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+# 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
+<i>visible frames</i>.
+
+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.)
+
+
+## <span id='invf'>Invocation</span> Functions and "debugger" Frames
+
+An <i>invocation function</i> 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 <i>older</i> 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 <i>older</i> 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.
+
+
+## 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.
+
+    * `"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.
+
+`this`
+:   The value of `this` for this frame (a debuggee value).
+
+`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`.
+
+`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 `this`value. 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.
+
+
+## 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.
+
+<code id="eval">eval(<i>code</i>, [<i>options</i>])</code>
+:   Evaluate <i>code</i> in the execution context of this frame, and return
+    a [completion value][cv] describing how it completed. <i>Code</i> is a
+    string. If this frame's `environment` property is `null`, 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>.
+
+<code>evalWithBindings(<i>code</i>, <i>bindings</i>, [<i>options</i>])</code>
+:   Like `eval`, but evaluate <i>code</i> in the environment of this frame,
+    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 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.