger|Debugger.Frame.Debugger frames]]. If the referent is not callable, throw a <code>TypeError</code>. This function follows the [[#Invocation_Functions_and_.22debugger.22_Frames|invocation function conventions]].

<dt>evalInGlobal(<i>code</i>)
<dd>If the referent is a global object, evaluate <i>code</i> in that global environment, and return a [[#Completion_Values|completion value]] describing how it completed. <i>Code</i> is a string. All extant handler methods, breakpoints, watchpoints, and so on remain active during the call. This function follows the [[#Invocation_Functions_and_.22debugger.22_Frames|invocation function conventions]]. If the referent is not a global object, throw a <code>TypeError</code> exception.

<i>Code</i> is interpreted as strict mode code when it contains a Use Strict Directive.

If <i>code</i> is not strict mode code, then variable declarations in <i>code</i> affect the referent global object. (In the terms used by the ECMAScript specification, the <code>VariableEnvironment</code> of the execution context for the eval code is the referent.)

<dt>evalInGlobalWithBindings(<i>code</i>, <i>bindings</i>)
<dd>Like <code>evalInGlobal</code>, but evaluate <i>code</i> using the referent as the variable object, but with a lexical environment extended with bindings from the object <i>bindings</i>. For each own enumerable property of <i>bindings</i> named <i>name</i> whose value is <i>value</i>, include a variable in the lexical environment in which <i>code</i> is evaluated named <i>name</i>, whose value is <i>value</i>. Each <i>value</i> must be a debuggee value. (This is not like a <code>with</code> statement: <i>code</i> may access, assign to, and delete the introduced bindings without having any effect on the <i>bindings</i> object.)

This method allows debugger code to introduce temporary bindings that are visible to the given debuggee code and which refer to debugger-held debuggee values, and do so without mutating any existing debuggee environment.

Note that, like <code>evalInGlobal</code>, declarations in the <i>code</i> passed to <code>evalInGlobalWithBindings</code> affect the referent global object, even as <i>code</i> is evaluated with <i>bindings</i> visible. (In the terms used by the ECMAScript specification, the <code>VariableEnvironment</code> of the execution context for the eval code is the referent, and the <i>bindings</i> appear in a new declarative environment, which is the eval code's <code>LexicalEnvironment</code>.)

<dt>asEnvironment()
<dd>If the referent is a global object, return the <code>Debugger.Environment</code> instance representing the referent as a variable environment for evaluating code. If the referent is not a global object, throw a <code>TypeError</code>.

<dt>setObjectWatchpoint(<i>handler</i>) <i>(future plan)</i>
<dd>Set a watchpoint on all the referent's own properties, reporting events by calling <i>handler</i>'s methods. Any previous watchpoint handler on this <code>Debugger.Object</code> instance is replaced. If <i>handler</i> is null, the referent is no longer watched. <i>Handler</i> may have the following methods, called under the given circumstances: 

<dl>
<dt>add(<i>frame</i>, <i>name</i>, <i>descriptor</i>)
<dd>A property named <i>name</i> has been added to the referent. <i>Descriptor</i> is a property descriptor of the sort accepted by <code>Debugger.Object.prototype.defineProperty</code>, giving the newly added property's attributes.
<dt>delete(<i>frame</i>, <i>name</i>)
<dd>The property named <i>name</i> is about to be deleted from the referent.
<dt>change(<i>frame</i>, <i>name</i>, <i>oldDescriptor</i>, <i>newDescriptor</i>)
<dd>The existing property named <i>name</i> on the referent is being changed from those given by <i>oldDescriptor</i> to those given by <i>newDescriptor</i>. This handler method is only called when attributes of the property other than its value are being changed; if only the value is changing, SpiderMonkey calls the handler's <code>set</code> method.
<dt>set(<i>frame</i>, <i>oldValue</i>, <i>newValue</i>)
<dd>The data property named <i>name</i> of the referent is about to have its value changed from <i>oldValue</i> to <i>newValue</i>.

SpiderMonkey only calls this method on assignments to data properties that will succeed; assignments to un-writable data properties fail without notifying the debugger.
<dt>extensionsPrevented(<i>frame</i>)
<dd>The referent has been made non-extensible, as if by a call to <code>Object.preventExtensions</code>.
</dl>

For all watchpoint handler methods:
<ul>
<li>Handler calls receive the handler object itself as the <code>this</code> value.
<li>The <i>frame</i> argument is the current stack frame, whose code is about to perform the operation on the object being reported.
<li>If the method returns <code>undefined</code>, then SpiderMonkey makes the announced change to the object, and continues execution normally. If the method returns an object:
<ul>
<li>If the object has a <code>superseded</code> property whose value is a true value, then SpiderMonkey does not make the announced change.
<li>If the object has a <code>resume</code> property, its value is taken as a [[#Resumption_Values|resumption value]], indicating how execution should proceed. (However, <code>return</code> resumption values are not supported.)
</ul>
<li>If a given method is absent from <i>handler</i>, then events of that sort are ignored. The watchpoint consults <i>handler</i>'s properties each time an event occurs, so adding methods to or removing methods from <i>handler</i> after setting the watchpoint enables or disables reporting of the corresponding events.
<li>Values passed to <i>handler</i>'s methods are debuggee values. Descriptors passed to <i>handler</i>'s methods are ordinary objects in the debugger's compartment, except for <code>value</code>, <code>get</code>, and <code>set</code> properties in descriptors, which are debuggee values; they are the sort of value expected by <code>Debugger.Object.prototype.defineProperty</code>.
<li>Watchpoint handler calls are cross-compartment, intra-thread calls: the call takes place in the same thread that changed the property, and in <i>handler</i>'s method's compartment (typically the same as the debugger's compartment).
</ul>

The new watchpoint belongs to the <code>Debugger</code> instance to which this <code>Debugger.Object</code> instance belongs; disabling the <code>Debugger</code> instance disables this watchpoint.

<dt>clearObjectWatchpoint() <i>(future plan)</i>
<dd>Remove any object watchpoint set on the referent.

<dt>setPropertyWatchpoint(<i>name</i>, <i>handler</i>) <i>(future plan)</i>
<dd>Set a watchpoint on the referent's property named <i>name</i>, reporting events by calling <i>handler</i>'s methods. Any previous watchpoint handler on this property for this <code>Debugger.Object</code> instance is replaced. If <i>handler</i> is null, the property is no longer watched. <i>Handler</i> is as described for <code>Debugger.Object.prototype.setObjectWatchpoint</code>, except that it does not receive <code>extensionsPrevented</code> events.

<dt>clearPropertyWatchpoint(<i>name</i>) <i>(future plan)</i>
<dd>Remove any watchpoint set on the referent's property named <i>name</i>.

</dl>

== Debugger.Environment ==

A <code>Debugger.Environment</code> instance represents a lexical environment, associating names with variables. Each <code>Debugger.Frame</code> instance representing a debuggee frame has an associated environment object describing the variables in scope in that frame; and each <code>Debugger.Object</code> instance representing a debuggee function has an environment object representing the environment the function has closed over.

ECMAScript environments form a tree, in which each local environment is parented by its enclosing environment (in ECMAScript terms, its 'outer' environment). We say an environment <i>binds</i> an identifier if that environment itself associates the identifier with a variable, independently of its outer environments. We say an identifier is <i>in scope</i> in an environment if the identifier is bound in that environment or any enclosing environment.

SpiderMonkey creates <code>Debugger.Environment</code> instances as needed as the debugger inspects stack frames and function objects; calling <code>Debugger.Environment</code> as a function or constructor raises a <code>TypeError</code> exception.

SpiderMonkey creates exactly one <code>Debugger.Environment</code> instance for each environment it presents via a given <code>Debugger</code> instance: if the debugger encounters the same environment through two different routes (perhaps two functions have closed over the same environment), SpiderMonkey presents the same <code>Debugger.Environment</code> instance to the debugger each time. This means that the debugger can use the <code>==</code> operator to recognize when two <code>Debugger.Environment</code> instances refer to the same environment in the debuggee, and place its own properties on a <code>Debugger.Environment</code> instance to store metadata about particular environments.

(If more than one <code>Debugger</code> instance is debugging the same code, each <code>Debugger</code> gets a separate <code>Debugger.Environment</code> instance for a given environment. This allows the code using each <code>Debugger</code> instance to place whatever properties it likes on its own <code>Debugger.Object</code> instances, without worrying about interfering with other debuggers.)

<code>Debugger.Environment</code> instances protect their referents from the garbage collector; as long as the <code>Debugger.Environment</code> instance is live, the referent remains live. Garbage collection has no visible effect on <code>Debugger.Environment</code> instances.

=== Accessor Properties of the Debugger.Environment Prototype Object === 

A <code>Debugger.Environment</code> instance inherits the following accessor properties from its prototype:

<dl>
<dt>type
<dd>The type of this environment object, one of the following values:
<ul>
<li>"declarative", indicating that the environment is a declarative environment record. Function calls, calls to <code>eval</code>, <code>let</code> blocks, <code>catch</code> blocks, and the like create declarative environment records.
<li>"object", indicating that the environment's bindings are the properties of an object. The global object and DOM elements appear in the chain of environments via object environments. (Note that <code>with</code> statements have their own environment type.)
<li>"with", indicating that the environment was introduced by a <code>with</code> statement.
</ul>

<dt>parent
<dd>The environment that encloses this one (the "outer" environment, in ECMAScript terminology), or <code>null</code> if this is the outermost environment.

<dt>object
<dd>A <code>Debugger.Object</code> instance referring to the object whose properties this environment reflects. If this is a declarative environment record, this accessor throws a <code>TypeError</code> (since declarative environment records have no such object). Both <code>"object"</code> and <code>"with"</code> environments have <code>object</code> properties that provide the object whose properties they reflect as variable bindings.

<dt>callee
<dd>If this environment represents the variable environment (the top-level environment within the function, which receives <code>var</code> definitions) for a call to a function <i>f</i>, then this property's value is a <code>Debugger.Object</code> instance referring to <i>f</i>. Otherwise, this property's value is <code>null</code>.

</dl>

=== Function Properties of the Debugger.Environment Prototype Object ===

The methods described below may only be called with a <code>this</code> value referring to a <code>Debugger.Environment</code> instance; they may not be used as methods of other kinds of objects.

<dl>
<dt>names()
<dd>Return an array of strings giving the names of the identifiers bound by this environment. The result does not include the names of identifiers bound by enclosing environments.

<dt>getVariable(<i>name</i>)
<dd>Return the value of the variable bound to <i>name</i> in this environment, or <code>undefined</code> if this environment does not bind <i>name</i>. <i>Name</i> must be a string that is a valid ECMAScript identifier name. The result is a debuggee value.

This is not an [[#Invocation_Functions_and_.22debugger.22_Frames|invocation function]]; if this call would cause debuggee code to run (say, because the environment is a <code>"with"</code> environment, and <i>name</i> refers to an accessor property of the <code>with</code> statement's operand), this call throws a [[#The_Debugger.DebuggeeWouldRun_Exception|<code>Debugger.DebuggeeWouldRun</code>]] exception.

<dt>setVariable(<i>name</i>, <i>value</i>)
<dd>Store <i>value</i> as the value of the variable bound to <i>name</i> in this environment, creating the variable if it does not exist. <i>Name</i> must be a string that is a valid ECMAScript identifier name; <i>value</i> must be a debuggee value.

If this call would add a new variable to this environment, but implementation restrictions prevent SpiderMonkey from extending the environment as requested, this call throws an <code>Error</code> exception.

This is not an [[#Invocation_Functions_and_.22debugger.22_Frames|invocation function]]; if this call would cause debuggee code to run, this call throws a [[#The_Debugger.DebuggeeWouldRun_Exception|<code>Debugger.DebuggeeWouldRun</code>]] exception.

<dt>getVariableDescriptor(<i>name</i>)
<dd>Return an property descriptor describing the variable bound to <i>name</i> in this environment, of the sort returned by <code>Debugger.Object.prototype.getOwnPropertyDescriptor</code>. <i>Name</i> must be a string whose value is a valid ECMAScript identifier name.

If this is an <code>"object"</code> or <code>"with"</code> environment record, this simply returns the descriptor for the given property of the environment's object. If this is a declarative environment record, this returns a descriptor reflecting the binding's mutability as the descriptor's <code>writable</code> property, and its deletability as the descriptor's <code>configurable</code> property. All declarative environment record bindings are marked as <code>enumerable</code>. <i>(This isn't great; the semantics of variables in declarative enviroments don't really match those of properties, so 
writing code that operates properly on descriptors for either kind may be difficult.)</i>

If there is no variable named <i>name</i> bound in this environment, throw a <code>ReferenceError</code>.

<dt>defineVariable(<i>name</i>, <i>descriptor</i>)
<dd>Create or reconfigure the variable bound to <i>name</i> in this environment according to <i>descriptor</i>. <i>Descriptor</i> is the sort of value returned by <code>getVariableDescriptor</code>. On success, return <code>undefined</code>; on failure, throw an appropriate exception. <i>Name</i> must be a string whose value is a valid ECMAScript identifier name.

If implementation restrictions prevent SpiderMonkey from creating or reconfiguring the variable as requested, this call throws an <code>Error</code> exception.

<dt>find(<i>name</i>)
<dd>Return a reference to the innermost environment, starting with this environment, that binds <i>name</i>. If <i>name</i> is not in scope in this environment, return <code>null</code>. <i>Name</i> must be a string whose value is a valid ECMAScript identifier name.

<dt>eval(<i>code</i>) <i>(future plan)</i>
<dd>Evaluate <i>code</i> in this environment, and return a [[#Completion_Values|completion value]] describing how it completed. <i>Code</i> is a string. All extant handler methods, breakpoints, watchpoints, and so on remain active during the call. This function follows the [[#Invocation_Functions_and_.22debugger.22_Frames|invocation function conventions]].

<i>Code</i> is interpreted as strict mode code when it contains a Use Strict Directive.

If <i>code</i> is not strict mode code, then variable declarations in <i>code</i> affect this environment. (In the terms used by the ECMAScript specification, the <code>VariableEnvironment</code> of the execution context for the eval code is the <code>VariableEnvironment</code> this <code>Debugger.Environment</code> instance represents.) If implementation restrictions prevent SpiderMonkey from extending this environment as requested, this call throws an <code>Error</code> exception.

<dt>evalWithBindings(<i>code</i>, <i>bindings</i>) <i>(future plan)</i>
<dd>Like <code>eval</code>, but evaluate <i>code</i> in this environment, extended with bindings from the object <i>bindings</i>. For each own enumerable property of <i>bindings</i> named <i>name</i> whose value is <i>value</i>, include a variable in the environment in which <i>code</i> is evaluated named <i>name</i>, whose value is <i>value</i>. Each <i>value</i> must be a debuggee value. (This is not like a <code>with</code> statement: <i>code</i> may access, assign to, and delete the introduced bindings without having any effect on the <i>bindings</i> object.)

This method allows debugger code to introduce temporary bindings that are visible to the given debuggee code and which refer to debugger-held debuggee values, and do so without mutating any existing debuggee environment.

Note that, like <code>eval</code>, declarations in the <i>code</i> passed to <code>evalWithBindings</code> affect this environment, even as <i>code</i> is evaluated with <i>bindings</i> visible. (In the terms used by the ECMAScript specification, the <code>VariableEnvironment</code> of the execution context for the eval code is the <code>VariableEnvironment</code> this environment represents, and the <i>bindings</i> appear in a new declarative environment, which is the eval code's <code>LexicalEnvironment</code>.) If implementation restrictions prevent SpiderMonkey from extending this environment as requested, this call throws an <code>Error</code> exception.
</dl>

== Future Work ==

<ul>
<li>Since common optimizations (say, the "null closure" closure representation) make environments that one would expect to be present, given the source code, unavailable at run time, <code>Debugger.Environment</code> should provide ways to reflect what is and is not available.
<li>It may be possible for <code>Debugger.Frame.prototype.environment</code> to return more complete environment chains than <code>Debugger.Object.prototype.environment</code>. This possibility should be documented, along with its effects on environment identity.
<li>The interface should provide clean, predictable ways to observe the effects of garbage collection. For example, perhaps it should provide an interface like [[http://www.cs.indiana.edu/~dyb/pubs/guardians-abstract.html R. Kent Dybvig's guardians]] for observing when objects and scripts become unreachable.
</ul>

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