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 # General Conventions

 This page describes general conventions used in the [`Debugger`][debugger] API,

 and defines some terminology used throughout the specification.

 ## Properties

 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

 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:

 <code>{ return: <i>value</i> }</code>

 :   The code completed normally, returning <i>value</i>. <i>Value</i> is a

     debuggee value.

 <code>{ yield: <i>value</i> }</code>

 :   <i>(Not yet implemented.)</i> The running code is a generator frame

     which has yielded <i>value</i>. <i>Value</i> is a debuggee value.

 <code>{ throw: <i>value</i> }</code>

 :   The code threw <i>value</i> as an exception. <i>Value</i> is a debuggee

     value.

 `null`

 :   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

 <code>{ throw: <i>stop</i> }</code> where <i>stop</i> 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:

 `undefined`

 :   The debuggee should continue execution normally.

 <code>{ return: <i>value</i> }</code>

 :   Return <i>value</i> immediately as the current value of the function.

     <i>Value</i> 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

     <i>value</i> 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.

 <code>{ yield: <i>value</i> }</code>

 :   <i>(Not yet implemented.)</i> Yield <i>value</i> immediately as the

     next value of the current frame, which must be a generator frame.

     <i>Value</i> is a debuggee value. The current frame must be a generator

     frame that has not yet completed in some other way. You may use `yield`

     resumption values to substitute a new value or one already yielded by a

     generator, or to make a generator yield additional values.

 <code>{ throw: <i>value</i> }</code>

 :   Throw <i>value</i> as an exception from the current bytecode

     instruction. <i>Value</i> must be a debuggee value.

 `null`

 :   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.

 ## 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][inv fr], 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.

 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:

   <i>cause</i> 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.