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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.
+
+### `scopeKind`
+If this is a declarative environment, a string describing the kind of scope
+which this environment is associated with, or `null` for other types of
+environments.  There is an assortment of possible scope kinds which can be
+generated, with a selection of possible values below.  Unlike the type
+accessor, the categorization this performs is specific to SpiderMonkey's
+implementation, and not derived from distinctions made in the ECMAScript
+language specification.
+
+* "function", indicating the top level body scope of a function for
+arguments and 'var' variables.
+
+* "function lexical", indicating the top level lexical scope in a function.
+
+### `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.
+
+### `calleeScript`
+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.Script`][script] instance referring to <i>f</i>'s script. 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 <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, in most cases.
+
+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`.
+
+Aside from the above case, this method can return something that is not a
+debuggee value in two other cases. If a function argument is missing, then it
+returns an ordinary JavaScript object whose `missingArgument` property is
+`true`. Finally, if a variable name is bound in the environment but not yet
+initialized (for example, if the debuggee is paused in the middle of an
+initializer expression) then it returns an ordinary JavaScript object whose
+`uninitialized` 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 <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.
+
+### `find(name)`
+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.
+
+
+[frame]: Debugger.Frame.md
+[object]: Debugger.Object.md
+[debugger-object]: Debugger.md
+[inv fr]: Debugger.Frame.html#invocation-functions-and-debugger-frames
+[wouldrun]: Conventions.html#the-debugger-debuggeewouldrun-exception