The Debugger Object

When called as a constructor, the Debugger object creates a new Debugger instance.

new Debugger([global, …]): Create a debugger object, and apply its addDebuggee method to each of the given global 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, 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.

allowUnobservedAsmJS

A boolean value indicating whether asm.js code running inside this Debugger instance’s debuggee globals is invisible to Debugger API handlers and breakpoints. Setting this to false inhibits the ahead-of-time asm.js compiler and forces asm.js code to run as normal JavaScript. This is an accessor property with a getter and setter. It is initially false in a freshly created Debugger instance.

Setting this flag to true is intended for uses of subsystems of the Debugger API (e.g, Debugger.Source) for purposes other than step debugging a target JavaScript program.

allowWasmBinarySource

A boolean value indicating whether WebAssembly sources will be available in binary form. The WebAssembly text generation will be disabled.

collectCoverageInfo

A boolean value indicating whether code coverage should be enabled inside each debuggee of this Debugger instance. Changing this flag value will recompile all JIT code to add or remove code coverage instrumentation. Changing this flag when any frame of the debuggee is currently active on the stack will produce an exception.

Setting this to true enables code coverage instrumentation, which can be accessed via the Debugger.Script getOffsetsCoverage function. In some cases, the code coverage might expose information which pre-date the modification of this flag. Code coverage reports are monotone, thus one can take a snapshot when the Debugger is enabled, and output the difference.

Setting this to false prevents this Debugger instance from requiring any code coverage instrumentation, but it does not guarantee that the instrumentation is not present.

uncaughtExceptionHook

Either null or a function that SpiderMonkey calls when a call to a debug event handler, breakpoint handler, or similar function throws some exception, which we refer to as debugger-exception here. Exceptions thrown in the debugger are not propagated to debuggee code; instead, SpiderMonkey calls this function, passing debugger-exception as its sole argument and the Debugger instance as the this value. This function should return a resumption value, which determines how the debuggee should continue.

If the uncaught exception hook itself throws an exception, uncaught-hook-exception, SpiderMonkey throws a new error object, confess-to-debuggee-exception, to the debuggee whose message blames the debugger, and includes textual descriptions of uncaught-hook-exception and the original debugger-exception.

If uncaughtExceptionHook’s value is null, SpiderMonkey throws an exception to the debuggee whose message blames the debugger, and includes a textual description of debugger-exception.

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 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.

onNewScript(script, global)

New code, represented by the Debugger.Script instance script, has been loaded in the scope of the debuggees.

This method’s return value is ignored.

onNewPromise(promise)

A new Promise object, referenced by the Debugger.Object instance promise, has been allocated in the scope of the debuggees. The Promise’s allocation stack can be obtained using the promiseAllocationStack accessor property of the Debugger.Object instance promise.

This handler method should return a resumption value specifying how the debuggee’s execution should proceed. However, note that a { return: value } resumption value is treated like undefined (“continue normally”); value is ignored.

onPromiseSettled(promise)

A Promise object, referenced by the Debugger.Object instance promise that was allocated within a debuggee scope, has settled (either fulfilled or rejected). The Promise’s state, fulfillment or rejection value, and the allocation and resolution stacks can be obtained using the Promise-related accessor properties of the Debugger.Object instance promise.

onDebuggerStatement(frame)

Debuggee code has executed a debugger statement in frame. This method should return a resumption value specifying how the debuggee’s execution should proceed.

onEnterFrame(frame)

The stack frame frame is about to begin executing code. (Naturally, frame is currently the youngest visible frame.) This method should return a resumption value specifying how the debuggee’s execution should proceed.

SpiderMonkey only calls onEnterFrame to report visible, non-"debugger" frames.

onExceptionUnwind(frame, value)

The exception value has been thrown, and has propagated to frame; frame is the youngest remaining stack frame, and is a debuggee frame. This method should return a resumption value 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, frame is popped, and the exception propagates to frame’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.)

This handler is not called when unwinding a frame due to an over-recursion or out-of-memory exception.

sourceHandler(ASuffusionOfYellow)

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.

onError(frame, report)

SpiderMonkey is about to report an error in frame. Report 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 line 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, global, has been created.

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.

addDebuggee(global)

Add the global object designated by global 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 instance referring to the designated global.

The value global 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 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 global argument. Thus, for example, a Debugger.Object referring to a second Debugger.Object which refers to a global does not designate that global for the purposes of this function.)

The global designated by global 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 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 instance this method returns does hold a strong reference to the added global.)

If this debugger is tracking allocation sites and cannot track allocation sites for global, this method throws an Error.

addAllGlobalsAsDebuggees()

This method is like addDebuggee, but adds all the global objects from all compartments to this Debugger instance’s set of debuggees. Note that it skips this debugger’s compartment.

If this debugger is tracking allocation sites and cannot track allocation sites for some global, this method throws an Error. Otherwise this method returns undefined.

This 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 addAllGlobalsAsDebuggees 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, addAllGlobalsAsDebuggees is only available to privileged code.

removeDebuggee(global)

Remove the global object designated by global from this Debugger instance’s set of debuggees. Return undefined.

This method interprets global using the same rules that addDebuggee does.

Removing a global as a debuggee from this Debugger clears all breakpoints that belong to that Debugger in that global.

removeAllDebuggees()

Remove all the global objects from this Debugger instance’s set of debuggees. Return undefined.

hasDebuggee(global)

Return true if the global object designated by global is a debuggee of this Debugger instance.

This method interprets global using the same rules that addDebuggee does.

getDebuggees()

Return an array of distinct Debugger.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 instance referring to the youngest visible frame currently on the calling thread’s stack, or null if there are no visible frames on the stack.

findSources([query]) (not yet implemented)

Return an array of all Debugger.Source instances matching query. Each source appears only once in the array. Query is an object whose properties restrict which sources are returned; a source must meet all the criteria given by query to be returned. If query is omitted, we return all sources of all debuggee scripts.

Query 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 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.

findScripts([query])

Return an array of Debugger.Script instances for all debuggee scripts matching query. Each instance appears only once in the array. Query is an object whose properties restrict which scripts are returned; a script must meet all the criteria given by query to be returned. If query is omitted, we return the Debugger.Script instances for all debuggee scripts.

Query may have the following properties:

url: The script’s url property must be equal to this value.
source: 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 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 query are optional. Passing an empty object returns all debuggee code scripts.

Note that the result may include Debugger.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.

findObjects([query])

Return an array of Debugger.Object instances referring to each live object allocated in the scope of the debuggee globals that matches query. Each instance appears only once in the array. Query is an object whose properties restrict which objects are returned; an object must meet all the criteria given by query to be returned. If query is omitted, we return the Debugger.Object instances for all objects allocated in the scope of debuggee globals.

The query object may have the following properties:

class: If present, only return objects whose internal [[Class]]’s name matches the given string. Note that in some cases, the prototype object for a given constructor has the same [[Class]] as the instances that refer to it, but cannot itself be used as a valid instance of the class. Code gathering objects by class name may need to examine them further before trying to use them.

All properties of query are optional. Passing an empty object returns all objects in debuggee globals.

Unlike findScripts, this function is deterministic and will never return Debugger.Objects referring to previously unreachable objects that had not been collected yet.

clearBreakpoint(handler)

Remove all breakpoints set in this Debugger instance that use handler as their handler. Note that, if breakpoints using other handler objects are set at the same location(s) as handler, they remain in place.

clearAllBreakpoints()

Remove all breakpoints set using this Debugger instance.

findAllGlobals()

Return an array of Debugger.Object instances referring to all the global objects present in this JavaScript instance.

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 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 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.

makeGlobalObjectReference(global)

Return the Debugger.Object whose referent is the global object designated by global, without adding the designated global as a debuggee. If global does not designate a global object, throw a TypeError. Determine which global is designated by global using the same rules as Debugger.prototype.addDebuggee.

adoptDebuggeeValue(value)

Given a debuggee value value owned by an arbitrary Debugger, return an equivalent debuggee value owned by this Debugger.

If value is a primitive value, return it unchanged. If value is a Debugger.Object owned by an arbitrary Debugger, return an equivalent Debugger.Object owned by this Debugger. Otherwise, if value is some other kind of object, and hence not a proper debuggee value, throw a TypeError instead.

Static methods of the Debugger Object

The functions described below are not called with a this value.

isCompilableUnit(source): Given a string of source code, designated by source, return false if the string might become a valid JavaScript statement with the addition of more lines. Otherwise return true. The intent is to support interactive compilation - accumulate lines in a buffer until isCompilableUnit is true, then pass it to the compiler.

Source Metadata

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: js/src/doc/Debugger/Debugger.md
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Debugger