Debugger.Frame.md

Debugger.Frame

A Debugger.Frame instance represents a [visible stack frame][vf]. Given a Debugger.Frame 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 [Debugger][debugger-object] instance, SpiderMonkey creates only one Debugger.Frame 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 == to decide whether two expressions refer to the same frame, and so on.

(If more than one [Debugger][debugger-object] instance is debugging the same code, each [Debugger][debugger-object] gets a separate Debugger.Frame instance for a given frame. This allows the code using each [Debugger][debugger-object] instance to place whatever properties it likes on its Debugger.Frame 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 Debugger.Frame instance referring to that frame becomes inactive: its live property becomes false, 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.

Visible Frames

When inspecting the call stack, [Debugger][debugger-object] 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 [Debugger][debugger-object] does reveal visible frames.

A frame is a visible frame if any of the following are true:

• it is running [debuggee code][dbg code];

• its immediate caller is a frame running debuggee code; or

• it is a ["debugger" frame][inv fr], representing the continuation of debuggee code invoked by the debugger.

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 invocation function is any function in this interface that allows the debugger to invoke code in the debuggee: Debugger.Object.prototype.call, Debugger.Frame.prototype.eval, 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:

1. Let older be the youngest visible frame on the stack, or null if there is no such frame. (This is never one of the the debugger's own frames; those never appear as Debugger.Frame instances.)

2. Push a "debugger" frame on the stack, with older as its older property.

3. Invoke the debuggee code as appropriate for the given invocation function, with the "debugger" frame as its continuation. For example, Debugger.Frame.prototype.eval pushes an "eval" frame for code it runs, whereas Debugger.Object.prototype.call pushes a "call" frame.

4. When the debuggee code completes, whether by returning, throwing an exception or being terminated, pop the "debugger" frame, and return an appropriate [completion value][cv] from the invocation function to the debugger.

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 "debugger" frame makes this continuation explicit, and makes it easier to find the extent of the stack created for the invocation.

Suspended Frames

Some frames can be suspended.

When a generator yields a value, or when an async function awaits a value, the current frame is suspended and removed from the stack, and other JS code has a chance to run. Later (if the awaited promise becomes resolved, for example), SpiderMonkey will resume the frame. It will be put back onto the stack, and execution will continue where it left off. Only generator and async function call frames can be suspended and resumed.

Currently, a frame's live property is false while it's suspended (bug 1448880).

SpiderMonkey uses the same Debugger.Frame object each time a generator or async function call is put back onto the stack. This means that the onStep handler can be used to step over yield and await.

The frame.onPop handler is called each time a frame is suspended, and the Debugger.onEnterFrame handler is called each time a frame is resumed. (This means these events can fire multiple times for the same Frame object, which is odd, but accurately conveys what's happening.)

Stepping Into Generators: The "Initial Yield"

When a debuggee generator is called, something weird happens. The .onEnterFrame hook fires, as though we're stepping into the generator. But the code inside the generator doesn't run. Instead it immediately returns. Then we sometimes get another .onEnterFrame event for the same generator. What's going on?

To explain this, we first have to describe how generator calls work, according to the ECMAScript language specification. Note that except for step 3, it's exactly like a regular function call.

1. An "execution context" (what we call a Frame) is pushed to the stack.
2. An environment is created (for arguments and local variables). Argument-default-value-expressions, if any, are evaluated.
3. A generator object is created, initially suspended at the start of the generator body.
4. The stack frame is popped, and the generator object is returned to the caller.

The JavaScript engine actually carries out these steps, in this order. So when a debuggee generator is called, here's what you'll observe:

1. The debugger.onEnterFrame hook fires.
2. The debugger can step through the argument-default-value code, if any.
3. The body of the generator does not run yet. Instead, a generator object is created and suspended (which does not fire any debugger events).
4. The frame.onPop hook fires, with a completion value of {return: (the new generator object) }.

In SpiderMonkey, this process of suspending and returning a new generator object is called the "initial yield".

If the caller then uses the generator's .next() method, which may or may not happen right away depending on the debuggee code, the suspended generator will be resumed, firing .onEnterFrame again.

Stepping into async functions — SpiderMonkey also performs an initial yield for async functions. This is not so easy to defend by citing the spec; we're just exposing internal SpiderMonkey implementation details. When the initial yield happens, you'll observe an extra .onPop returning a generator object that SpiderMonkey will use internally, followed immediately by a second .onEnterFrame event for the same frame.

Accessor Properties of the Debugger.Frame Prototype Object

A Debugger.Frame instance inherits the following accessor properties from its prototype:

type : A string describing what sort of frame this is:

* `"call"`: a frame running a function call. (We may not be able to obtain
  frames for calls to host functions.)

* `"eval"`: a frame running code passed to `eval`.

* `"global"`: a frame running global code (JavaScript that is neither of
  the above).

* `"module"`: a frame running code at the top level of a module.

* `"wasmcall"`: a frame running a WebAssembly function call.

* `"debugger"`: a frame for a call to user code invoked by the debugger
  (see the `eval` method below).

implementation : A string describing which tier of the JavaScript engine this frame is executing in:

* `"interpreter"`: a frame running in the interpreter.

* `"baseline"`: a frame running in the unoptimizing, baseline JIT.

* `"ion"`: a frame running in the optimizing JIT.

* `"wasm"`: a frame running in WebAssembly baseline JIT.

this : The value of this for this frame (a debuggee value). For a wasmcall frame, this property throws a TypeError.

older : The next-older visible frame, in which control will resume when this frame completes. If there is no older frame, this is null.

depth : The depth of this frame, counting from oldest to youngest; the oldest frame has a depth of zero.

live : True if the frame this Debugger.Frame instance refers to is still on the stack; false if it has completed execution or been popped in some other way.

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

offset : The offset of the bytecode instruction currently being executed in script, or undefined if the frame's script property is null. For a wasmcall frame, this property throws a TypeError.

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

callee : The function whose application created this frame, as a debuggee value, or null if this is not a "call" frame.

generator : True if this frame is a generator frame, false otherwise.

constructing : True if this frame is for a function called as a constructor, false otherwise.

arguments : The arguments passed to the current frame, or null if this is not a "call" frame. When non-null, this is an object, allocated in the same global as the debugger, with Array.prototype on its prototype chain, a non-writable length 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.

Handler Methods of Debugger.Frame Instances

Each Debugger.Frame 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.

Debugger.Frame instances inherit the following handler method properties:

onStep : This property must be either undefined 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 Debugger.Frame instance as the thisvalue. The function should return a [resumption value][rv] 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 [`Debugger`][debugger-object] instances each have
`Debugger.Frame` instances for a given stack frame with `onStep`
handlers set, their handlers are run in an unspecified order. If any
`onStep` handler forces the frame to return early (by returning a
resumption value other than `undefined`), any remaining debuggers'
`onStep` handlers do not run.

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

onPop : This property must be either undefined or a function. If it is a function, SpiderMonkey calls it just before this frame is popped, passing a [completion value][cv] indicating how this frame's execution completed, and providing this Debugger.Frame instance as the this value. The function should return a [resumption value][rv] indicating how execution should proceed. On newly created frames, this property's value is undefined.

When this handler is called, this frame's current execution location, as
reflected in its `offset` and `environment` properties, is the operation
which caused it to be unwound. In frames returning or throwing an
exception, the location is often a return or a throw statement. In frames
propagating exceptions, the location is a call.

When an `onPop` call reports the completion of a construction call
(that is, a function called via the `new` 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 `new` expression, which will be the value of
the frame's `this` property. (In ECMAScript terms, the `onPop` handler
receives the value returned by the `[[Call]]` method, not the value
returned by the `[[Construct]]` method.)

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

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

If multiple [`Debugger`][debugger-object] instances each have
`Debugger.Frame` instances for a given stack frame with `onPop`
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 handler is not called on `"debugger"` frames. It is also not called
when unwinding a frame due to an over-recursion or out-of-memory
exception.

The `onPop` handler is typically called only once for a given frame,
after which the frame becomes inactive. However, in the case of
[generators and async functions](suspended), `onPop` fires each time
the frame is suspended.

Function Properties of the Debugger.Frame Prototype Object

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

eval(code, [options]) : Evaluate code in the execution context of this frame, and return a [completion value][cv] describing how it completed. Code is a string. If this frame's environment property is null or type property is wasmcall, throw a TypeError. All extant handler methods, breakpoints, and so on remain active during the call. This function follows the [invocation function conventions][inv fr].

<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 `VariableEnvironment` of the
execution context for the eval code is the `VariableEnvironment` 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.

If given, <i>options</i> should be an object whose properties specify
details of how the evaluation should occur. The `eval` method
recognizes the following properties:

<code>url</code>
:   The filename or URL to which we should attribute <i>code</i>. If this
    property is omitted, the URL defaults to `"debugger eval code"`.

<code>lineNumber</code>
:   The line number at which the evaluated code should be claimed to begin
    within <i>url</i>.

evalWithBindings(code, bindings, [options]) : Like eval, but evaluate code in the environment of this frame, extended with bindings from the object bindings. For each own enumerable property of bindings named name whose value is value, include a variable in the environment in which code is evaluated named name, whose value is value. Each value must be a debuggee value. (This is not like a with statement: code may access, assign to, and delete the introduced bindings without having any effect on the bindings 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 `eval`, declarations in the <i>code</i> passed to
`evalWithBindings` 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 `VariableEnvironment`
of the execution context for the eval code is the `VariableEnvironment`
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 `LexicalEnvironment`.) If implementation restrictions
prevent SpiderMonkey from extending this frame's environment as
requested, this call throws an `Error` exception.

The <i>options</i> argument is as for
[`Debugger.Frame.prototype.eval`][fr eval], described above.
Also like `eval`, if this frame's `environment` property is `null` or
`type` property is `wasmcall`, throw a `TypeError`.