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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 <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 `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 <i>f</i>, then this property's value is a
+    [`Debugger.Object`][object] instance referring to <i>f</i>. Otherwise,
+    this property's value is `null`.
+
+
+
+## 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.
+
+<code>getVariable(<i>name</i>)</code>
+:   Return the value of the variable bound to <i>name</i> in this
+    environment, or `undefined` 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.
+
+    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.
+
+<code>setVariable(<i>name</i>, <i>value</i>)</code>
+:   Store <i>value</i> as the value of the variable bound to <i>name</i> in
+    this environment. <i>Name</i> must be a string that is a valid
+    ECMAScript identifier name; <i>value</i> 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.
+
+<code>getVariableDescriptor(<i>name</i>)</code>
+:   Return an property descriptor describing the variable bound to
+    <i>name</i> in this environment, of the sort returned by
+    `Debugger.Object.prototype.getOwnPropertyDescriptor`. <i>Name</i> must
+    be a string whose value is a valid ECMAScript identifier name.
+
+    If this is an `"object"` or `"with"` 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
+    `writable` property, and its deletability as the descriptor's
+    `configurable` property. All declarative environment record bindings are
+    marked as `enumerable`. <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 this environment binds no variable named <i>name</i>, throw a
+    `ReferenceError`.
+
+<code>defineVariable(<i>name</i>, <i>descriptor</i>)</code>
+:   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 `getVariableDescriptor`. On success, return
+    `undefined`; 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 `Error`
+    exception.
+
+<code>deleteVariable(<i>name</i>)</code>
+:   Delete this environment's binding for <i>name</i>.
+
+    If this environment binds no variable named <i>name</i>, throw a
+    `ReferenceError`.
+
+    If implementation restrictions prevent SpiderMonkey from deleting the
+    variable as requested, this call throws an `Error` exception.
+
+<code>find(<i>name</i>)</code>
+:   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 `null`. <i>Name</i> must be a string whose
+    value is a valid ECMAScript identifier name.
+
+<code>eval(<i>code</i>)</code> <i>(future plan)</i>
+:   Evaluate <i>code</i> in this environment, and return a
+    [completion value][cv] 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 function conventions][inv fr].
+
+    <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 `VariableEnvironment` of the execution
+    context for the eval code is the `VariableEnvironment` this
+    `Debugger.Environment` instance represents.) If implementation
+    restrictions prevent SpiderMonkey from extending this environment as
+    requested, this call throws an `Error` exception.
+
+<code>evalWithBindings(<i>code</i>, <i>bindings</i>)</code> <i>(future plan)</i>
+:   Like `eval`, 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 `with`
+    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 `eval`, declarations in the <i>code</i> passed to
+    `evalWithBindings` affect this environment, even as <i>code</i> is
+    evaluated with <i>bindings</i> visible. (In the terms used by the
+    ECMAScript specification, the `VariableEnvironment` of the execution
+    context for the eval code is the `VariableEnvironment` this environment
+    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
+    environment as requested, this call throws an `Error` exception.
+
+