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 # The Debugger Object

 When called as a constructor, the `Debugger` object creates a new

 `Debugger` instance.

 <code>new Debugger([<i>global</i>, ...])</code>

 :   Create a debugger object, and apply its [`addDebuggee`][add] method to

     each of the given <i>global</i> objects to add them as the initial

     debuggees.

 ## Accessor Properties of the Debugger Prototype Object

 A `Debugger` instance inherits the following accessor properties from

 its prototype:

 `enabled`

 :   A boolean value indicating whether this `Debugger` instance's handlers,

     breakpoints, watchpoints, and the like are currently enabled. It is an

     accessor property with a getter and setter: assigning to it enables or

     disables this `Debugger` instance; reading it produces true if the

     instance is enabled, or false otherwise. This property is initially

     `true` in a freshly created `Debugger` instance.

     This property gives debugger code a single point of control for

     disentangling itself from the debuggee, regardless of what sort of

     events or handlers or "points" we add to the interface.

 `uncaughtExceptionHook`

 :   Either `null` or a function that SpiderMonkey calls when a call to a

     debug event handler, breakpoint handler, watchpoint handler, or similar

     function throws some exception, which we refer to as

     <i>debugger-exception</i> here. Exceptions thrown in the debugger are

     not propagated to debuggee code; instead, SpiderMonkey calls this

     function, passing <i>debugger-exception</i> as its sole argument and

     the `Debugger` instance as the `this` value. This function should

     return a [resumption value][rv], which determines how the debuggee

     should continue.

     If the uncaught exception hook itself throws an exception,

     <i>uncaught-hook-exception</i>, SpiderMonkey throws a new error object,

     <i>confess-to-debuggee-exception</i>, to the debuggee whose message

     blames the debugger, and includes textual descriptions of

     <i>uncaught-hook-exception</i> and the original

     <i>debugger-exception</i>.

     If `uncaughtExceptionHook`'s value is `null`, SpiderMonkey throws an

     exception to the debuggee whose message blames the debugger, and

     includes a textual description of <i>debugger-exception</i>.

     Assigning anything other than a callable value or `null` to this

     property throws a `TypeError` exception.

     (This is not an ideal way to handle debugger bugs, but the hope here is

     that some sort of backstop, even if imperfect, will make life easier for

     debugger developers. For example, an uncaught exception hook may have

     access to browser-level features like the `alert` function, which this

     API's implementation does not, making it possible to present debugger

     errors to the developer in a way suited to the context.)

 ## Debugger Handler Functions

 Each `Debugger` instance inherits accessor properties with which you can

 store handler functions for SpiderMonkey to call when given events occur

 in debuggee code.

 When one of the events described below occurs in debuggee code, the engine

 pauses the debuggee and calls the corresponding debugging handler on each

 `Debugger` instance that is observing the debuggee. The handler functions

 receive the `Debugger` instance as their `this` value. Most handler

 functions can return a [resumption value][rv] indicating how the debuggee's

 execution should proceed.

 On a new `Debugger` instance, each of these properties is initially

 `undefined`. Any value assigned to a debugging handler must be either a

 function or `undefined`; otherwise a `TypeError` is thrown.

 Handler functions run in the same thread in which the event occurred.

 They run in the compartment to which they belong, not in a debuggee

 compartment.

 <code>onNewScript(<i>script</i>, <i>global</i>)</code>

 :   New code, represented by the [`Debugger.Script`][script] instance

     <i>script</i>, has been loaded in the scope of the debuggee global

     object <i>global</i>. <i>global</i> is a [`Debugger.Object`][object]

     instance whose referent is the global object.

     This method's return value is ignored.

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

 :   Debuggee code has executed a <i>debugger</i> statement in <i>frame</i>.

     This method should return a [resumption value][rv] specifying how the

     debuggee's execution should proceed.

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

 :   The stack frame <i>frame</i> is about to begin executing code.

     (Naturally, <i>frame</i> is currently the youngest

     [visible frame][vf].) This method should return

     a [resumption value][rv] specifying how the debuggee's execution should

     proceed.

     SpiderMonkey only calls `onEnterFrame` to report

     [visible][vf], non-`"debugger"` frames.

 <code>onThrow(<i>frame</i>, <i>value</i>) <i>(future plan)</i></code>

 :   The exception <i>value</i> is being thrown by <i>frame</i>, which is

     running debuggee code. This method should return a

     [resumption value][rv] specifying how the debuggee's execution should

     proceed. If it returns `undefined`, the exception is thrown as normal.

     A call to the `onThrow` handler is typically followed by one or more

     calls to the `onExceptionUnwind` handler.

     *(pending discussion)* If the debuggee executes

     `try { throw 0; } finally { f(); }` and `f()` executes without error,

     the `onThrow` handler is called only once. The debugger is not notified

     when the exception is set aside in order to execute the `finally` block,

     nor when it is restored after the `finally` block completes normally.

     *(An alternative design here would be: onException(status, frame, value)

     where status is one of the strings "throw", "unwind", "catch",

     "finally", "rethrow". JS\_SaveExceptionState would trigger a "finally"

     event, JS\_RestoreExceptionState would trigger a "rethrow",

     JS\_ClearPendingException would trigger a "catch"; not sure what

     JS\_DropExceptionState or a return/throw from a finally block should

     do.)*

 <code>onExceptionUnwind(<i>frame</i>, <i>value</i>)</code>

 :   The exception <i>value</i> has been thrown, and has propagated to

     <i>frame</i>; <i>frame</i> is the youngest remaining stack frame, and is a

     debuggee frame. This method should return a [resumption value][rv]

     specifying how the debuggee's execution should proceed. If it returns

     `undefined`, the exception continues to propagate as normal: if control in

     `frame` is in a `try` block, control jumps to the corresponding `catch` or

     `finally` block; otherwise, <i>frame</i> is popped, and the exception

     propagates to <i>frame</i>'s caller.

     When an exception's propagation causes control to enter a `finally`

     block, the exception is temporarily set aside. If the `finally` block

     finishes normally, the exception resumes propagation, and the debugger's

     `onExceptionUnwind` handler is called again, in the same frame. (The

     other possibility is for the `finally` block to exit due to a `return`,

     `continue`, or `break` statement, or a new exception. In those cases the

     old exception does not continue to propagate; it is discarded.)

 <code>sourceHandler(<i>ASuffusionOfYellow</i>)</code>

 :   This method is never called. If it is ever called, a contradiction has

     been proven, and the debugger is free to assume that everything is true.

 <code>onError(<i>frame</i>, <i>report</i>)</code>

 :   SpiderMonkey is about to report an error in <i>frame</i>. <i>Report</i>

     is an object describing the error, with the following properties:

     `message`

     :   The fully formatted error message.

     `file`

     :   If present, the source file name, URL, etc. (If this property is

         present, the <i>line</i> property will be too, and vice versa.)

     `line`

     :   If present, the source line number at which the error occurred.

     `lineText`

     :   If present, this is the source code of the offending line.

     `offset`

     :   The index of the character within lineText at which the error occurred.

     `warning`

     :   Present and true if this is a warning; absent otherwise.

     `strict`

     :   Present and true if this error or warning is due to the strict option

         (not to be confused with ES strict mode)

     `exception`

     :   Present and true if an exception will be thrown; absent otherwise.

     `arguments`

     :   An array of strings, representing the arguments substituted into the

         error message.

     This method's return value is ignored.

 `onNewGlobalObject(global)`

 :   A new global object, <i>global</i>, has been created. The application

     embedding the JavaScript implementation may provide details about what

     kind of global it is via <code><i>global</i>.hostAnnotations</code>.

     This handler method should return a [resumption value][rv] specifying how

     the debuggee's execution should proceed. However, note that a <code>{ return:

     <i>value</i> }</code> resumption value is treated like `undefined` ("continue

     normally"); <i>value</i> is ignored. (Allowing the handler to substitute

     its own value for the new global object doesn't seem useful.)

     This handler method is only available to debuggers running in privileged

     code ("chrome", in Firefox). Most functions provided by this `Debugger`

     API observe activity in only those globals that are reachable by the

     API's user, thus imposing capability-based restrictions on a

     `Debugger`'s reach. However, the `onNewGlobalObject` method allows the

     API user to monitor all global object creation that occurs anywhere

     within the JavaScript system (the "JSRuntime", in SpiderMonkey terms),

     thereby escaping the capability-based limits. For this reason,

     `onNewGlobalObject` is only available to privileged code.

 ## Function Properties of the Debugger Prototype Object

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

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

 other kinds of objects.

 <code id="addDebuggee">addDebuggee(<i>global</i>)</code>

 :   Add the global object designated by <i>global</i> to the set of global

     objects this `Debugger` instance is debugging. If the designated global

     is already a debuggee, this has no effect. Return this `Debugger`'s

     [`Debugger.Object`][object] instance referring to the designated global.

     The value <i>global</i> may be any of the following:

     * A global object.

     * An HTML5 `WindowProxy` object (an "outer window", in Firefox

       terminology), which is treated as if the `Window` object of the

       browsing context's active document (the "inner window") were passed.

     * A cross-compartment wrapper of an object; we apply the prior rules to

       the wrapped object.

     * A [`Debugger.Object`][object] instance belonging to this `Debugger` instance;

       we apply the prior rules to the referent.

     * Any other sort of value is treated as a `TypeError`. (Note that each

       rule is only applied once in the process of resolving a given

       <i>global</i> argument. Thus, for example, a [`Debugger.Object`][object]

       referring to a second [`Debugger.Object`][object] which refers to a global does

       not designate that global for the purposes of this function.)

     The global designated by <i>global</i> must be in a different

     compartment than this `Debugger` instance itself. If adding the

     designated global's compartment would create a cycle of debugger and

     debuggee compartments, this method throws an error.

     This method returns the [`Debugger.Object`][object] instance whose referent is

     the designated global object.

     The `Debugger` instance does not hold a strong reference to its

     debuggee globals: if a debuggee global is not otherwise reachable, then

     it is dropped from the `Debugger`'s set of debuggees. (Naturally, the

     [`Debugger.Object`][object] instance this method returns does hold a strong

     reference to the added global.)

 <code>removeDebuggee(<i>global</i>)</code>

 :   Remove the global object designated by <i>global</i> from this

     `Debugger` instance's set of debuggees. Return `undefined`.

     This method interprets <i>global</i> using the same rules that

     [`addDebuggee`][add] does.

 <code>hasDebuggee(<i>global</i>)</code>

 :   Return `true` if the global object designated by <i>global</i> is a

     debuggee of this `Debugger` instance.

     This method interprets <i>global</i> using the same rules that

     [`addDebuggee`][add] does.

 `getDebuggees()`

 :   Return an array of distinct [`Debugger.Object`][object] instances whose referents

     are all the global objects this `Debugger` instance is debugging.

     Since `Debugger` instances don't hold strong references to their

     debuggee globals, if a debuggee global is otherwise unreachable, it may

     be dropped at any moment from the array this method returns.

 `getNewestFrame()`

 :   Return a [`Debugger.Frame`][frame] instance referring to the youngest

     [visible frame][vf] currently on the calling thread's stack, or `null`

     if there are no visible frames on the stack.

 <code>findSources([<i>query</i>]) <i>(not yet implemented)</i></code>

 :   Return an array of all [`Debugger.Source`][source] instances matching

     <i>query</i>. Each source appears only once in the array. <i>Query</i>

     is an object whose properties restrict which sources are returned; a

     source must meet all the criteria given by <i>query</i> to be returned.

     If <i>query</i> is omitted, we return all sources of all debuggee

     scripts.

     <i>Query</i> may have the following properties:

     `url`

     :   The source's `url` property must be equal to this value.

     `global`

     :   The source must have been evaluated in the scope of the given global

         object. If this property's value is a [`Debugger.Object`][object] instance

         belonging to this `Debugger` instance, then its referent is used. If the

         object is not a global object, then the global in whose scope it was

         allocated is used.

     Note that the result may include sources that can no longer ever be

     used by the debuggee: say, eval code that has finished running, or

     source for unreachable functions. Whether such sources appear can be

     affected by the garbage collector's behavior, so this function's result

     is not entirely deterministic.

 <code>findScripts([<i>query</i>])</code>

 :   Return an array of [`Debugger.Script`][script] instances for all debuggee scripts

     matching <i>query</i>. Each instance appears only once in the array.

     <i>Query</i> is an object whose properties restrict which scripts are

     returned; a script must meet all the criteria given by <i>query</i> to

     be returned. If <i>query</i> is omitted, we return the [`Debugger.Script`][script]

     instances for all debuggee scripts.

     <i>Query</i> may have the following properties:

     `url`

     :   The script's `url` property must be equal to this value.

     `source` <i>(not yet implemented)</i>

     :   The script's `source` property must be equal to this value.

     `line`

     :   The script must at least partially cover the given source line. If this

         property is present, the `url` property must be present as well.

     `column`

     :   The script must include given column on the line given by the `line`

         property. If this property is present, the `url` and `line` properties

         must both be present as well.

     `innermost`

     :   If this property is present and true, the script must be the innermost

         script covering the given source location; scripts of enclosing code are

         omitted.

     `global`

     :   The script must be in the scope of the given global object. If this

         property's value is a [`Debugger.Object`][object] instance belonging to this

         `Debugger` instance, then its referent is used. If the object is not a

         global object, then the global in whose scope it was allocated is used.

     All properties of <i>query</i> are optional. Passing an empty object

     returns all debuggee code scripts.

     Note that the result may include [`Debugger.Script`][script] instances for

     scripts that can no longer ever be used by the debuggee, say, those for

     eval code that has finished running, or unreachable functions. Whether

     such scripts appear can be affected by the garbage collector's

     behavior, so this function's behavior is not entirely deterministic.

 <code>clearBreakpoint(<i>handler</i>)</code>

 :   Remove all breakpoints set in this `Debugger` instance that use

     <i>handler</i> as their handler. Note that, if breakpoints using other

     handler objects are set at the same location(s) as <i>handler</i>, they

     remain in place.

 `clearAllBreakpoints()`

 :   Remove all breakpoints set using this `Debugger` instance.

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

 :   Clear all watchpoints owned by this `Debugger` instance.

 `findAllGlobals()`

 :   Return an array of [`Debugger.Object`][object] instances referring to all the

     global objects present in this JavaScript instance. The application may

     provide details about what kind of globals they are via the

     [`Debugger.Object`][object] instances' `hostAnnotations` accessors.

     The results of this call can be affected in non-deterministic ways by

     the details of the JavaScript implementation. The array may include

     [`Debugger.Object`][object] instances referring to global objects that are not

     actually reachable by the debuggee or any other code in the system.

     (Naturally, once the function has returned, the array's

     [`Debugger.Object`][object] instances strongly reference the globals they refer

     to.)

     This handler method is only available to debuggers running in privileged

     code ("chrome", in Firefox). Most functions provided by this `Debugger`

     API observe activity in only those globals that are reachable by the

     API's user, thus imposing capability-based restrictions on a

     `Debugger`'s reach. However, `findAllGlobals` allows the API user to

     find all global objects anywhere within the JavaScript system (the

     "JSRuntime", in SpiderMonkey terms), thereby escaping the

     capability-based limits. For this reason, `findAllGlobals` is only

     available to privileged code.

 <code>makeGlobalObjectReference(<i>global</i>)</code>

 :   Return the [`Debugger.Object`][object] whose referent is the global object

     designated by <i>global</i>, without adding the designated global as a

     debuggee. If <i>global</i> does not designate a global object, throw a

     `TypeError`. Determine which global is designated by <i>global</i>

     using the same rules as [`Debugger.prototype.addDebuggee`][add].