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 # Debugger.Object

 A `Debugger.Object` instance represents an object in the debuggee,

 providing reflection-oriented methods to inspect and modify its referent.

 The referent's properties do not appear directly as properties of the

 `Debugger.Object` instance; the debugger can access them only through

 methods like `Debugger.Object.prototype.getOwnPropertyDescriptor` and

 `Debugger.Object.prototype.defineProperty`, ensuring that the debugger will

 not inadvertently invoke the referent's getters and setters.

 SpiderMonkey creates exactly one `Debugger.Object` instance for each

 debuggee object it presents to a given [`Debugger`][debugger-object]

 instance: if the debugger encounters the same object through two different

 routes (perhaps two functions are called on the same object), SpiderMonkey

 presents the same `Debugger.Object` instance to the debugger each time.

 This means that the debugger can use the `==` operator to recognize when

 two `Debugger.Object` instances refer to the same debuggee object, and

 place its own properties on a `Debugger.Object` instance to store metadata

 about particular debuggee objects.

 JavaScript code in different compartments can have different views of the

 same object. For example, in Firefox, code in privileged compartments sees

 content DOM element objects without redefinitions or extensions made to

 that object's properties by content code. (In Firefox terminology,

 privileged code sees the element through an "xray wrapper".) To ensure that

 debugger code sees each object just as the debuggee would, each

 `Debugger.Object` instance presents its referent as it would be seen from a

 particular compartment. This "viewing compartment" is chosen to match the

 way the debugger came across the referent. As a consequence, a single

 [`Debugger`][debugger-object] instance may actually have several

 `Debugger.Object` instances: one for each compartment from which the

 referent is viewed.

 If more than one [`Debugger`][debugger-object] instance is debugging the

 same code, each [`Debugger`][debugger-object] gets a separate

 `Debugger.Object` instance for a given object. This allows the code using

 each [`Debugger`][debugger-object] instance to place whatever properties it

 likes on its own `Debugger.Object` instances, without worrying about

 interfering with other debuggers.

 While most `Debugger.Object` instances are created by SpiderMonkey in the

 process of exposing debuggee's behavior and state to the debugger, the

 debugger can use `Debugger.Object.prototype.makeDebuggeeValue` to create

 `Debugger.Object` instances for given debuggee objects, or use

 `Debugger.Object.prototype.copy` and `Debugger.Object.prototype.create` to

 create new objects in debuggee compartments, allocated as if by particular

 debuggee globals.

 `Debugger.Object` instances protect their referents from the garbage

 collector; as long as the `Debugger.Object` instance is live, the referent

 remains live. This means that garbage collection has no visible effect on

 `Debugger.Object` instances.

 ## Accessor Properties of the Debugger.Object prototype

 A `Debugger.Object` instance inherits the following accessor properties

 from its prototype:

 `proto`

 :   The referent's prototype (as a new `Debugger.Object` instance), or

     `null` if it has no prototype.

 `class`

 :   A string naming the ECMAScript `[[Class]]` of the referent.

 `callable`

 :   `true` if the referent is a callable object (such as a function or a

     function proxy); false otherwise.

 `name`

 :   The name of the referent, if it is a named function. If the referent is

     an anonymous function, or not a function at all, this is `undefined`.

     This accessor returns whatever name appeared after the `function`

     keyword in the source code, regardless of whether the function is the

     result of instantiating a function declaration (which binds the

     function to its name in the enclosing scope) or evaluating a function

     expression (which binds the function to its name only within the

     function's body).

 `displayName`

 :   The referent's display name, if the referent is a function with a

     display name. If the referent is not a function, or if it has no display

     name, this is `undefined`.

     If a function has a given name, its display name is the same as its

     given name. In this case, the `displayName` and `name` properties are

     equal.

     If a function has no name, SpiderMonkey attempts to infer an appropriate

     name for it given its context. For example:

         function f() {}          // display name: f (the given name)

         var g = function () {};  // display name: g

         o.p = function () {};    // display name: o.p

         var q = {

           r: function () {}      // display name: q.r

         };

     Note that the display name may not be a proper JavaScript identifier,

     or even a proper expression: we attempt to find helpful names even when

     the function is not immediately assigned as the value of some variable

     or property. Thus, we use <code><i>a</i>/<i>b</i></code> to refer to

     the <i>b</i> defined within <i>a</i>, and <code><i>a</i>&lt;</code> to

     refer to a function that occurs somewhere within an expression that is

     assigned to <i>a</i>. For example:

         function h() {

           var i = function() {};    // display name: h/i

           f(function () {});        // display name: h/<

         }

         var s = f(function () {});  // display name: s<

 `parameterNames`

 :   If the referent is a debuggee function, the names of the its parameters,

     as an array of strings. If the referent is not a debuggee function, or

     not a function at all, this is `undefined`.

     If the referent is a host function for which parameter names are not

     available, return an array with one element per parameter, each of which

     is `undefined`.

     If the referent is a function proxy, return an empty array.

     If the referent uses destructuring parameters, then the array's elements

     reflect the structure of the parameters. For example, if the referent is

     a function declared in this way:

         function f(a, [b, c], {d, e:f}) { ... }

     then this `Debugger.Object` instance's `parameterNames` property would

     have the value:

         ["a", ["b", "c"], {d:"d", e:"f"}]

 `script`

 :   If the referent is a function that is debuggee code, this is that

     function's script, as a [`Debugger.Script`][script] instance. If the

     referent is a function proxy or not debuggee code, this is `undefined`.

 `environment`

 :   If the referent is a function that is debuggee code, a

     [`Debugger.Environment`][environment] instance representing the lexical

     environment enclosing the function when it was created. If the referent

     is a function proxy or not debuggee code, this is `undefined`.

 `proxyHandler`

 :   If the referent is a proxy whose handler object was allocated by

     debuggee code, this is its handler object—the object whose methods are

     invoked to implement accesses of the proxy's properties. If the referent

     is not a proxy whose handler object was allocated by debuggee code, this

     is `null`.

 `proxyCallTrap`

 :   If the referent is a function proxy whose handler object was allocated

     by debuggee code, this is its call trap function—the function called

     when the function proxy is called. If the referent is not a function

     proxy whose handler object was allocated by debuggee code, this is

     `null`.

 `proxyConstructTrap`

 :   If the referent is a function proxy whose handler object was allocated

     by debuggee code, its construction trap function—the function called

     when the function proxy is called via a `new` expression. If the

     referent is not a function proxy whose handler object was allocated by

     debuggee code, this is `null`.

 `global`

 :   A `Debugger.Object` instance referring to the global object in whose

     scope the referent was allocated. This does not unwrap cross-compartment

     wrappers: if the referent is a wrapper, the result refers to the

     wrapper's global, not the wrapped object's global. The result refers to

     the global directly, not via a wrapper.

 `hostAnnotations`

 :   A JavaScript object providing further metadata about the referent, or

     `null` if none is available. The metadata object is in the same

     compartment as this `Debugger.Object` instance. The same metadata

     object is returned each time for a given `Debugger.Object` instance.

     A typical JavaScript embedding provides "host objects" to expose

     application-specific functionality to scripts. The `hostAnnotations`

     accessor consults the embedding for additional information about the

     referent that might be of interest to the debugger. The returned

     object's properties' meanings are up to the embedding. For example, a

     web browser might provide host annotations for global objects to

     distinguish top-level windows, iframes, and internal JavaScript scopes.

     By convention, host annotation objects have a string-valued `"type"`

     property that, taken together with the object's class, indicate what

     sort of thing the referent is. The host annotation object's other

     properties provide further details, as appropriate for the type. For

     example, in Firefox, a metadata object for a JavaScript Module's global

     object might look like this:

         { "type":"jsm", "uri":"resource:://gre/modules/XPCOMUtils.jsm" }

     Firefox provides [DebuggerHostAnnotationsForFirefox annotations] for its

     host objects.

 ## Function Properties of the Debugger.Object prototype

 The functions described below may only be called with a `this` value

 referring to a `Debugger.Object` instance; they may not be used as methods

 of other kinds of objects. The descriptions use "referent" to mean "the

 referent of this `Debugger.Object` instance".

 Unless otherwise specified, these methods are not

 [invocation functions][inv fr]; if a call would cause debuggee code to run

 (say, because it gets or sets an accessor property whose handler is

 debuggee code, or because the referent is a proxy whose traps are debuggee

 code), the call throws a [`Debugger.DebuggeeWouldRun`][wouldrun] exception.

 <code>getProperty(<i>name</i>)</code>

 :   Return the value of the referent's property named <i>name</i>, or

     `undefined` if it has no such property. <i>Name</i> must be a string.

     The result is a debuggee value.

 <code>setProperty(<i>name</i>, <i>value</i>)</code>

 :   Store <i>value</i> as the value of the referent's property named

     <i>name</i>, creating the property if it does not exist. <i>Name</i>

     must be a string; <i>value</i> must be a debuggee value.

 <code>getOwnPropertyDescriptor(<i>name</i>)</code>

 :   Return a property descriptor for the property named <i>name</i> of the

     referent. If the referent has no such property, return `undefined`.

     (This function behaves like the standard

     `Object.getOwnPropertyDescriptor` function, except that the object being

     inspected is implicit; the property descriptor returned is allocated as

     if by code scoped to the debugger's global object (and is thus in the

     debugger's compartment); and its `value`, `get`, and `set` properties,

     if present, are debuggee values.)

 `getOwnPropertyNames()`

 :   Return an array of strings naming all the referent's own properties, as

     if <code>Object.getOwnPropertyNames(<i>referent</i>)</code> had been

     called in the debuggee, and the result copied in the scope of the

     debugger's global object.

 <code>defineProperty(<i>name</i>, <i>attributes</i>)</code>

 :   Define a property on the referent named <i>name</i>, as described by

     the property descriptor <i>descriptor</i>. Any `value`, `get`, and

     `set` properties of <i>attributes</i> must be debuggee values. (This

     function behaves like `Object.defineProperty`, except that the target

     object is implicit, and in a different compartment from the function

     and descriptor.)

 <code>defineProperties(<i>properties</i>)</code>

 :   Add the properties given by <i>properties</i> to the referent. (This

     function behaves like `Object.defineProperties`, except that the target

     object is implicit, and in a different compartment from the

     <i>properties</i> argument.)

 <code>deleteProperty(<i>name</i>)</code>

 :   Remove the referent's property named <i>name</i>. Return true if the

     property was successfully removed, or if the referent has no such

     property. Return false if the property is non-configurable.

 `seal()`

 :   Prevent properties from being added to or deleted from the referent.

     Return this `Debugger.Object` instance. (This function behaves like the

     standard `Object.seal` function, except that the object to be sealed is

     implicit and in a different compartment from the caller.)

 `freeze()`

 :   Prevent properties from being added to or deleted from the referent, and

     mark each property as non-writable. Return this `Debugger.Object`

     instance. (This function behaves like the standard `Object.freeze`

     function, except that the object to be sealed is implicit and in a

     different compartment from the caller.)

 `preventExtensions()`

 :   Prevent properties from being added to the referent. (This function

     behaves like the standard `Object.preventExtensions` function, except

     that the object to operate on is implicit and in a different compartment

     from the caller.)

 `isSealed()`

 :   Return true if the referent is sealed—that is, if it is not extensible,

     and all its properties have been marked as non-configurable. (This

     function behaves like the standard `Object.isSealed` function, except

     that the object inspected is implicit and in a different compartment

     from the caller.)

 `isFrozen()`

 :   Return true if the referent is frozen—that is, if it is not extensible,

     and all its properties have been marked as non-configurable and

     read-only. (This function behaves like the standard `Object.isFrozen`

     function, except that the object inspected is implicit and in a

     different compartment from the caller.)

 `isExtensible()`

 :   Return true if the referent is extensible—that is, if it can have new

     properties defined on it. (This function behaves like the standard

     `Object.isExtensible` function, except that the object inspected is

     implicit and in a different compartment from the caller.)

 <code>copy(<i>value</i>)</code>

 :   Apply the HTML5 "structured cloning" algorithm to create a copy of

     <i>value</i> in the referent's global object (and thus in the referent's

     compartment), and return a `Debugger.Object` instance referring to the

     copy.

     Note that this returns primitive values unchanged. This means you can

     use `Debugger.Object.prototype.copy` as a generic "debugger value to

     debuggee value" conversion function—within the limitations of the

     "structured cloning" algorithm.

 <code>create(<i>prototype</i>, [<i>properties</i>])</code>

 :   Create a new object in the referent's global (and thus in the

     referent's compartment), and return a `Debugger.Object` referring to

     it. The new object's prototype is <i>prototype</i>, which must be an

     `Debugger.Object` instance. The new object's properties are as given by

     <i>properties</i>, as if <i>properties</i> were passed to

     `Debugger.Object.prototype.defineProperties`, with the new

     `Debugger.Object` instance as the `this` value.

 <code>makeDebuggeeValue(<i>value</i>)</code>

 :   Return the debuggee value that represents <i>value</i> in the debuggee.

     If <i>value</i> is a primitive, we return it unchanged; if <i>value</i>

     is an object, we return the `Debugger.Object` instance representing

     that object, wrapped appropriately for use in this `Debugger.Object`'s

     referent's compartment.

     Note that, if <i>value</i> is an object, it need not be one allocated

     in a debuggee global, nor even a debuggee compartment; it can be any

     object the debugger wishes to use as a debuggee value.

     As described above, each `Debugger.Object` instance presents its

     referent as viewed from a particular compartment. Given a

     `Debugger.Object` instance <i>d</i> and an object <i>o</i>, the call

     <code><i>d</i>.makeDebuggeeValue(<i>o</i>)</code> returns a

     `Debugger.Object` instance that presents <i>o</i> as it would be seen

     by code in <i>d</i>'s compartment.

 <code>decompile([<i>pretty</i>])</code>

 :   If the referent is a function that is debuggee code, return the

     JavaScript source code for a function definition equivalent to the

     referent function in its effect and result, as a string. If

     <i>pretty</i> is present and true, produce indented code with line

     breaks. If the referent is not a function that is debuggee code, return

     `undefined`.

 <code>call(<i>this</i>, <i>argument</i>, ...)</code>

 :   If the referent is callable, call it with the given <i>this</i> value

     and <i>argument</i> values, and return a [completion value][cv]

     describing how the call completed. <i>This</i> should be a debuggee

     value, or `{ asConstructor: true }` to invoke the referent as a

     constructor, in which case SpiderMonkey provides an appropriate `this`

     value itself. Each <i>argument</i> must be a debuggee value. All extant

     handler methods, breakpoints, watchpoints, and so on remain active

     during the call. If the referent is not callable, throw a `TypeError`.

     This function follows the [invocation function conventions][inv fr].

 <code>apply(<i>this</i>, <i>arguments</i>)</code>

 :   If the referent is callable, call it with the given <i>this</i> value

     and the argument values in <i>arguments</i>, and return a

     [completion value][cv] describing how the call completed. <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> must either be an

     array (in the debugger) of debuggee values, or `null` or `undefined`,

     which are treated as an empty array. All extant handler methods,

     breakpoints, watchpoints, and so on remain active during the call. If

     the referent is not callable, throw a `TypeError`. This function

     follows the [invocation function conventions][inv fr].

 <code>evalInGlobal(<i>code</i>, [<i>options</i>])</code>

 :   If the referent is a global object, evaluate <i>code</i> in that global

     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].

     If the referent is not a global object, throw a `TypeError` exception.

     <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 the referent global object. (In the terms used by the

     ECMAScript specification, the `VariableEnvironment` of the execution

     context for the eval code is the referent.)

     The <i>options</i> argument is as for [`Debugger.Frame.prototype.eval`][fr eval].

 <code>evalInGlobalWithBindings(<i>code</i>, <i>bindings</i>, [<i>options</i>])</code>

 :   Like `evalInGlobal`, but evaluate <i>code</i> using the referent as the

     variable object, but with a lexical 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 lexical 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 `evalInGlobal`, if the code passed to

     `evalInGlobalWithBindings` is not strict mode code, then any

     declarations it contains affect the referent global object, even as

     <i>code</i> is evaluated in an environment extended according to

     <i>bindings</i>. (In the terms used by the ECMAScript specification, the

     `VariableEnvironment` of the execution context for non-strict eval code

     is the referent, and the <i>bindings</i> appear in a new declarative

     environment, which is the eval code's `LexicalEnvironment`.)

     The <i>options</i> argument is as for [`Debugger.Frame.prototype.eval`][fr eval].

 `asEnvironment()`

 :   If the referent is a global object, return the [`Debugger.Environment`][environment]

     instance representing the referent as a variable environment for

     evaluating code. If the referent is not a global object, throw a

     `TypeError`.

 <code>setObjectWatchpoint(<i>handler</i>)</code> <i>(future plan)</i>

 :   Set a watchpoint on all the referent's own properties, reporting events

     by calling <i>handler</i>'s methods. Any previous watchpoint handler on

     this `Debugger.Object` instance is replaced. If <i>handler</i> is null,

     the referent is no longer watched. <i>Handler</i> may have the following

     methods, called under the given circumstances:

     <code>add(<i>frame</i>, <i>name</i>, <i>descriptor</i>)</code>

     :   A property named <i>name</i> has been added to the referent.

         <i>Descriptor</i> is a property descriptor of the sort accepted by

         `Debugger.Object.prototype.defineProperty`, giving the newly added

         property's attributes.

     <code>delete(<i>frame</i>, <i>name</i>)</code>

     :   The property named <i>name</i> is about to be deleted from the referent.

     <code>change(<i>frame</i>, <i>name</i>, <i>oldDescriptor</i>, <i>newDescriptor</i>)</code>

     :   The existing property named <i>name</i> on the referent is being changed

         from those given by <i>oldDescriptor</i> to those given by

         <i>newDescriptor</i>. This handler method is only called when attributes

         of the property other than its value are being changed; if only the

         value is changing, SpiderMonkey calls the handler's `set` method.

     <code>set(<i>frame</i>, <i>oldValue</i>, <i>newValue</i>)</code>

     :   The data property named <i>name</i> of the referent is about to have its

         value changed from <i>oldValue</i> to <i>newValue</i>.

         SpiderMonkey only calls this method on assignments to data properties

         that will succeed; assignments to un-writable data properties fail

         without notifying the debugger.

     <code>extensionsPrevented(<i>frame</i>)</code>

     :   The referent has been made non-extensible, as if by a call to

         `Object.preventExtensions`.

     For all watchpoint handler methods:

     * Handler calls receive the handler object itself as the `this` value.

     * The <i>frame</i> argument is the current stack frame, whose code is

       about to perform the operation on the object being reported.

     * If the method returns `undefined`, then SpiderMonkey makes the announced

       change to the object, and continues execution normally. If the method

       returns an object:

     * If the object has a `superseded` property whose value is a true value,

       then SpiderMonkey does not make the announced change.

     * If the object has a `resume` property, its value is taken as a

       [resumption value][rv], indicating how

       execution should proceed. (However, `return` resumption values are not

       supported.)

     * If a given method is absent from <i>handler</i>, then events of that

       sort are ignored. The watchpoint consults <i>handler</i>'s properties

       each time an event occurs, so adding methods to or removing methods from

       <i>handler</i> after setting the watchpoint enables or disables

       reporting of the corresponding events.

     * Values passed to <i>handler</i>'s methods are debuggee values.

       Descriptors passed to <i>handler</i>'s methods are ordinary objects in

       the debugger's compartment, except for `value`, `get`, and `set`

       properties in descriptors, which are debuggee values; they are the sort

       of value expected by `Debugger.Object.prototype.defineProperty`.

     * Watchpoint handler calls are cross-compartment, intra-thread calls: the

       call takes place in the same thread that changed the property, and in

       <i>handler</i>'s method's compartment (typically the same as the

       debugger's compartment).

     The new watchpoint belongs to the [`Debugger`][debugger-object] instance to which this

     `Debugger.Object` instance belongs; disabling the [`Debugger`][debugger-object] instance

     disables this watchpoint.

 `clearObjectWatchpoint()` <i>(future plan)</i>

 :   Remove any object watchpoint set on the referent.

 <code>setPropertyWatchpoint(<i>name</i>, <i>handler</i>)</code> <i>(future plan)</i>

 :   Set a watchpoint on the referent's property named <i>name</i>, reporting

     events by calling <i>handler</i>'s methods. Any previous watchpoint

     handler on this property for this `Debugger.Object` instance is

     replaced. If <i>handler</i> is null, the property is no longer watched.

     <i>Handler</i> is as described for

     `Debugger.Object.prototype.setObjectWatchpoint`, except that it does not

     receive `extensionsPrevented` events.

 <code>clearPropertyWatchpoint(<i>name</i>)</code> <i>(future plan)</i>

 :   Remove any watchpoint set on the referent's property named <i>name</i>.

 `unwrap()`

 :   If the referent is a wrapper that this `Debugger.Object`'s compartment

     is permitted to unwrap, return a `Debugger.Object` instance referring to

     the wrapped object. If we are not permitted to unwrap the referent,

     return `null`. If the referent is not a wrapper, return this

     `Debugger.Object` instance unchanged.

 `unsafeDereference()`

 :   Return the referent of this `Debugger.Object` instance.

     If the referent is an inner object (say, an HTML5 `Window` object),

     return the corresponding outer object (say, the HTML5 `WindowProxy`

     object). This makes `unsafeDereference` more useful in producing values

     appropriate for direct use by debuggee code, without using [invocation functions][inv fr].

     This method pierces the membrane of `Debugger.Object` instances meant to

     protect debugger code from debuggee code, and allows debugger code to

     access debuggee objects through the standard cross-compartment wrappers,

     rather than via `Debugger.Object`'s reflection-oriented interfaces. This

     method makes it easier to gradually adapt large code bases to this

     Debugger API: adapted portions of the code can use `Debugger.Object`

     instances, but use this method to pass direct object references to code

     that has not yet been updated.