source-map.bs

<pre class='metadata'>
Title: Source Map
H1: Source Map
Shortname: source-map
Level: 1
Status: STAGE0
URL: https://source-map.github.io/source-map-spec/
Editor: Armin Ronacher, Sentry
Former Editor: Victor Porof, Google
Former Editor: John Lenz, Google
Former Editor: Nick Fitzgerald, Mozilla
Previous Version: https://docs.google.com/document/d/1U1RGAehQwRypUTovF1KRlpiOFze0b-_2gc6fAH0KY0k/edit?pli=1#
Repository: source-map/source-map-spec
Abstract: A specification for mapping transpiled source code (primarily JavaScript) back to the original sources.  This specification is a living document and describes a hardened version of the Source Map v3 specification.
Markup Shorthands: markdown yes
Group: tc39
</pre>

<pre class=link-defaults>
spec:html; type:element;
    text:a
    text:script
    text:style
    text:title
    text:link
spec:bikeshed-1; type:dfn; for:railroad; text:optional
</pre>

<pre class="biblio">
{
  "VLQ": {
    "href": "https://en.wikipedia.org/wiki/Variable-length_quantity",
    "title": "Variable-length quantity",
    "publisher": "Wikipedia",
    "status": "reference article"
  },
  "base64": {
    "href": "https://www.ietf.org/rfc/rfc4648.txt",
    "id": "rfc4648",
    "publisher": "IETF",
    "status": "Standards Track",
    "title": "The Base16, Base32, and Base64 Data Encodings"
  },
  "URL": {
    "href": "https://url.spec.whatwg.org/",
    "publisher": "WhatWG",
    "status": "Living Standard",
    "title": "URL Standard"
  },
  "EvalSourceURL": {
    "href": "https://web.archive.org/web/20120814122523/http://blog.getfirebug.com/2009/08/11/give-your-eval-a-name-with-sourceurl/",
    "publisher": "Firebug",
    "status": "archive",
    "title": "Give your eval a name with //@ sourceURL"
  },
  "V2Format": {
    "href": "https://docs.google.com/document/d/1xi12LrcqjqIHTtZzrzZKmQ3lbTv9mKrN076UB-j3UZQ/edit?hl=en_US",
    "publisher": "Google",
    "title": "Source Map Revision 2 Proposal"
  }
}
</pre>

## License

This work is licensed under a [Creative Commons Attribution-ShareAlike 3.0 Unported License](http://creativecommons.org/licenses/by-sa/3.0/).

## Introduction

This document is a draft version of a hardened version of the Source Map v3
specification.  In its current form, it's not a defined standard and is subject to
modifications.  If you want to get involved you will find more information under
the following GitHub repositories:

* [Spec Repository](https://github.com/source-map/source-map-spec/): holds the different
    specifications (v1, v2, v3 and the hardened v3 draft you are looking at)
* [RFC Repository](https://github.com/source-map/source-map-rfc/): meta
    repository for change suggestions to the specification.

Background {#background}
========================

The original source map format (v1) was created by Joseph Schorr for use by
Closure Inspector to enable source-level debugging of optimized JavaScript code
(although the format itself is language agnostic).  However, as the size of the
projects using the source maps expanded the verbosity of the format started to
become a problem.  The v2 ([[V2Format]]) was created by trading some simplicity
and flexibility to reduce the overall size of the source map. Even with the
changes made with the v2 version of the format, the source map file size was
limiting its usefulness.  The v3 format is based on suggestions made by
Pavel Podivilov (Google).

<mark>This document codifies the prior art that is Source Map v3 but is more specific
about the precise meanings of the specification.</mark>

Terminology {#terminology}
==========================

<dfn>Generated Code</dfn> is the code which is generated
by the compiler or transpiler.

<dfn>Original Source</dfn> is the source code which has not
been passed through the compiler.

<dfn>Base64 VLQ</dfn>: [[VLQ]] is a [[base64]] value, where the most significant
bit (the 6th bit) is used as the continuation bit, and the "digits" are encoded
into the string least significant first, and where the least significant bit of
the first digit is used as the sign bit.

Note: The values that can be represented by the VLQ Base64 encoded are limited to
32-bit quantities until some use case for larger values is presented.

<dfn>Source Mapping URL</dfn> refers to the URL referencing
the location of a source map from the [=Generated code=].

<ins><dfn>Column</dfn> is the 0 (zero) indexed offsets within a line of the
generated code measured.  The definition for columns in source maps can depend on
the content type.  For JavaScript and CSS based source maps are defined to be in
UTF-16 code units analogous to JavaScript string indexes.  That means that
"A" (`LATIN CAPITAL LETTER A`) measures 1 code unit, and "🔥" (`FIRE`) measures
2 code units.  For WebAssembly, columns are defined as byte offsets from the beginning 
of the binary content (and there is only one group representing a line).  Source maps
for other content types might diverge from this.
</ins>

General Goals {#general-goals}
==============================

<mark>The goals for the v3 format of Source Maps</mark>

* Reduce the overall size to improve parse time, memory consumption, and download time.
* Support source-level debugging allowing bidirectional mapping
* Support server-side stack trace deobfuscation

Source Map Format {#source-map-format}
======================================

The source map is a JSON document containing a top-level JSON object with the
following structure:

```json
{
  "version" : 3,
  "file": "out.js",
  "sourceRoot": "",
  "sources": ["foo.js", "bar.js"],
  "sourcesContent": [null, null],
  "names": ["src", "maps", "are", "fun"],
  "mappings": "A,AAAB;;ABCDE;"
}
```

Note: The previous specification suggested an order to the keys in this file, but
for practical reasons, the order cannot be defined in many JSON generators and
has never been enforced.

<dfn><code>version</code></dfn> is the version field which is always the number
`3` as an integer.

<dfn><code>file</code></dfn> an optional name of the generated code
that this source map is associated with.  <mark>It's not specified if this can
be a URL, relative path name, or just a base name. As such has a mostly informal
character.</mark>

<dfn><code>sourceRoot</code></dfn> an optional source root,
useful for relocating source files on a server or removing repeated values in
the "sources" entry.  This value is prepended to the individual entries in the
"source" field.

<dfn><code>sources</code></dfn> is a list of original sources
used by the "mappings" entry.  Each entry is either a string that is a
(potentially relative) URL or `null` if the source name is not known.

<dfn><code>sourcesContent</code></dfn> an optional list
of source content (that is the [=Original Source=]), useful when the "source"
can't be hosted.  The contents are listed in the same order as the [=sources=].
`null` may be used if some original sources should be retrieved by name.

<dfn><code>names</code></dfn> a list of symbol names used by the [=mappings=] entry.

<dfn><code>mappings</code></dfn> a string with the encoded mapping data (see [[#mappings-structure]]).

Mappings Structure {#mappings-structure}
----------------------------------------

The [=mappings=] data is broken down as follows:

- each group representing a line in the generated file is separated by a semicolon (`;`)
- each segment is separated by a comma (`,`)
- each segment is made up of 1, 4, or 5 variable length fields.

The fields in each segment are:

1. The zero-based starting [=column=] of the line in the generated code that the segment represents.
    If this is the first field of the first segment, or the first segment following a new generated
    line (`;`), then this field holds the whole [=Base64 VLQ=].  Otherwise, this field contains
    a [=Base64 VLQ=] that is relative to the previous occurrence of this field.  _Note that this
    is different than the fields below because the previous value is reset after every generated line._

2. If present, a zero-based index into the [=sources=] list. This field is a [=Base64 VLQ=]
    relative to the previous occurrence of this field, unless this is the first occurrence of this
    field, in which case the whole value is represented.

3. If present, the zero-based starting line in the original source is represented.  This field is a
    [=Base64 VLQ=] relative to the previous occurrence of this field, unless this is the first
    occurrence of this field, in which case the whole value is represented.  Always present if there
    is a source field.

4. If present, the zero-based starting [=column=] of the line in the source represented.  This
    field is a [=Base64 VLQ=] relative to the previous occurrence of this field unless this
    is the first occurrence of this field, in which case the whole value is represented.  Always
    present if there is a source field.

5. If present, the zero-based index into the [=names=] list associated with this segment.  This
    field is a base 64 VLQ relative to the previous occurrence of this field unless this
    is the first occurrence of this field, in which case the whole value is represented.

Note: This encoding reduces the source map size by 50% relative to the V2 format in tests performed
using Google Calendar.

Resolving Sources {#resolving-sources}
--------------------------------------

If the sources are not absolute URLs after prepending the [=sourceRoot=], the sources are
resolved relative to the SourceMap (like resolving the script `src` attribute in an HTML document).

Encoding {#encoding}
--------------------

For simplicity, the character set encoding is always UTF-8.

Compression {#compression}
--------------------------

The file is allowed to be GZIP compressed.   It is not expected that in-browser consumers of
the source map will support GZIP compression directly but that they will consume an
uncompressed map that may be GZIP'd for transport.

Extensions {#extensions}
------------------------

Additional fields may be added to the top-level source map provided the fields begin with the
`x_` naming convention.  It is expected that the extensions would be classified by the
organization providing the extension, such as `x_google_linecount`.    Field names outside
the `x_` namespace are reserved for future revisions.  It is recommended that fields be
namespaced by domain, i.e. `x_com_google_gwt_linecount`.

Known Extensions
----------------

<dfn export><code>x_google_linecount</code></dfn> The number of lines represented by this source map. 

<dfn export><code>x_google_ignoreList</code></dfn> Identifies third-party sources (such as framework
code or bundler-generated code), allowing developers to avoid code that they don't want to see
or step through, without having to configure this beforehand.

It refers to the [=sources=] array and lists the indices of all the known third-party sources
in that source map.  When parsing the source map, developer tools can use this to determine
sections of the code that the browser loads and runs that could be automatically ignore-listed.

Notes on File Offsets
---------------------

Using file offsets was considered but rejected in favor of using line/column data to avoid becoming
misaligned with the original due to platform-specific line endings. 

Index Map
=========

To support concatenating generated code and other common post-processing,
an alternate representation of a map is supported:

```json
{
  "version" : 3,
  "file": "app.js",
  "sections": [
    {
      "offset": {"line": 0, "column": 0},
      "map": {
        "version" : 3,
        "file": "section.js",
        "sources": ["foo.js", "bar.js"],
        "names": ["src", "maps", "are", "fun"],
        "mappings": "AAAA,E;;ABCDE;"
      }
    },
    {
      "offset": {"line": 100, "column": 10},
      "map": {
        "version" : 3,
        "file": "another_section.js",
        "sources": ["more.js"],
        "names": ["more", "is", "better"],
        "mappings": "AAAA,E;AACA,C;ABCDE;"
      }
    }
  ]
}
```

The index map follows the form of the standard map.  Like the regular source map,
the file format is JSON with a top-level object.  It shares the [=version=] and
[=file=] field from the regular source map, but gains a new [=sections=] field.

<dfn><code>sections</code></dfn> is an array of JSON objects that itself has two
fields [=offset=] and [=map=].

## Section

<dfn><code>offset</dfn></code> is an object with two fields, `line` and `column`,
that represent the offset into generated code that the referenced source map
represents.

<dfn><code>map</code></dfn> is an embedded complete source map object.
An embedded map does not inherit any values from the containing index map.

The sections must be sorted by starting position and the represented sections
may not overlap.

Conventions {#conventions}
==========================

The following conventions should be followed when working with source maps or
when generating them.

Source Map Naming {#source-map-naming}
--------------------------------------

Optionally, a source map will have the same name as the generated file but with a `.map`
extension.  For example, for `page.js` a source map named `page.js.map` would be generated. 

Linking generated code to source maps {#linking-generated-code}
---------------------------------------------------------------

While the source map format is intended to be language and platform agnostic, it is useful
to have some conventions for the expected use-case of web server-hosted javascript.

There are two suggested ways to link source maps to the output.  The first requires a server
support to add a HTTP header and the second requires an annotation in the source.

The HTTP header should supply the source map URL reference as:
 
```
SourceMap: <url>
```

Note: previous revisions of this document recommended a header name of `X-SourceMap`.  This
is now deprecated; `SourceMap` is now expected.

The generated code should include a line at the end of the source, with the following form:

```
//# sourceMappingURL=<url>
```

Note: The prefix for this annotation was initially `//@` however this conflicts with Internet
Explorer's Conditional Compilation and was changed to `//#`.  It is reasonable for tools to
also accept `//@` but `//#` is preferred.

This recommendation works well for JavaScript, it is expected that other source files will
have different conventions.  For instance, for CSS `/*# sourceMappingURL=<url> */` is proposed.

`<url>` is a URL as defined in [[URL]]; in particular,
characters outside the set permitted to appear in URIs must be percent-encoded
and it may be a data URI.  Using a data URI along with [=sourcesContent=] allows
for a completely self-contained source map.

<ins>The HTTP `SourceMap` header has precedence over source annotation, and if both are present,
header URL should be used to resolve the source map file.</ins>

Regardless of the method used to retrieve the [=Source Mapping URL=] the same
process is used to resolve it, which is as follows:

When the [=Source Mapping URL=] is not absolute, then it is relative to the generated code's
<dfn>source origin</dfn>. The [=source origin=] is determined by one of the following cases:

- If the generated source is not associated with a script element that has a `src`
    attribute and there exists a `//# sourceURL` comment in the generated code, that
    comment should be used to determine the [=source origin=]. Note: Previously, this was
    `//@ sourceURL`, as with `//@ sourceMappingURL`, it is reasonable to accept both
    but `//#` is preferred.

- If the generated code is associated with a script element and the script element has
    a `src` attribute, the `src` attribute of the script element will be the [=source origin=].

- If the generated code is associated with a script element and the script element does
    not have a `src` attribute, then the [=source origin=] will be the page's origin.

- If the generated code is being evaluated as a string with the `eval()` function or
    via `new Function()`, then the [=source origin=] will be the page's origin.

Linking eval'd code to named generated code
-------------------------------------------

There is an existing convention that should be supported for the use of source maps with
eval'd code, it has the following form:

```
//# sourceURL=foo.js
```

It is described in [[EvalSourceURL]].

Language Neutral Stack Mapping Notes
====================================

Stack tracing mapping without knowledge of the source language is not covered by this document.

Multi-level Mapping Notes
=========================

It is getting more common to have tools generate sources from some DSL (templates) or compile
TypeScript -> JavaScript -> minified JavaScript, resulting in multiple translations before the
final source map is created.  This problem can be handled in one of two ways.  The easy but
lossy way is to ignore the intermediate steps in the process for the purposes of debugging,
the source location information from the translation is either ignored (the intermediate
translation is considered the “Original Source”) or the source location information is carried
through (the intermediate translation hidden).  The more complete way is to support multiple
levels of mapping: if the Original Source also has a source map reference, the user is given
the choice of using that as well.

However, It is unclear what a "source map reference" looks like in anything other than JavaScript.
More specifically, what a source map reference looks like in a language that doesn't support
JavaScript-style single-line comments.

JSON over HTTP Transport
========================

For historic reasons, when delivering source maps over HTTP, servers may prepend a line
starting with the string `)]}'` to the source map.  A client fetching a source map via
HTTP thus should check if the response starts with this string and then ignore the
first line.

```
)]}'garbage here
{"version": 3, ...}
```

Is to be interpreted as

```
{"version": 3, ...}
```