General Conventions

This page describes general conventions used in the Debugger API, and defines some terminology used throughout the specification.


Properties of objects that comprise the Debugger interface, and those that the interface creates, follow some general conventions:

  • Instances and prototypes are extensible; you can add your own properties and methods to them.

  • Properties are configurable. This applies to both “own” and prototype properties, and to both methods and data properties. (Leaving these properties open to redefinition will hopefully make it easier for JavaScript debugger code to cope with bugs, bug fixes, and changes in the interface over time.)

  • Method properties are writable.

  • We prefer inherited accessor properties to own data properties. Both are read using the same syntax, but inherited accessors seem like a more accurate reflection of what’s going on. Unless otherwise noted, these properties have getters but no setters, as they cannot meaningfully be assigned to.

Debuggee Values

The Debugger interface follows some conventions to help debuggers safely inspect and modify the debuggee’s objects and values. Primitive values are passed freely between debugger and debuggee; copying or wrapping is handled transparently. Objects received from the debuggee (including host objects like DOM elements) are fronted in the debugger by Debugger.Object instances, which provide reflection-oriented methods for inspecting their referents; see Debugger.Object, below.

Of the debugger’s objects, only Debugger.Object instances may be passed to the debuggee: when this occurs, the debuggee receives the Debugger.Object’s referent, not the Debugger.Object instance itself.

In the descriptions below, the term “debuggee value” means either a primitive value or a Debugger.Object instance; it is a value that might be received from the debuggee, or that could be passed to the debuggee.

Debuggee Code

Each Debugger instance maintains a set of global objects that, taken together, comprise the debuggee. Code evaluated in the scope of a debuggee global object, directly or indirectly, is considered debuggee code. Similarly:

  • a debuggee frame is a frame running debuggee code;

  • a debuggee function is a function that closes over a debuggee global object (and thus the function’s code is debuggee code);

  • a debuggee environment is an environment whose outermost enclosing environment is a debuggee global object; and

  • a debuggee script is a script containing debuggee code.

Completion Values

The Debugger API often needs to convey the result of running some JS code. For example, suppose you get a frame.onPop callback telling you that a method in the debuggee just finished. Did it return successfully? Did it throw? What did it return? The debugger passes the onPop handler a completion value that tells what happened.

A completion value is one of these:

  • { return: value }

    The code completed normally, returning value. Value is a debuggee value.

  • { throw: value, stack: stack }

    The code threw value as an exception. Value is a debuggee value. stack is a SavedFrame representing the location from which the value was thrown, and may be missing.

  • null

    The code was terminated, as if by the “slow script” ribbon.

Generators and async functions add a wrinkle: they can suspend themselves (with yield or await), which removes their frame from the stack. Later, the generator or async frame might be returned to the stack and continue running where it left off. Does it count as “completion” when a generator suspends itself?

The Debugger API says yes. yield and await do trigger the frame.onPop handler, passing a completion value that explains why the frame is being suspended. The completion value gets an extra .yield or .await property, to distinguish this kind of completion from a normal return.

{ return: value, yield: true }

where value is a debuggee value for the iterator result object, like { value: 1, done: false }, for the yield.

When a generator function is called, it first evaluates any default argument expressions and destructures its arguments. Then its frame is suspended, and the new generator object is returned to the caller. This initial suspension is reported to any onPop handlers as a completion value of the form:

{ return: generatorObject, yield: true, initial: true }

where generatorObject is a debuggee value for the generator object being returned to the caller.

When an async function awaits a promise, its suspension is reported to any onPop handlers as a completion value of the form:

{ return: promise, await: true }

where promise is a debuggee value for the promise being returned to the caller.

The first time a call to an async function awaits, returns, or throws, a promise of its result is returned to the caller. Subsequent resumptions of the async call, if any, are initiated directly from the job queue’s event loop, with no calling frame on the stack. Thus, if needed, an onPop handler can distinguish an async call’s initial suspension, which returns the promise, from any subsequent suspensions by checking the Debugger.Frame’s older property: if that is null, the call was resumed directly from the event loop.

Async generators are a combination of async functions and generators that can use both yield and await expressions. Suspensions of async generator frames are reported using any combination of the completion values above.

Resumption Values

As the debuggee runs, the Debugger interface calls various debugger-provided handler functions to report the debuggee’s behavior. Some of these calls can return a value indicating how the debuggee’s execution should continue; these are called resumption values. A resumption value has one of the following forms:

  • undefined

    The debuggee should continue execution normally.

  • { return: value }

    Force the top frame of the debuggee to return value immediately, as if by executing a return statement. Value must be a debuggee value. (Most handler functions support this, except those whose descriptions say otherwise.) See the list of special cases below.

  • { throw: value }

    Throw value as an exception from the current bytecode instruction. Value must be a debuggee value. Note that unlike completion values, resumption values do not specify a stack. When initiating an exceptional return from a handler, the current debuggee stack will be used. If a handler wants to avoid modifying the stack of an already-thrown exception, it should return undefined.

  • null

    Terminate the debuggee, as if it had been cancelled by the “slow script” dialog box.

In some places, the JS language treats return statements specially or doesn’t allow them at all. So there are a few special cases.

  • An arrow function without curly braces can’t contain a return statement, but { return: value } works anyway, returning the specified value.

    Likewise, if the top frame of the debuggee is not in a function at all—that is, it’s running toplevel code in a script tag, or eval code—then value is returned even though return statements aren’t legal in that kind of code. (In the case of a script tag, the browser discards the return value.)

  • If the debuggee is in a function that was called as a constructor (that is, via a new expression), then value serves as the value returned by the function’s body, not that produced by the new expression: if the value is not an object, the new expression returns the frame’s this value.

    Similarly, if the function is the constructor for a subclass, then a non-object value may result in a TypeError.

  • Returning from a generator simulates a return, not a yield; there is no way to force a debuggee generator to yield.

    The way generators execute is rather odd. When a generator-function is first called, it is put onto the stack and runs just a few bytecode instructions (or more, if the generator-function has any default argument values to compute), then performs the “initial suspend”. At that point, a new generator object is created and returned to the caller. Thereafter, the caller may cause execution of the generator to resume at any time, by calling, and the generator may pause itself again using yield.

    JS generators normally can’t return before the “initial suspend”—there’s no place to put a return statement—but { return: value } there works anyway, replacing the generator object that the initial suspend would normally create and return.

    Returning from a generator that’s been resumed via (or one of the other methods) closes the generator, and the or other method returns a new object of the form { done: true, value: value }.

If a debugger hook function throws an exception, rather than returning a resumption value, we never propagate such an exception to the debuggee; instead, we call the associated Debugger instance’s uncaughtExceptionHook property, as described below.


Timestamps are expressed in units of milliseconds since an arbitrary, but fixed, epoch. The resolution of timestamps is generally greater than milliseconds, though no specific resolution is guaranteed.

The Debugger.DebuggeeWouldRun Exception

Some debugger operations that appear to simply inspect the debuggee’s state may actually cause debuggee code to run. For example, reading a variable might run a getter function on the global or on a with expression’s operand; and getting an object’s property descriptor will run a handler trap if the object is a proxy. To protect the debugger’s integrity, only methods whose stated purpose is to run debuggee code can do so. These methods are called invocation functions, and they follow certain common conventions to report the debuggee’s behavior safely. For other methods, if their normal operation would cause debuggee code to run, they throw an instance of the Debugger.DebuggeeWouldRun exception.

If there are debugger frames on stack from multiple Debugger instances, the thrown exception is an instance of the topmost locking debugger’s global’s Debugger.DebuggeeWouldRun.

A Debugger.DebuggeeWouldRun exception may have a cause property, providing more detailed information on why the debuggee would have run. The cause property’s value is one of the following strings:

  • "proxy": Carrying out the operation would have caused a proxy handler to run. |

  • "getter": Carrying out the operation would have caused an object property getter to run. |

  • "setter": Carrying out the operation would have caused an object property setter to run. |

If the system can’t determine why control attempted to enter the debuggee, it will leave the exception’s cause property undefined.