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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.

 Stack frames that represent the control state of generator-iterator objects

 behave in a special way, described in [Generator Frames][generator] below.

 ## 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.)

 ## Invocation 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).

     * `"debugger"`: a frame for a call to user code invoked by the debugger

       (see the `eval` method below).

 `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`. (On a

     suspended generator frame, the value of this property is `null`; see

     [Generator Frames][generator].)

 `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 (or, in the case of generator-iterator objects, has not yet

     finished its iteration); 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 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.

     <i>(Not yet implemented.)</i> When a generator frame yields a value,

     SpiderMonkey calls its `Debugger.Frame` instance's `onPop` handler

     method, if present, passing a `yield` resumption value; however, the

     `Debugger.Frame` instance remains live.

     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 property is ignored on `"debugger"` frames.

 `onResume`

 :   This property must be either `undefined` or a function. If it is a

     function, SpiderMonkey calls it if the current frame is a generator

     frame whose execution has just been resumed. The function should return

     a [resumption value][rv] indicating how execution should proceed. On

     newly created frames, this property's value is `undefined`.

     If the program resumed the generator by calling its `send` method and

     passing a value, then <i>value</i> is that value. Otherwise,

     <i>value</i> is `undefined`.

 ## 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, 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, 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.

 <code>pop(<i>completion</i>)</code> <i>(future plan)</i>

 :   Pop this frame (and any younger frames) from the stack as if this frame

     had completed as specified by the completion value <i>completion</i>.

     Note that this does <i>not</i> resume the debuggee's execution; it

     merely adjusts the debuggee's state to what it would be if this frame's

     execution had completed. The debuggee will only resume execution when

     you return from the handler method that brought control to the debugger

     originally.

     This cannot remove any `"call"` frames for calls to host functions from

     the stack. (We might be able to make this work eventually, but it will

     take some cleverness.)

 <code>replaceCall(<i>function</i>, <i>this</i>, <i>arguments</i>)</code> <i>(future plan)</i>

 :   Pop any younger frames from the stack, and then change this frame into

     a frame for a call to <i>function</i>, with the given <i>this</i> value

     and <i>arguments</i>. <i>This</i> should be a debuggee value, or

     `{ asConstructor: true }` to invoke <i>function</i> as a constructor,

     in which case SpiderMonkey provides an appropriate `this` value itself.

     <i>Arguments</i> should be an array of debuggee values. This frame must

     be a `"call"` frame.

     This can be used as a primitive in implementing some forms of a "patch

     and continue" debugger feature.

     Note that this does <i>not</i> resume the debuggee's execution; it

     merely adjusts the debuggee's state to what it would be if this frame

     were about to make this call. The debuggee will only resume execution

     when you return from the handler method that brought control to the

     debugger originally.

     Like `pop`, this cannot remove `"call"` frames for calls to host

     functions from the stack.

 ## Generator Frames

 <i>Not all behavior described in this section has been implemented

 yet.</i>

 SpiderMonkey supports generator-iterator objects, which produce a series of

 values by repeatedly suspending the execution of a function or expression.

 For example, calling a function that uses `yield` produces a

 generator-iterator object, as does evaluating a generator expression like

 `(i*i for each (i in [1,2,3]))`.

 A generator-iterator object refers to a stack frame with no fixed

 continuation frame. While the generator's code is running, its continuation

 is whatever frame called its `next` method; while the generator is

 suspended, it has no particular continuation frame; and when it resumes

 again, the continuation frame for that resumption could be different from

 that of the previous resumption.

 A `Debugger.Frame` instance representing a generator frame differs from an

 ordinary stack frame as follows:

 * A generator frame's `generator` property is true.

 * A generator frame disappears from the stack each time the generator

   yields a value and is suspended, and reappears atop the stack when it is

   resumed to produce the generator's next value. The same `Debugger.Frame`

   instance refers to the generator frame until it returns, throws an

   exception, or is terminated.

 * A generator frame's `older` property refers to the frame's continuation

   frame while the generator is running, and is `null` while the generator

   is suspended.

 * A generator frame's `depth` property reflects the frame's position on

   the stack when the generator is resumed, and is `null` while the

   generator is suspended.

 * A generator frame's `live` property remains true until the frame

   returns, throws an exception, or is terminated. Thus, generator frames

   can be live while not present on the stack.

 The other `Debugger.Frame` methods and accessor properties work as

 described on generator frames, even when the generator frame is suspended.

 You may examine a suspended generator frame's variables, and use its

 `script` and `offset` members to see which `yield` it is suspended at.

 A `Debugger.Frame` instance referring to a generator-iterator frame has a

 strong reference to the generator-iterator object; the frame (and its

 object) will live as long as the `Debugger.Frame` instance does. However,

 when the generator function returns, throws an exception, or is terminated,

 thus ending the iteration, the `Debugger.Frame` instance becomes inactive

 and its `live` property becomes `false`, just as would occur for any other

 sort of frame that is popped. A non-live `Debugger.Frame` instance no

 longer holds a strong reference to the generator-iterator object.