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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 “ownâ€� 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

When a debuggee stack frame completes its execution, or when some sort of debuggee call initiated by the debugger finishes, the Debugger interface provides a value describing how the code completed; these are called completion values. A completion value has one of the following forms:

{ return: value }
The code completed normally, returning value. Value is a debuggee value.
{ throw: value }
The code threw value as an exception. Value is a debuggee value.
The code was terminated, as if by the “slow script� dialog box.

If control reaches the end of a generator frame, the completion value is { throw: stop } where stop is a Debugger.Object object representing the StopIteration object being thrown.

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:

The debuggee should continue execution normally.
{ return: value }
Return value immediately as the current value of the function. Value must be a debuggee value. (Most handler functions support this, except those whose descriptions say otherwise.) If the function 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.
{ throw: value }
Throw value as an exception from the current bytecode instruction. Value must be a debuggee value.
Terminate the debuggee, as if it had been cancelled by the “slow script� dialog box.

If a function that would normally return a resumption value to indicate how the debuggee should continue instead throws an exception, 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:

cause value meaning
“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.

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 Contributors to this page: jimblandy, erxin
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