Debugger.Object

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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 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.

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 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 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.

isBoundFunction

true if the referent is a bound function; false otherwise.

isArrowFunction

true if the referent is an arrow function; false otherwise.

boundTargetFunction

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

boundThis

If the referent is a bound function, this is the this value it was bound to. If the referent is not a bound function, this is undefined.

boundArguments

If the referent is a bound 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 not a bound function, 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.

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.

getProperty(name)

Return the value of the referent’s property named name, or undefined if it has no such property. Name must be a string. The result is a debuggee value.

setProperty(name, value)

Store value as the value of the referent’s property named name, creating the property if it does not exist. Name must be a string; value must be a debuggee value.

getOwnPropertyDescriptor(name)

Return a property descriptor for the property named name 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.

defineProperty(name, attributes)

Define a property on the referent named name, 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(name)

Remove the referent’s property named name. 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.)

copy(value)

Apply the HTML5 “structured cloning” algorithm to create a copy of value 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.

create(prototype, [properties])

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 prototype, which must be an Debugger.Object instance. The new object’s properties are as given by properties, as if properties were passed to Debugger.Object.prototype.defineProperties, with the new Debugger.Object instance as the this value.

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.

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, 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.

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, 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.

evalInGlobal(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, watchpoints, and so on remain active during the call. 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.

If code is not strict mode code, then variable declarations in code 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 options argument is as for Debugger.Frame.prototype.eval.

evalInGlobalWithBindings(code, bindings, [options])

Like evalInGlobal, 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 evalInGlobal, if the code passed to evalInGlobalWithBindings is not strict mode code, then 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 non-strict eval 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.

asEnvironment()

If the referent is a global object, return the Debugger.Environment instance representing the referent as a variable environment for evaluating code. If the referent is not a global object, throw a TypeError.

setObjectWatchpoint(handler) (future plan)

Set a watchpoint on all the referent’s own properties, reporting events by calling handler’s methods. Any previous watchpoint handler on this Debugger.Object instance is replaced. If handler is null, the referent is no longer watched. Handler may have the following methods, called under the given circumstances:

add(frame, name, descriptor)

A property named name has been added to the referent. Descriptor is a property descriptor of the sort accepted by Debugger.Object.prototype.defineProperty, giving the newly added property’s attributes.

delete(frame, name)

The property named name is about to be deleted from the referent.

change(frame, name, oldDescriptor, newDescriptor)

The existing property named name on the referent is being changed from those given by oldDescriptor to those given by newDescriptor. 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.

set(frame, oldValue, newValue)

The data property named name of the referent is about to have its value changed from oldValue to newValue.

SpiderMonkey only calls this method on assignments to data properties that will succeed; assignments to un-writable data properties fail without notifying the debugger.

extensionsPrevented(frame)

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 frame 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, indicating how execution should proceed. (However, return resumption values are not supported.)

  • If a given method is absent from handler, then events of that sort are ignored. The watchpoint consults handler’s properties each time an event occurs, so adding methods to or removing methods from handler after setting the watchpoint enables or disables reporting of the corresponding events.

  • Values passed to handler’s methods are debuggee values. Descriptors passed to handler’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 handler’s method’s compartment (typically the same as the debugger’s compartment).

The new watchpoint belongs to the Debugger instance to which this Debugger.Object instance belongs; disabling the Debugger instance disables this watchpoint.

clearObjectWatchpoint() (future plan)

Remove any object watchpoint set on the referent.

setPropertyWatchpoint(name, handler) (future plan)

Set a watchpoint on the referent’s property named name, reporting events by calling handler’s methods. Any previous watchpoint handler on this property for this Debugger.Object instance is replaced. If handler is null, the property is no longer watched. Handler is as described for Debugger.Object.prototype.setObjectWatchpoint, except that it does not receive extensionsPrevented events.

clearPropertyWatchpoint(name) (future plan)

Remove any watchpoint set on the referent’s property named name.

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.

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