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Debugger.Environment.md

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Debugger.Environment

A Debugger.Environment instance represents a lexical environment, associating names with variables. Each [Debugger.Frame][frame] instance representing a debuggee frame has an associated environment object describing the variables in scope in that frame; and each [Debugger.Object][object] 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 binds an identifier if that environment itself associates the identifier with a variable, independently of its outer environments. We say an identifier is in scope in an environment if the identifier is bound in that environment or any enclosing environment.

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

SpiderMonkey creates exactly one Debugger.Environment instance for each environment it presents via a given [Debugger][debugger-object] 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 Debugger.Environment instance to the debugger each time. This means that the debugger can use the == operator to recognize when two Debugger.Environment instances refer to the same environment in the debuggee, and place its own properties on a Debugger.Environment instance to store metadata about particular environments.

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

If a Debugger.Environment instance's referent is not a debuggee environment, then attempting to access its properties (other than inspectable) or call any its methods throws an instance of Error.

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

Accessor Properties of the Debugger.Environment Prototype Object

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

inspectable : True if this environment is a debuggee environment, and can therefore be inspected. False otherwise. All other properties and methods of Debugger.Environment instances throw if applied to a non-inspectable environment.

type : The type of this environment object, one of the following values:

* "declarative", indicating that the environment is a declarative
  environment record. Function calls, calls to `eval`, `let` blocks,
  `catch` blocks, and the like create declarative environment records.

* "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 `with`
  statements have their own environment type.)

* "with", indicating that the environment was introduced by a `with`
  statement.

parent : The environment that encloses this one (the "outer" environment, in ECMAScript terminology), or null if this is the outermost environment.

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

callee : If this environment represents the variable environment (the top-level environment within the function, which receives var definitions) for a call to a function f, then this property's value is a [Debugger.Object][object] instance referring to f. Otherwise, this property's value is null.

optimizedOut : True if this environment is optimized out. False otherwise. For example, functions whose locals are never aliased may present optimized-out environments. When true, getVariable returns an ordinary JavaScript object whose optimizedOut property is true on all bindings, and setVariable throws a ReferenceError.

Function Properties of the Debugger.Environment Prototype Object

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

names() : 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.

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

JavaScript engines often omit variables from environments, to save space
and reduce execution time. If the given variable should be in scope, but
`getVariable` is unable to produce its value, it returns an ordinary
JavaScript object (not a [`Debugger.Object`][object] instance) whose
`optimizedOut` property is `true`.

This is not an [invocation function][inv fr];
if this call would cause debuggee code to run (say, because the
environment is a `"with"` environment, and <i>name</i> refers to an
accessor property of the `with` statement's operand), this call throws a
[`Debugger.DebuggeeWouldRun`][wouldrun]
exception.

setVariable(name, value) : Store value as the value of the variable bound to name in this environment. Name must be a string that is a valid ECMAScript identifier name; value must be a debuggee value.

If this environment binds no variable named <i>name</i>, throw a
`ReferenceError`.

This is not an [invocation function][inv fr];
if this call would cause debuggee code to run, this call throws a
[`Debugger.DebuggeeWouldRun`][wouldrun]
exception.

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