# Debugger.Source

A `Debugger.Source` instance represents a piece of JavaScript source
code: its properties provide the source code itself as a string, and
describe where it came from. Each [`Debugger.Script`][script] instance refers to
the `Debugger.Source` instance holding the source code from which it was
produced.

If a single piece of source code contains both top-level code and
function definitions, perhaps with nested functions, then the
[`Debugger.Script`][script] instances for those all refer to the same
`Debugger.Source` instance. Each script indicates the substring of the
overall source to which it corresponds.

A `Debugger.Source` instance may represent only a portion of a larger
source document. For example, an HTML document can contain JavaScript in
multiple `<script>` elements and event handler content attributes.
In this case, there may be either a single `Debugger.Source` instance
for the entire HTML document, with each [`Debugger.Script`][script] referring to
its substring of the document; or there may be a separate
`Debugger.Source` instance for each `<script>` element and
attribute. The choice is left up to the implementation.

If a given piece of source code is presented to the JavaScript
implementation more than once, with the same origin metadata, the
JavaScript implementation may generate a fresh `Debugger.Source`
instance to represent each presentation, or it may use a single
`Debugger.Source` instance to represent them all.

Each [`Debugger`][debugger-object] instance has a separate collection of `Debugger.Source`
instances representing the source code that has been presented to the
system.

A debugger may place its own properties on `Debugger.Source` instances,
to store metadata about particular pieces of source code.


## Accessor Properties of the Debugger.Source Prototype Object

A `Debugger.Source` instance inherits the following accessor properties
from its prototype:

`text`
:   The JavaScript source code, as a string. The value satisfies the
    `Program`, `FunctionDeclaration`, or `FunctionExpression` productions in
    the ECMAScript standard.

`enclosingStart` <i>(future plan)</i>
:   The position within the enclosing document at which this source's text
    starts. This is a zero-based character offset. (The length of this
    script within the enclosing document is `source.length`.)

`lineCount` <i>(future plan)</i>
:   The number of lines in the source code. If there are characters after
    the last newline in the source code, those count as a final line;
    otherwise, `lineCount` is equal to the number of newlines in the source
    code.

`url`
:   The URL from which this source was loaded, if this source was loaded
    from a URL. Otherwise, this is `undefined`. Source may be loaded from a
    URL in the following ways:

    * The URL may appear as the `src` attribute of a `<script>` element
      in markup text.

    * The URL may be passed to the `Worker` web worker constructor, or the web
      worker `importScripts` function.

    * The URL may be the name of a XPCOM JavaScript module or subscript.

    (Note that code passed to `eval`, the `Function` constructor, or a
    similar function is <i>not</i> considered to be loaded from a URL; the
    `url` accessor on `Debugger.Source` instances for such sources should
    return `undefined`.)

`element`
:   The [`Debugger.Object`][object] instance referring to the DOM element to which
    this source code belongs, if any, or `undefined` if it belongs to no DOM
    element. Source belongs to a DOM element in the following cases:

    * Source belongs to a `<script>` element if it is the element's text
      content (that is, it is written out as the body of the `<script>`
      element in the markup text), or is the source document referenced by its
      `src` attribute.

    * Source belongs to a DOM element if it is an event handler content
      attribute (that is, if it is written out in the markup text as an
      attribute value).

    * Source belongs to a DOM element if it was assigned to one of the
      element's event handler IDL attributes as a string. (Note that one may
      assign both strings and functions to DOM elements' event handler IDL
      attributes. If one assigns a function, that function's script's source
      does <i>not</i> belong to the DOM element; the function's definition
      must appear elsewhere.)

    (If the sources attached to a DOM element change, the `Debugger.Source`
    instances representing superceded code still refer to the DOM element;
    this accessor only reflects origins, not current relationships.)

`elementAttributeName`
:   If this source belongs to a DOM element because it is an event handler
    content attribute or an event handler IDL attribute, this is the name of
    that attribute, a string. Otherwise, this is `undefined`.

`introductionType`
:   A string indicating how this source code was introduced into the system.
    This accessor returns one of the following values:

    * `"eval"`, for code passed to `eval`.

    * `"Function"`, for code passed to the `Function` constructor.

    * `"Worker"`, for code loaded by calling the Web worker constructor—the
      worker's main script.

    * `"importScripts"`, for code by calling `importScripts` in a web worker.

    * `"eventHandler"`, for code assigned to DOM elements' event handler IDL
      attributes as a string.

    * `"scriptElement"`, for code belonging to `<script>` elements.

    * `"javascriptURL"`, for code presented in `javascript:` URLs.

    * `"setTimeout"`, for code passed to `setTimeout` as a string.

    * `"setInterval"`, for code passed to `setInterval` as a string.

    * `undefined`, if the implementation doesn't know how the code was
      introduced.

`introductionScript`, `introductionScriptOffset` <i>(future plan)</i>
:   If this source was introduced by calling a function from debuggee code,
    then `introductionScript` is the [`Debugger.Script`][script] instance referring to
    the script containing that call, and `introductionScriptOffset` is the
    call's bytecode offset within that script. Otherwise, these are both
    `undefined`. Taken together, these properties indicate the location of
    the introducing call.

    For the purposes of these accessors, assignments to accessor properties
    are treated as function calls. Thus, setting a DOM element's event
    handler IDL attribute by assigning to the corresponding JavaScript
    property creates a source whose `introductionScript` and
    `introductionScriptOffset` refer to the property assignment.

    Since a `<script>` element parsed from a web page's original HTML
    was not introduced by any scripted call, its source's
    `introductionScript` and `introductionScriptOffset` accessors both
    return `undefined`.

    If a `<script>` element was dynamically inserted into a document,
    then these accessors refer to the call that actually caused the script
    to run—usually the call that made the element part of the document.
    Thus, they do <i>not</i> refer to the call that created the element;
    stored the source as the element's text child; made the element a child
    of some uninserted parent node that was later inserted; or the like.

    Although the main script of a worker thread is introduced by a call to
    `Worker` or `SharedWorker`, these accessors always return `undefined` on
    such script's sources. A worker's main script source and the call that
    created the worker are always in separate threads, but [`Debugger`][debugger-object] is an
    inherently single-threaded facility: its debuggees must all run in the
    same thread. Since the global that created the worker is in a different
    thread, it is guaranteed not to be a debuggee of the [`Debugger`][debugger-object] instance
    that owns this source; and thus the creating call is never "in debuggee
    code". Relating a worker to its creator, and other multi-threaded
    debugging concerns, are out of scope for [`Debugger`][debugger-object].



## Function Properties of the Debugger.Source Prototype Object

<code>substring(<i>start</i>, [<i>end</i>])</code> <i>(future plan)</i>
:   Return a substring of this instance's `source` property, starting at
    <i>start</i> and extending to, but not including, <i>end</i>. If
    <i>end</i> is `undefined`, the substring returned extends to the end of
    the source.

    Both indices are zero-based. If either is `NaN` or negative, it is
    replaced with zero. If either is greater than the length of the source,
    it is replaced with the length of the source. If <i>start</i> is larger
    than <i>end</i>, they are swapped. (This is meant to be consistent with
    the way `String.prototype.substring` interprets its arguments.)

<code>lineToPosition(<i>line</i>)</code> <i>(future plan)</i>
:   Return an object of the form
    <code>{ line:<i>line</i>, start:<i>start</i>, length:<i>length</i> }</code>, where
    <i>start</i> is the character index within `source` of the first
    character of line number <i>line</i>, and <i>length</i> is the length of
    that line in characters, including the final newline, if any. The first
    line is numbered one. If <i>line</i> is negative or greater than the
    number of lines in this `Debugger.Source` instance, then return `null`.

<code>positionToLine(<i>start</i>)</code> <i>(future plan)</i>
:   Return an object of the form
    <code>{ line:<i>line</i>, start:<i>start</i>, length:<i>length</i> }</code>, where
    <i>line</i> is the line number containing the character position
    <i>start</i>, and <i>length</i> is the length of that line in
    characters, including the final newline, if any. The first line is
    numbered one. If <i>start</i> is negative or greater than the length of
    the source code, then return `null`.

<code>findScripts(<i>query</i>)</code> <i>(future plan)</i>
:   Return an array of [`Debugger.Script`][script] instances for all debuggee scripts
    matching <i>query</i> that are produced from this `Debugger.Source`
    instance. Aside from the restriction to scripts produced from this
    source, <i>query</i> is interpreted as for
    `Debugger.prototype.findScripts`.


