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 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 instance may actually have several Debugger.Object instances: one for each compartment from which the referent is viewed.

If more than one Debugger instance is debugging the same code, each Debugger gets a separate Debugger.Object instance for a given object. This allows the code using each Debugger 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. This accessor may throw if the referent is a scripted proxy or some other sort of exotic object (an opaque wrapper, for example).

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 a/b to refer to the b defined within a, and a< to refer to a function that occurs somewhere within an expression that is assigned to a. 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 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 function uses destructuring parameters, the corresponding array elements are undefined. 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", undefined, undefined]

script

If the referent is a function that is debuggee code, this is that function’s script, as a Debugger.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 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.

isError

true if the referent is any potentially wrapped Error; false otherwise.

errorMessageName

If the referent is an error created with an engine internal message template this is a string which is the name of the template; undefined otherwise.

errorLineNumber

If the referent is an Error object, this is the line number at which the referent was created; undefined otherwise.

errorColumnNumber

If the referent is an Error object, this is the column number at which the referent was created; undefined otherwise.

isBoundFunction

If the referent is a debuggee function, returns true if the referent is a bound function; false otherwise. If the referent is not a debuggee function, or not a function at all, returns undefined instead.

isArrowFunction

If the referent is a debuggee function, returns true if the referent is an arrow function; false otherwise. If the referent is not a debuggee function, or not a function at all, returns undefined instead.

isGeneratorFunction

If the referent is a debuggee function, returns true if the referent was created with a function* expression or statement, or false if it is some other sort of function. If the referent is not a debuggee function, or not a function at all, this is undefined. (This is always equal to obj.script.isGeneratorFunction, assuming obj.script is a Debugger.Script.)

isAsyncFunction

If the referent is a debuggee function, returns true if the referent is an async function, defined with an async function expression or statement, or false if it is some other sort of function. If the referent is not a debuggee function, or not a function at all, this is undefined. (This is always equal to obj.script.isAsyncFunction, assuming obj.script is a Debugger.Script.)

isClassConstructor

If the referent is a debuggee function, returns true if the referent is a class, or false if it is some other sort of function. If the referent is not a debuggee function, or not a function at all, this is undefined. (This is always equal to obj.script.isClassConstructor, assuming obj.script is a Debugger.Script.)

isPromise

true if the referent is a Promise; false otherwise.

boundTargetFunction

If the referent is a bound debuggee function, this is its target function— the function that was bound to a particular this object. If the referent is either not a bound function, not a debuggee function, or not a function at all, this is undefined.

boundThis

If the referent is a bound debuggee function, this is the this value it was bound to. If the referent is either not a bound function, not a debuggee function, or not a function at all, this is undefined.

boundArguments

If the referent is a bound debuggee function, this is an array (in the Debugger object’s compartment) that contains the debuggee values of the arguments object it was bound to. If the referent is either not a bound function, not a debuggee function, or not a function at all, this is undefined.

isProxy

If the referent is a (scripted) proxy, either revoked or not, return true. If the referent is not a (scripted) proxy, return false.

proxyTarget

If the referent is a non-revoked (scripted) proxy, return a Debugger.Object instance referring to the ECMAScript [[ProxyTarget]] of the referent. If the referent is a revoked (scripted) proxy, return null. If the referent is not a (scripted) proxy, return undefined.

proxyHandler

If the referent is a non-revoked (scripted) proxy, return a Debugger.Object instance referring to the ECMAScript [[ProxyHandler]] of the referent. If the referent is a revoked (scripted) proxy, return null. If the referent is not a (scripted) proxy, return undefined.

promiseState

If the referent is a Promise, return a string indicating whether the Promise is pending, or has been fulfilled or rejected. This string takes one of the following values:

  • "pending", if the Promise is pending.

  • "fulfilled", if the Promise has been fulfilled.

  • "rejected", if the Promise has been rejected.

If the referent is not a Promise, throw a TypeError.

promiseValue

Return a debuggee value representing the value the Promise has been fulfilled with.

If the referent is not a Promise, or the Promise has not been fulfilled, throw a TypeError.

promiseReason

Return a debuggee value representing the value the Promise has been rejected with.

If the referent is not a Promise, or the Promise has not been rejected, throw a TypeError.

promiseAllocationSite

If the referent is a Promise, this is the JavaScript execution stack captured at the time of the promise’s allocation. This can return null if the promise was not created from script. If the referent is not a Promise, throw a TypeError exception.

promiseResolutionSite

If the referent is a Promise, this is the JavaScript execution stack captured at the time of the promise’s resolution. This can return null if the promise was not resolved by calling its resolve or reject resolving functions from script. If the referent is not a Promise, throw a TypeError exception.

promiseID

If the referent is a Promise, this is a process-unique identifier for the Promise. With e10s, the same id can potentially be assigned to multiple Promise instances, if those instances were created in different processes. If the referent is not a Promise, throw a TypeError exception.

promiseDependentPromises

If the referent is a Promise, this is an Array of Debugger.Objects referring to the promises directly depending on the referent Promise. These are:

  1. Return values of then() calls on the promise.

  2. Return values of Promise.all() if the referent Promise was passed in as one of the arguments.

  3. Return values of Promise.race() if the referent Promise was passed in as one of the arguments.

Once a Promise is settled, it will generally notify its dependent promises and forget about them, so this is most useful on pending promises.

Note that the Array only contains the promises that directly depend on the referent Promise. It does not contain promises that depend on promises that depend on the referent Promise.

If the referent is not a Promise, throw a TypeError exception.

promiseLifetime

If the referent is a Promise, this is the number of milliseconds elapsed since the Promise was created. If the referent is not a Promise, throw a TypeError exception.

promiseTimeToResolution

If the referent is a Promise, this is the number of milliseconds elapsed between when the Promise was created and when it was resolved. If the referent hasn’t been resolved or is not a Promise, throw a TypeError exception.

allocationSite

If object allocation site tracking was enabled when this Debugger.Object’s referent was allocated, return the JavaScript execution stack captured at the time of the allocation. Otherwise, return null.

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; 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 exception.

These methods may throw if the referent is not a native object. Even simple accessors like isExtensible may throw if the referent is a proxy or some sort of exotic object like an opaque wrapper.

getProperty(key, [receiver])

Return a completion value with “return” being the value of the referent’s property named key, or undefined if it has no such property. key must be a string or symbol; receiver must be a debuggee value. If the property is a getter, it will be evaluated with receiver as the receiver, defaulting to the Debugger.Object if omitted. All extant handler methods, breakpoints, and so on remain active during the call.

setProperty(key, value<, [receiver])

Store value as the value of the referent’s property named key, creating the property if it does not exist. key must be a string or symbol; value and receiver must be debuggee values. If the property is a setter, it will be evaluated with receiver as the receiver, defaulting to the Debugger.Object if omitted. Return a completion value with “return” being a success/failure boolean, or else “throw” being the exception thrown during property assignment. All extant handler methods, breakpoints, and so on remain active during the call.

getOwnPropertyDescriptor(key)

Return a property descriptor for the property named key 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 Object.getOwnPropertyNames(referent) had been called in the debuggee, and the result copied in the scope of the debugger’s global object.

getOwnPropertyNamesLength()

Return the number of the referent’s own properties.

getOwnPropertySymbols()

Return an array of strings naming all the referent’s own symbols, as if Object.getOwnPropertySymbols(referent) had been called in the debuggee, and the result copied in the scope of the debugger’s global object.

defineProperty(key, attributes)

Define a property on the referent named key, as described by the property descriptor descriptor. Any value, get, and set properties of attributes 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.)

defineProperties(properties)

Add the properties given by properties to the referent. (This function behaves like Object.defineProperties, except that the target object is implicit, and in a different compartment from the properties argument.)

deleteProperty(key)

Remove the referent’s property named key. 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.)

makeDebuggeeValue(value)

Return the debuggee value that represents value in the debuggee. If value is a primitive, we return it unchanged; if value 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 value 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 d and an object o, the call d.makeDebuggeeValue(o) returns a Debugger.Object instance that presents o as it would be seen by code in d’s compartment.

makeDebuggeeNativeFunction(value)

If value is a native function in the debugger’s compartment, create an equivalent function for the same native in the debuggee’s realm, and return a Debugger.Object instance for the new function. The new function can be accessed by code in the debuggee without going through a cross compartment wrapper.

isSameNative(value)

If value is a native function in the debugger’s compartment, return whether the referent is a native function for the same C++ native.

decompile([pretty])

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 pretty is present and true, produce indented code with line breaks. If the referent is not a function that is debuggee code, return undefined.

call(this, argument, ...)

If the referent is callable, call it with the given this value and argument values, and return a completion value describing how the call completed. This 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 argument must be a debuggee value. All extant handler methods, breakpoints, and so on remain active during the call. If the referent is not callable, throw a TypeError. This function follows the invocation function conventions.

apply(this, arguments)

If the referent is callable, call it with the given this value and the argument values in arguments, and return a completion value describing how the call completed. This should be a debuggee value, or { asConstructor: true } to invoke function as a constructor, in which case SpiderMonkey provides an appropriate this value itself. Arguments 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, and so on remain active during the call. If the referent is not callable, throw a TypeError. This function follows the invocation function conventions.

executeInGlobal(code, [options])

If the referent is a global object, evaluate code in that global environment, and return a completion value describing how it completed. Code is a string. All extant handler methods, breakpoints, and so on remain active during the call (pending note below). This function follows the invocation function conventions. If the referent is not a global object, throw a TypeError exception.

Code is interpreted as strict mode code when it contains a Use Strict Directive.

This evaluation is semantically equivalent to executing statements at the global level, not an indirect eval. Regardless of code being strict mode code, variable declarations in code affect the referent global object.

The options argument is as for Debugger.Frame.prototype.eval.

Note: If this method is called on an object whose owner Debugger object has an onNativeCall handler, only hooks on objects associated with that debugger will be called during the evaluation.

executeInGlobalWithBindings(code, bindings, [options])

Like executeInGlobal, but evaluate code using the referent as the variable object, but with a lexical environment extended with bindings from the object bindings. For each own enumerable property of bindings named name whose value is value, include a variable in the lexical environment in which code is evaluated named name, whose value is value. Each value must be a debuggee value. (This is not like a with statement: code may access, assign to, and delete the introduced bindings without having any effect on the bindings 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 executeInGlobal, any declarations it contains affect the referent global object, even as code is evaluated in an environment extended according to bindings. (In the terms used by the ECMAScript specification, the VariableEnvironment of the execution context for code is the referent, and the bindings appear in a new declarative environment, which is the eval code’s LexicalEnvironment.)

The options argument is as for Debugger.Frame.prototype.eval.

Note: If this method is called on an object whose owner Debugger object has an onNativeCall handler, only hooks on objects associated with that debugger will be called during the evaluation.

createSource(options)

If the referent is a global object, return a new JavaScript source in the global’s realm which has its properties filled in according to the options object. If the referent is not a global object, throw a TypeError exception. The options object can have the following properties:

  • text: String contents of the JavaScript in the source.

  • url: URL the resulting source should be associated with.

  • startLine: Starting line of the source.

  • sourceMapURL: Optional URL specifying the source’s source map URL. If not specified, the source map URL can be filled in if specified by the source’s text.

  • isScriptElement: Optional boolean which will set the source’s introductionType to "inlineScript" if specified. Otherwise, the source’s introductionType will be undefined.

asEnvironment()

If the referent is a global object, return the Debugger.Environment instance representing the referent’s global lexical scope. The global lexical scope’s enclosing scope is the global object. If the referent is not a global object, throw a TypeError.

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.

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.

forceLexicalInitializationByName(binding)

If binding is in an uninitialized state initialize it to undefined and return true, otherwise do nothing and return false.

getPromiseReactions

If the referent is a Promise or a cross-compartment wrapper of one, this returns an array of objects describing the reaction records added to the promise. There are several different sorts of reaction records:

  • The array entry for a reaction record added with then or catch has the form { resolve: F, reject: F, result: P }, where each F is a Debugger.Object referring to a function object, and P is the promise that will be resolved with the result of calling them.

    The resolve and reject properties may be absent in some cases. A call to then can omit the rejection handler, and a call to catch omits the resolution handler. Furthermore, various promise facilities create records like this as internal implementation details, creating handlers that are not representable as JavaScript functions.

  • When a promise P1 is resolved to another promise P2 (such that resolving P2 resolves P1 in the same way) that adds a reaction record to P2. The array entry for that reaction record is simply the Debugger.Object representing P1.

    Note that, if P1 and P2 are in different compartments, resolving P1 to P2 creates the same sort of reaction record as a call to then or catch, with P1 stored only in a private slot of the resolve and reject functions, and not directly available from the reaction record.

  • An await expression calls PromiseResolve on its operand to obtain a promise P, and then adds a reaction record to P that resumes the suspended call appropriately. The array entry for that reaction record is a Debugger.Frame representing the suspended call.

    If the await’s operand A is a native promise with the standard constructor, then PromiseResolve simply returns A unchanged, and the reaction record for resuming the suspended call is added to A’s list. But if A is some other sort of ‘thenable’, then PromiseResolve creates a new promise and enqueues a job to call A’s then method; this may produce more indirect chains from awaitees to awaiters.

  • JS::AddPromiseReactions and JS::AddPromiseReactionsIgnoringUnhandledRejection create a reaction record whose promise field is null.