====================
Debugger.Environment
====================

A ``Debugger.Environment`` instance represents a lexical environment, associating names with variables. Each :doc:`Debugger.Frame <../debugger.frame/index>` instance representing a debuggee frame has an associated environment object describing the variables in scope in that frame; and each :doc:`Debugger.Object <../debugger.object/index>` 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 :doc:`Debugger <../debugger/index>` 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 :doc:`Debugger <../debugger/index>` instance is debugging the same code, each :doc:`Debugger <../debugger/index>` gets a separate ``Debugger.Environment`` instance for a given environment. This allows the code using each :doc:`Debugger <../debugger/index>` instance to place whatever properties it likes on its own :doc:`Debugger.Object <../debugger.object/index>` 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
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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 :doc:`Debugger.Object <../debugger.object/index>` 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 :doc:`Debugger.Object <../debugger.object/index>` 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
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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 :doc:`Debugger.Object <../debugger.object/index>` instance) whose ``optimizedOut`` property is ``true``.

  This is not an :ref:`invocation function <debugger-api-debugger-frame-invf>`; if this call would cause debuggee code to run (say, because the environment is a ``"with"`` environment, and *name* refers to an accessor property of the ``with`` statement’s operand), this call throws a ``Debugger.DebuggeeWouldRun`` 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 *name*, throw a ``ReferenceError``.

  This is not an :ref:`invocation function <debugger-api-debugger-frame-invf>`; if this call would cause debuggee code to run, this call throws a ``Debugger.DebuggeeWouldRun`` 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.


Source Metadata
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Generated from file:
  js/src/doc/Debugger/Debugger.Environment.md

Watermark:
 sha256:3d6f67939e351803d5d7fe201ed38c4aaf766caf032f255e168df1f1c6fe73cb

Changeset:
  `7ae377917236 <https://hg.mozilla.org/mozilla-central/rev/7ae377917236>`_
