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<!DOCTYPE html>
<html lang="en-GB-oxendict">
<meta charset="utf-8">
<link rel="icon" href="img/favicon.ico">
<style>
#metadata-block {
margin: 4em 0;
padding: 10px;
border: 1px solid #ee8421;
}
#metadata-block h1 {
font-size: 1.5em;
margin-top: 0;
}
#metadata-block > ul {
list-style-type: none;
margin: 0; padding: 0;
}
#ecma-logo {
width: 500px;
}
/* TODO: Get proper Ecmarkup support for examples */
emu-note[example] {
--note-type: example;
--note-label: "Example";
--note-text-color: #574b0f;
--note-border-color: #e0cb52;
--note-background-color: #fcfaee;
}
emu-note[issue] {
--note-type: issue;
--note-label: "Issue";
--note-border-color: #e05252;
--note-background-color: #fbe9e9;
}
emu-note[issue], emu-note[example] {
color: black;
counter-increment: var(--note-type);
border-left-color: var(--note-border-color);
background-color: var(--note-background-color);
padding: 0.5em;
&::before {
content: var(--note-label) "\00A0" counter(var(--note-type));
color: var(--note-text-color, black);
text-transform: uppercase;
}
& .note {
display: none;
}
}
emu-clause[example][normative-optional] .attributes-tag emu-xref {
display: none;
}
emu-clause[example][normative-optional] .attributes-tag::before {
content: "Example";
}
dfn code {
font-style: normal;
}
emu-annex#sec-external-definitions dd dfn {
font-style: normal;
}
</style>
<pre class="metadata">
title: Source map format specification
shortname: ECMA-426
status: draft
location: https://tc39.es/ecma426/
</pre>
<p><img src="img/ecma-logo.svg" id="ecma-logo" alt="Ecma International logo"></p>
<div id="metadata-block">
<h1>About this Specification</h1>
<p>The document at <a href="https://tc39.es/ecma426/">https://tc39.es/ecma426/</a> is the most accurate and up-to-date source map specification. It contains the content of the most recently published snapshot plus any modifications that will be included in the next snapshot.</p>
<h1>Contributing to this Specification</h1>
<p>This specification is developed on GitHub. There are a number of ways to contribute to the development of this specification:</p>
<ul>
<li>GitHub Repository: <a href="https://github.com/tc39/ecma426">https://github.com/tc39/ecma426</a></li>
<li>Issues: <a href="https://github.com/tc39/ecma426/issues">All Issues</a>, <a href="https://github.com/tc39/ecma426/issues/new">File a New Issue</a></li>
<li>Pull Requests: <a href="https://github.com/tc39/ecma426/pulls">All Pull Requests</a>, <a href="https://github.com/tc39/ecma426/pulls/new">Create a New Pull Request</a></li>
<li>Test Suite: <a href="https://github.com/tc39/source-map-tests/">tc39/source-map-tests</a></li>
<li>
Editors:
<ul>
<li><a href="https://github.com/takikawa">Asumu Takikawa</a> (Igalia)</li>
</ul>
</li>
</ul>
<p>Refer to the <emu-xref href="#sec-colophon">colophon</emu-xref> for more information on how this document is created.</p>
</div>
<emu-intro id="sec-intro">
<h1>Introduction</h1>
<p>This Ecma Standard defines the Source map format, used for mapping transpiled source code back to the original sources.</p>
<p>The source map format has the following goals:</p>
<ul>
<li>Support source-level debugging allowing bidirectional mapping</li>
<li>Support server-side stack trace deobfuscation</li>
</ul>
<p>The <emu-not-ref>original source</emu-not-ref> 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 source maps expanded, the verbosity of the format started to become a problem. The v2 format (Source Map Revision 2 Proposal) 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).</p>
<p>The source map format does not have version numbers anymore, and it is instead hard-coded to always be "3".</p>
<p>In 2023-2024, the source map format was developed into a more precise Ecma standard, with significant contributions from many people. Further iteration on the source map format is expected to come from TC39-TG4.</p>
<p>
Asumu Takikawa, Nicolò Ribaudo, Jon Kuperman<br>
ECMA-426, 1<sup>st</sup> edition, Project Editors
</p>
</emu-intro>
<emu-clause id="sec-scope">
<h1>Scope</h1>
<p>This Standard defines the source map format, used by different types of developer tools to improve the debugging experience of code compiled to JavaScript, WebAssembly, and CSS.</p>
</emu-clause>
<emu-clause id="sec-conformance">
<h1>Conformance</h1>
<p>A conforming source map document is a JSON document that conforms to the structure detailed in this specification.</p>
<p>A conforming source map generator should generate documents which are conforming source map documents, and can be decoded by the algorithms in this specification without reporting any errors (even those which are specified as optional).</p>
<p>A conforming source map consumer should implement the algorithms specified in this specification for retrieving (where applicable) and decoding source map documents. A conforming consumer is permitted to ignore errors or report them without terminating where the specification indicates that an algorithm may optionally report an error.</p>
</emu-clause>
<emu-clause id="sec-references">
<h1>References</h1>
<p>The following documents are referred to in the text in such a way that some or all of their content constitutes requirements of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.</p>
<!-- All documents listed here must also be listed in the "Biography" annex. -->
<emu-clause id="sec-references-normative">
<h1>Normative References</h1>
<p>
ECMA-262, <i>ECMAScript® Language Specification</i>.<br>
<a href="https://tc39.es/ecma262/">https://tc39.es/ecma262/</a>
</p>
<p>
ECMA-404, <i>The JSON Data Interchange Format</i>.<br>
<a href="https://www.ecma-international.org/publications-and-standards/standards/ecma-404/">https://www.ecma-international.org/publications-and-standards/standards/ecma-404/</a>
</p>
</emu-clause>
<emu-clause id="sec-references-informative">
<h1>Informative References</h1>
<!-- NOTE: These references should actually be normative, but Ecma is allergic to normatively referencing web specs. As a rule of thumb, Ecma/IEEE/Unicode specs can go in the "normative" section, while the rest should go here. -->
<p>
IETF RFC 4648, <i>The Base16, Base32, and Base64 Data Encodings</i>.<br>
<a href="https://datatracker.ietf.org/doc/html/rfc4648">https://datatracker.ietf.org/doc/html/rfc4648</a>
</p>
<p>
<i>WebAssembly Core Specification</i>.<br>
<a href="https://www.w3.org/TR/wasm-core-2/">https://www.w3.org/TR/wasm-core-2/</a>
</p>
<p>
WHATWG <i>Encoding</i>.<br>
<a href="https://encoding.spec.whatwg.org/">https://encoding.spec.whatwg.org/</a>
</p>
<p>
WHATWG <emu-not-ref><i>Fetch</i></emu-not-ref>.<br>
<a href="https://fetch.spec.whatwg.org/">https://fetch.spec.whatwg.org/</a>
</p>
<p>
WHATWG <i>Infra</i>.<br>
<a href="https://infra.spec.whatwg.org/">https://infra.spec.whatwg.org/</a>
</p>
<p>
WHATWG <emu-not-ref><i>URL</i></emu-not-ref>.<br>
<a href="https://url.spec.whatwg.org/">https://url.spec.whatwg.org/</a>
</p>
</emu-clause>
</emu-clause>
<emu-clause id="sec-notational-conventions">
<h1>Notational Conventions</h1>
<p>This specification follows the same notational conventions as defined by <a href="https://tc39.es/ecma262/#sec-notational-conventions">ECMA-262 (Notational conventions)</a>, with the extensions defined in this section.</p>
<emu-clause id="sec-algorithm-conventions">
<h1>Algorithm Conventions</h1>
<emu-clause id="sec-implicit-completions">
<h1>Implicit Completions</h1>
<p>All abstract operations declared in this specification are implicitly assumed to either return a normal completion containing the algorithm's declared return type, or a throw completion. For example, an abstract operation declared as</p>
<blockquote>
<emu-clause id="example-get-the-answer" type="abstract operation" example>
<h1>
GetTheAnswer (
_input_: an integer,
): an integer
</h1>
<dl class="header"></dl>
</emu-clause>
</blockquote>
<p>is equivalent to:</p>
<blockquote>
<emu-clause id="example-get-the-answer-2" type="abstract operation" example>
<h1>
GetTheAnswer2 (
_input_: an integer,
): either a normal completion containing an integer or a throw completion
</h1>
<dl class="header"></dl>
</emu-clause>
</blockquote>
<p>All calls to abstract operations that return completion records are implicitly assumed to be wrapped by a ReturnIfAbrupt macro, unless they are explicitly wrapped by an explicit Completion call. For example:</p>
<emu-alg example>
1. Let _result_ be GetTheAnswer(_value_).
1. Let _second_ be Completion(GetTheAnswer(_value_)).
</emu-alg>
<p>is equivalent to:</p>
<emu-alg example>
1. Let _result_ be ReturnIfAbrupt(GetTheAnswer(_value_)).
1. Let _second_ be Completion(GetTheAnswer(_value_)).
</emu-alg>
</emu-clause>
<emu-clause id="sec-optional-errors">
<h1>Optional Errors</h1>
<p>Whenever an algorithm is to <dfn>optionally report an error</dfn>, an implementation may choose one of the following behaviours:</p>
<ul>
<li>Continue executing the rest of the algorithm.</li>
<li>Report an error to the user (for example, in the browser console), and continue executing the rest of the algorithm.</li>
<li>Return a ThrowCompletion.</li>
</ul>
<p>An implementation can choose different behaviours for different optional errors.</p>
</emu-clause>
</emu-clause>
<emu-clause id="sec-grammar-notation">
<h1>Grammar Notation</h1>
<p>This specification follows the same grammar notation convention as defined by <a href="https://tc39.es/ecma262/#sec-grammar-notation">ECMA-262 (Grammar Notation)</a>, with the following caveats:</p>
<ul>
<li>The terminal symbols of grammars defined in this specification are individual code points. This is similar to ECMA-262's <a href="https://tc39.es/ecma262/#sec-lexical-and-regexp-grammars">lexical grammar</a>, rather than to ECMA-262's <a href="https://tc39.es/ecma262/#sec-syntactic-grammar">syntactic grammar</a>.</li>
<li>This specification does not use <a href="https://tc39.es/ecma262/#sec-grammatical-parameters">grammatical parameters</a> or <a href="https://tc39.es/ecma262/#sec-lookahead-restrictions">lookahead restrictions</a>, to reduce the grammar definitions complexity.</li>
</ul>
</emu-clause>
</emu-clause>
<emu-clause id="sec-terms-and-definitions">
<h1>Terms and Definitions</h1>
<p>For the purposes of this document, the following terms and definitions apply.</p>
<dl>
<dt><dfn id="sec-terms-and-definitions-generated-code">generated code</dfn></dt>
<dd>
<p>code which is generated by the compiler or transpiler.</p>
</dd>
<dt><dfn id="sec-terms-and-definitions-original-source">original source</dfn></dt>
<dd>
<p>source code which has not been passed through a compiler or transpiler.</p>
</dd>
<dt><dfn id="sec-terms-and-definitions-source-mapping-url">source map URL</dfn></dt>
<dd>
<p>URL referencing the location of a source map from the generated code.</p>
</dd>
<dt><dfn id="sec-terms-and-definitions-colun">column</dfn></dt>
<dd>
<p>zero-based indexed offset within a line of the generated code, computed as UTF-16 code units for JavaScript and CSS source maps, and as byte indices in the binary content (represented as a single line) for WebAssembly source maps.</p>
<emu-note>That means that "A" (`LATIN CAPITAL LETTER A`) measures as 1 code unit, and "🔥" (`FIRE`) measures as 2 code units. Source maps for other content types may diverge from this.</emu-note>
</dd>
</dl>
</emu-clause>
<emu-clause id="sec-base64-vlq">
<h1>base64 VLQ</h1>
<p>A <dfn id="sec-base64-vlq">base64 VLQ</dfn> is a <emu-xref href="#sec-references-informative">base64</emu-xref>-encoded variable-length quantity, 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.</p>
<emu-note>The values that can be represented by the base64 VLQ encoding are limited to 32-bit quantities until some use case for larger values is presented. This means that values exceeding 32-bits are invalid and implementations may reject them. The sign bit is counted towards the limit, but the continuation bits are not.</emu-note>
<emu-note example>
The string `"iB"` represents a base64 VLQ with two digits. The first digit `"i"` encodes the bit pattern `0x100010`, which has a continuation bit of `1` (the VLQ continues), a sign bit of `0` (non-negative), and the value bits `0x0001`. The second digit `B` encodes the bit pattern `0x000001`, which has a continuation bit of `0`, no sign bit, and value bits `0x00001`. The decoding of this VLQ string is the number 17.
</emu-note>
<emu-note example>
The string `"V"` represents a base64 VLQ with one digit. The digit `"V"` encodes the bit pattern `0x010101`, which has a continuation bit of `0` (no continuation), a sign bit of `1` (negative), and the value bits `0x1010`. The decoding of this VLQ string is the number -10.
</emu-note>
<p>A base64 VLQ adheres to the following lexical grammar:</p>
<emu-grammar type="definition">
Vlq ::
VlqDigitList
VlqDigitList ::
TerminalDigit
ContinuationDigit VlqDigitList
TerminalDigit ::
`A` `B` `C` `D` `E` `F` `G` `H` `I` `J` `K` `L` `M` `N` `O` `P` `Q` `R`
`S` `T` `U` `V` `W` `X` `Y` `Z` `a` `b` `c` `d` `e` `f`
ContinuationDigit ::
`g` `h` `i` `j` `k` `l` `m` `n` `o` `p` `q` `r` `s` `t` `u` `v` `w` `x`
`y` `z` `0` `1` `2` `3` `4` `5` `6` `7` `8` `9` `+` `/`
</emu-grammar>
<emu-clause id="sec-VLQSignedValue" type="sdo">
<h1>VLQSignedValue ( ): an integer</h1>
<dl class="header"></dl>
<emu-grammar>
Vlq :: VlqDigitList
</emu-grammar>
<emu-alg>
1. Let _unsigned_ be the VLQUnsignedValue of |VlqDigitList|.
1. If _unsigned_ modulo 2 = 1, let _sign_ be -1.
1. Else, let _sign_ be 1.
1. Let _value_ be floor(_unsigned_ / 2).
1. If _value_ is 0 and _sign_ is -1, return -2<sup>31</sup>.
1. [id="step-VLQSignedValue-boundary-check"] If _value_ is ≥ 2<sup>31</sup>, throw an error.
1. Return _sign_ × _value_.
</emu-alg>
<emu-note>
The check in step <emu-xref href="#step-VLQSignedValue-boundary-check"></emu-xref> is needed because _unsigned_ is the VLQUnsignedValue of |VlqDigitList|, not of |Vlq|.
</emu-note>
</emu-clause>
<emu-clause id="sec-VLQUnsignedValue" type="sdo">
<h1>VLQUnsignedValue ( ): an non-negative integer</h1>
<dl class="header"></dl>
<emu-grammar>
Vlq :: VlqDigitList
</emu-grammar>
<emu-alg>
1. Let _value_ be the VLQUnsignedValue of |VlqDigitList|.
1. If _value_ is ≥ 2<sup>32</sup>, throw an error.
1. Return _value_.
</emu-alg>
<emu-grammar>
VlqDigitList ::
ContinuationDigit VlqDigitList
</emu-grammar>
<emu-alg>
1. Let _left_ be the VLQUnsignedValue of |ContinuationDigit|.
1. Let _right_ be the VLQUnsignedValue of |VlqDigitList|.
1. Return _left_ + _right_ × 2<sup>5</sup>.
</emu-alg>
<emu-grammar>
TerminalDigit ::
`A` `B` `C` `D` `E` `F` `G` `H` `I` `J` `K` `L` `M` `N` `O` `P` `Q` `R`
`S` `T` `U` `V` `W` `X` `Y` `Z` `a` `b` `c` `d` `e` `f`
</emu-grammar>
<emu-alg>
1. Let _digit_ be the character matched by this production.
1. Let _value_ be the integer corresponding to _digit_, according to the <emu-xref href="#sec-references-informative">base64</emu-xref> encoding as defined by IETF RFC 4648.
1. Assert: _value_ < 32.
1. Return _value_.
</emu-alg>
<emu-grammar>
ContinuationDigit ::
`g` `h` `i` `j` `k` `l` `m` `n` `o` `p` `q` `r` `s` `t` `u` `v` `w` `x`
`y` `z` `0` `1` `2` `3` `4` `5` `6` `7` `8` `9` `+` `/`
</emu-grammar>
<emu-alg>
1. Let _digit_ be the character matched by this production.
1. Let _value_ be the integer corresponding to _digit_, according to the <emu-xref href="#sec-references-informative">base64</emu-xref> encoding as defined by IETF RFC 4648.
1. Assert: 32 ≤ _value_ < 64.
1. Return _value_ - 32.
</emu-alg>
</emu-clause>
</emu-clause>
<emu-clause id="sec-json-values-utilities">
<h1>JSON values utilities</h1>
<p>While this specification's algorithms are defined on top of ECMA-262 internals, it is meant to be easily implementable by non-JavaScript platforms. This section contains utilities for working with JSON values, abstracting away ECMA-262 details from the rest of the document.</p>
<p>A <dfn id="type-json-value" variants="JSON values">JSON value</dfn> is either a JSON object, a JSON array, a <emu-xref href="#sec-ecmascript-language-types-string-type">String</emu-xref>, a <emu-xref href="#sec-ecmascript-language-types-number-type">Number</emu-xref>, a <emu-xref href="#sec-ecmascript-language-types-boolean-type">Boolean</emu-xref>, or <emu-xref href="#sec-ecmascript-language-types-null-type">*null*</emu-xref>.</p>
<p>A <dfn id="type-json-object">JSON object</dfn> is an Object such that each of its properties:</p>
<ul>
<li>is a data property,</li>
<li>has a String key,</li>
<li>has JSON value value.</li>
</ul>
<p>A <dfn id="type-json-array">JSON array</dfn> is a JSON object such that:</p>
<ul>
<li>it has a property whose key is *"length"* and whose value is a Number,</li>
<li>all its other properties' keys are integer indices.</li>
</ul>
<emu-clause id="sec-ParseJSON" type="abstract operation">
<h1>
ParseJSON (
_string_: a String,
): a JSON value
</h1>
<dl class="header"></dl>
<emu-alg>
1. Let _result_ be Call(<emu-xref href="#sec-json.parse">%JSON.parse%</emu-xref>, *null*, « _string_ »).
1. Assert: _result_ is a JSON value.
1. Return _result_.
</emu-alg>
<emu-note type="editor">
This abstract operation is in the process of being exposed by ECMA-262 itself, at <a href="https://github.com/tc39/ecma262/pull/3540">tc39/ecma262#3540</a>.
</emu-note>
</emu-clause>
<emu-clause id="sec-JSONObjectGet" type="abstract operation">
<h1>
JSONObjectGet (
_object_: a JSON object,
_key_: a String,
): a JSON value or ~missing~
</h1>
<dl class="header">
<dt>description</dt>
<dd>It returns the value associated with the specified _key_ in _object_.</dd>
</dl>
<emu-alg>
1. If _object_ does not have an own property with key _key_, return ~missing~.
1. Let _prop_ be _object_'s own property whose key is _key_.
1. Return _prop_'s [[Value]] attribute.
</emu-alg>
</emu-clause>
<emu-clause id="sec-JSONArrayIterate" type="abstract operation">
<h1>
JSONArrayIterate (
_array_: a JSON array,
): a List of JSON values
</h1>
<dl class="header">
<dt>description</dt>
<dd>It returns a List containing all elements of _array_, so that it can be iterated by algorithms using "For each".</dd>
</dl>
<emu-alg>
1. Let _length_ be JSONObjectGet(_array_, *"length"*).
1. Assert: _length_ is a non-negative integral Number.
1. Let _list_ be a new empty List.
1. Let _i_ be 0.
1. Repeat, while _i_ < ℝ(_length_),
1. Let _value_ be JSONObjectGet(_array_, ToString(𝔽(_i_))).
1. Assert: _value_ is not ~missing~.
1. Append _value_ to _list_.
1. Set _i_ to _i_ + 1.
1. Return _list_.
</emu-alg>
</emu-clause>
<emu-clause id="sec-StringSplit" type="abstract operation">
<h1>
StringSplit (
_string_: a String,
_separators_: a List of String,
): a List of Strings
</h1>
<dl class="header">
<dt>description</dt>
<dd>It splits the string in substrings separated by any of the elements of _separators_. If multiple separators match, those appearing first in _separators_ have higher priority.</dd>
</dl>
<emu-alg>
1. Let _parts_ be a new empty List.
1. Let _strLen_ be the length of _string_.
1. Let _lastStart_ be 0.
1. Let _i_ be 0.
1. Repeat, while _i_ < _strLen_,
1. Let _matched_ be *false*.
1. For each String _sep_ of _separators_, do
1. Let _sepLen_ be the length of _sep_.
1. Let _candidate_ be the substring of _string_ from _i_ to min(_i_ + _sepLen_, _strLen_).
1. If _candidate_ = _sep_ and _matched_ is *false*, then
1. Let _chunk_ be the substring of _string_ from _lastStart_ to _i_.
1. Append _chunk_ to _parts_.
1. Set _lastStart_ to _i_ + _sepLen_.
1. Set _i_ to _i_ + _sepLen_.
1. Set _matched_ to *true*.
1. If _matched_ is *false*, set _i_ to _i_ + 1.
1. Let _chunk_ be the substring of _string_ from _lastStart_ to _strLen_.
1. Append _chunk_ to _parts_.
1. Return _parts_.
</emu-alg>
</emu-clause>
</emu-clause>
<emu-clause id="sec-position-types">
<h1>Position types</h1>
<emu-clause id="sec-position-record-type">
<h1>Position Record</h1>
<p>A <dfn>Position Record</dfn> is a tuple of a non-negative line and non-negative column number:</p>
<emu-table id="table-position-record-fields" caption="Position Record Fields">
<table>
<thead>
<tr>
<th>Field Name</th>
<th>Value Type</th>
</tr>
</thead>
<tr>
<td>[[Line]]</td>
<td>a non-negative integral Number</td>
</tr>
<tr>
<td>[[Column]]</td>
<td>a non-negative integral Number</td>
</tr>
</table>
</emu-table>
</emu-clause>
<emu-clause id="sec-original-position-record-type">
<h1>Original Position Record</h1>
<p>A <dfn variants="Original Position Records">Original Position Record</dfn> is a tuple of a Decoded Source Record, a non-negative line and non-negative column number. It is similar to a Position Record but describes a source position in a concrete original source file.</p>
<emu-table id="table-original-position-record-fields" caption="Original Position Record Fields">
<table>
<thead>
<tr>
<th>Field Name</th>
<th>Value Type</th>
</tr>
</thead>
<tr>
<td>[[Source]]</td>
<td>a Decoded Source Record</td>
</tr>
<tr>
<td>[[Line]]</td>
<td>a non-negative integral Number</td>
</tr>
<tr>
<td>[[Column]]</td>
<td>a non-negative integral Number</td>
</tr>
</table>
</emu-table>
</emu-clause>
<emu-clause id="sec-ComparePositions" type="abstract operation">
<h1>
ComparePositions (
_first_: a Position Record or a Original Position Record,
_second_: a Position Record or a Original Position Record,
): ~lesser~, ~equal~ or ~greater~
</h1>
<dl class="header">
<dt>description</dt>
<dd>It returns ~lesser~, ~equal~ or ~greater~ depending whether _first_ occurs before, is equal or occurs after _second_ respectively. The [[Source]] field of Original Position Records are ignored.</dd>
</dl>
<emu-alg>
1. If _first_.[[Line]] < _second_.[[Line]], return ~lesser~.
1. If _first_.[[Line]] > _second_.[[Line]], return ~greater~.
1. Assert: _first_.[[Line]] is equal to _second_.[[Line]].
1. If _first_.[[Column]] < _second_.[[Column]], return ~lesser~.
1. If _first_.[[Column]] > _second_.[[Column]], return ~greater~.
1. Return ~equal~.
</emu-alg>
</emu-clause>
</emu-clause>
<emu-clause id="sec-source-map-format">
<h1>Source map format</h1>
<p>A source map is a JSON document containing a top-level JSON object with the following structure:</p>
<pre><code class="json">
{
"version" : 3,
"file": "out.js",
"sourceRoot": "",
"sources": ["foo.js", "bar.js"],
"sourcesContent": [null, null],
"names": ["src", "maps", "are", "fun"],
"mappings": "A,AAAB;;ABCDE"
"ignoreList": [0]
}
</code></pre>
<ul>
<li>The <dfn id="json-version"><code>version</code> field</dfn> shall always be the number 3 as an integer. The source map may be rejected if the field has any other value.</li>
<li>The <dfn id="json-file"><code>file</code> field</dfn> is an optional name of the generated code that this source map is associated with. It's not specified if this can be an URL, relative path name, or just a base name. Source map generators may choose the appropriate interpretation for their contexts of use.</li>
<li>The <dfn id="json-sourceRoot"><code>sourceRoot</code> field</dfn> is an optional source root string, used 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 sources field.</li>
<li>The <dfn id="json-sources"><code>sources</code> field</dfn> is a list of original sources used by the mappings field. Each entry is either a string that is a (potentially relative) URL or *null* if the source name is not known.</li>
<li>The <dfn id="json-sourcesContent"><code>sourcesContent</code> field</dfn> is an optional list of source content (i.e. the original source) strings, used when the source cannot be hosted. The contents are listed in the same order as in the sources field. Entries may be *null* if some <emu-xref href="#sec-terms-and-definitions-original-source">original sources</emu-xref> should be retrieved by name.</li>
<li>The <dfn id="json-names"><code>names</code> field</dfn> is an optional list of symbol names which may be used by the mappings field.</li>
<li>The <dfn id="json-mappings"><code>mappings</code> field</dfn> is a string with the encoded mapping data (see section <emu-xref href="#sec-mappings"></emu-xref>).</li>
<li>The <dfn id="json-ignoreList"><code>ignoreList</code> field</dfn> is an optional list of indices of files that should be considered third party code, such as framework code or bundler-<emu-not-ref>generated code</emu-not-ref>. This allows developer tools to avoid code that developers likely don't want to see or step through, without requiring developers to configure this beforehand. It refers to the sources field and lists the indices of all the known third-party sources in the source map. Some browsers may also use the deprecated `x_google_ignoreList` field if `ignoreList` is not present.</li>
</ul>
<emu-clause id="sec-decoding-source-maps">
<h1>Decoding source maps</h1>
<p>A <dfn id="decoded-source-map-record" variants="Decoded Source Map Records">Decoded Source Map Record</dfn> has the following fields:</p>
<emu-table id="table-decoded-source-map-fields" caption="Fields of Decoded Source Map Records">
<table>
<thead>
<tr>
<th>
Field Name
</th>
<th>
Value Type
</th>
</tr>
</thead>
<tr>
<td>[[File]]</td>
<td>a String or *null*</td>
</tr>
<tr>
<td>[[Sources]]</td>
<td>a List of Decoded Source Records</td>
</tr>
<tr>
<td>[[Mappings]]</td>
<td>a List of Decoded Mapping Records</td>
</tr>
</table>
</emu-table>
<p>A <dfn id="decoded-source-record" variants="Decoded Source Records">Decoded Source Record</dfn> has the following fields:</p>
<emu-table id="table-decoded-source-fields" caption="Fields of Decoded Source Records">
<table>
<thead>
<tr>
<th>
Field Name
</th>
<th>
Value Type
</th>
</tr>
</thead>
<tr>
<td>[[URL]]</td>
<td>a URL or *null*</td>
</tr>
<tr>
<td>[[Content]]</td>
<td>a String or *null*</td>
</tr>
<tr>
<td>[[Ignored]]</td>
<td>a Boolean</td>
</tr>
</table>
</emu-table>
<emu-clause id="sec-ParseSourceMap" type="abstract operation">
<h1>
ParseSourceMap (
_string_: a String,
_baseURL_: an URL,
): a Decoded Source Map Record
</h1>
<dl class="header"></dl>
<emu-alg>
1. Let _json_ be ParseJSON(_string_).
1. If _json_ is not a JSON object, throw an error.
1. If JSONObjectGet(_json_, *"sections"*) is not ~missing~, then
1. Return DecodeIndexSourceMap(_json_, _baseURL_).
1. Return DecodeSourceMap(_json_, _baseURL_).
</emu-alg>
</emu-clause>
<emu-clause id="sec-DecodeSourceMap" type="abstract operation">
<h1>
DecodeSourceMap (
_json_: a JSON object,
_baseURL_: an URL,
): a Decoded Source Map Record
</h1>
<dl class="header"></dl>
<emu-alg>
1. If JSONObjectGet(_json_, *"version"*) is not *3*<sub>𝔽</sub>, optionally report an error.
1. Let _mappingsField_ be JSONObjectGet(_json_, *"mappings"*).
1. If _mappingsField_ is not a String, throw an error.
1. If JSONObjectGet(_json_, *"sources"*) is not a JSON array, throw an error.
1. Let _fileField_ be GetOptionalString(_json_, *"file"*).
1. Let _sourceRootField_ be GetOptionalString(_json_, *"sourceRoot"*).
1. Let _sourcesField_ be GetOptionalListOfOptionalStrings(_json_, *"sources"*).
1. Let _sourcesContentField_ be GetOptionalListOfOptionalStrings(_json_, *"sourcesContent"*).
1. Let _ignoreListField_ be GetOptionalListOfArrayIndexes(_json_, *"ignoreList"*).
1. Let _sources_ be DecodeSourceMapSources(_baseURL_, _sourceRootField_, _sourcesField_, _sourcesContentField_, _ignoreListField_).
1. Let _namesField_ be GetOptionalListOfStrings(_json_, *"names"*).
1. Let _mappings_ be DecodeMappings(_mappingsField_, _namesField_, _sources_).
1. Return the Decoded Source Map Record { [[File]]: _fileField_, [[Sources]]: _sources_, [[Mappings]]: _mappings_ }.
</emu-alg>
<emu-clause id="sec-GetOptionalString" type="abstract operation">
<h1>
GetOptionalString (
_object_: a JSON object,
_key_: a String,
): a String or *null*
</h1>
<dl class="header"></dl>
<emu-alg>
1. Let _value_ be JSONObjectGet(_object_, _key_).
1. If _value_ is a String, return _value_.
1. If _value_ is not ~missing~, optionally report an error.
1. Return *null*.
</emu-alg>
</emu-clause>
<emu-clause id="sec-GetOptionalListOfStrings" type="abstract operation">
<h1>
GetOptionalListOfStrings (
_object_: a JSON object,
_key_: a String,
): a List of Strings
</h1>
<dl class="header"></dl>
<emu-alg>
1. Let _list_ be a new empty List.
1. Let _values_ be JSONObjectGet(_object_, _key_).
1. If _values_ is ~missing~, return _list_.
1. If _values_ is not a JSON array, then
1. Optionally report an error.
1. Return _list_.
1. For each element _item_ of JSONArrayIterate(_values_), do
1. If _item_ is a String, then
1. Append _item_ to _list_.
1. Else,
1. Optionally report an error.
1. Append *""* to _list_.
1. Return _list_.
</emu-alg>
</emu-clause>
<emu-clause id="sec-GetOptionalListOfOptionalStrings" type="abstract operation">
<h1>
GetOptionalListOfOptionalStrings (
_object_: a JSON object,
_key_: a String,
): a List of either Strings or *null*
</h1>
<dl class="header"></dl>
<emu-alg>
1. Let _list_ be a new empty List.
1. Let _values_ be JSONObjectGet(_object_, _key_).
1. If _values_ is ~missing~, return _list_.
1. If _values_ is not a JSON array, then
1. Optionally report an error.
1. Return _list_.
1. For each element _item_ of JSONArrayIterate(_values_), do
1. If _item_ is a String, then
1. Append _item_ to _list_.
1. Else,
1. If _item_ ≠ *null*, optionally report an error.
1. Append *null* to _list_.
1. Return _list_.
</emu-alg>
</emu-clause>
<emu-clause id="sec-GetOptionalListOfArrayIndexes" type="abstract operation">
<h1>
GetOptionalListOfArrayIndexes (
_object_: an Object,
_key_: a String,
): a List of non-negative integers
</h1>
<dl class="header"></dl>
<emu-alg>
1. Let _list_ be a new empty List.
1. Let _values_ be JSONObjectGet(_object_, _key_).
1. If _values_ is ~missing~, return _list_.
1. If _values_ is not a JSON array, then
1. Optionally report an error.
1. Return _list_.
1. For each element _item_ of JSONArrayIterate(_values_), do
1. If _item_ is an integral Number, _item_ ≠ *+0*<sub>𝔽</sub>, and _item_ ≥ *+0*<sub>𝔽</sub>, then
1. Append ℝ(_item_) to _list_.
1. Else,
1. If _item_ ≠ *null*, optionally report an error.
1. Append *null* to _list_.
1. Return _list_.
</emu-alg>
<!--
TODO:
Should the "If _item_ ≠ *null*, optionally report an error" step be
removed, so that the list only contains numbers?
-->
</emu-clause>
</emu-clause>
</emu-clause>
<emu-clause id="sec-mappings">
<h1>Mappings structure</h1>
<p>The mappings field data is broken down as follows:</p>
<ul>
<li>each group representing a line in the generated file is separated by a semicolon (`;`)</li>
<li>each segment is separated by a comma (`,`)</li>
<li>each segment is made up of 1, 4, or 5 variable length fields.</li>
</ul>
<p>The fields in each segment are:</p>
<ol>
<li>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. <em>Note that this is different from the subsequent fields below because the previous value is reset after every generated line.</em></li>
<li>If present, the zero-based index into the sources list. This field contains a base64 VLQ relative to the previous occurrence of this field, unless it is the first occurrence of this field, in which case the whole value is represented.</li>
<li>If present, the zero-based starting line in the original source. This field contains a base64 VLQ relative to the previous occurrence of this field, unless it is the first occurrence of this field, in which case the whole value is represented. Shall be present if there is a source field.</li>
<li>If present, the zero-based starting column of the line in the original source. This field contains a base64 VLQ relative to the previous occurrence of this field, unless it is the first occurrence of this field, in which case the whole value is represented. Shall be present if there is a source field.</li>
<li>If present, the zero-based index into the names list associated with this segment. This field contains a base64 VLQ relative to the previous occurrence of this field, unless it is the first occurrence of this field, in which case the whole value is represented.</li>
</ol>
<emu-note>The purpose of this encoding is to reduce the source map size. VLQ encoding reduced source maps by 50% relative to the Source Map Revision 2 Proposal in tests performed using Google Calendar.</emu-note>
<emu-note>Segments with one field are intended to represent generated code that is unmapped because there is no corresponding original source code, such as code that is generated by a compiler. Segments with four fields represent mapped code where a corresponding name does not exist. Segments with five fields represent mapped code that also has a mapped name.</emu-note>
<emu-note>Using <emu-not-ref>file</emu-not-ref> offsets was considered but rejected in favor of using line/<emu-not-ref>column</emu-not-ref> data to avoid becoming misaligned with the original due to platform-specific line endings.</emu-note>
<p>A <dfn id="decoded-mapping-record" variants="Decoded Mapping Records">Decoded Mapping Record</dfn> has the following fields:</p>
<emu-table id="table-decoded-mapping-fields" caption="Fields of Decoded Mapping Records">
<table>
<thead>
<tr>
<th>
Field Name
</th>
<th>
Value Type
</th>
</tr>
</thead>
<tr>
<td>[[GeneratedPosition]]</td>
<td>a Position Record</td>
</tr>
<tr>
<td>[[OriginalPosition]]</td>
<td>a Original Position Record or *null*</td>
</tr>
<tr>
<td>[[Name]]</td>
<td>a String or *null*</td>
</tr>
</table>
</emu-table>
<emu-clause id="sec-mappings-grammar">
<h1>Mappings grammar</h1>
<p>The mappings String must adhere to the following grammar:</p>
<emu-grammar type="definition">
MappingsField :
LineList
LineList :
Line
Line `;` LineList
Line :
MappingList?
MappingList :
Mapping
Mapping `,` MappingList
Mapping :
GeneratedColumn
GeneratedColumn OriginalSource OriginalLine OriginalColumn Name?
GeneratedColumn :
Vlq
OriginalSource :
Vlq
OriginalLine :
Vlq
OriginalColumn :
Vlq
Name :
Vlq
</emu-grammar>
<p>A <dfn id="decode-mapping-state-record" variants="Decode Mapping State Records">Decode Mapping State Record</dfn> has the following fields:</p>
<emu-table id="table-decode-mapping-state-fields" caption="Fields of Decode Mapping State Records">
<table>
<thead>
<tr>
<th>
Field Name
</th>
<th>
Value Type
</th>
</tr>
</thead>
<tr>
<td>[[GeneratedLine]]</td>
<td>a non-negative integer</td>
</tr>
<tr>
<td>[[GeneratedColumn]]</td>
<td>a non-negative integer</td>
</tr>
<tr>
<td>[[SourceIndex]]</td>
<td>a non-negative integer</td>
</tr>
<tr>
<td>[[OriginalLine]]</td>
<td>a non-negative integer</td>
</tr>
<tr>
<td>[[OriginalColumn]]</td>
<td>a non-negative integer</td>
</tr>
<tr>
<td>[[NameIndex]]</td>
<td>a non-negative integer</td>
</tr>
</table>
</emu-table>
<emu-clause id="sec-DecodeMappingsField" type="sdo">
<h1>
DecodeMappingsField (
_state_: a Decode Mapping State Record,
_mappings_: a List of Decoded Mapping Records,
_names_: a List of Strings,
_sources_: a List of Decoded Source Records,
)
</h1>
<dl class="header"></dl>
<emu-grammar>
LineList :
Line `;` LineList
</emu-grammar>
<emu-alg>
1. Perform DecodeMappingsField of |Line| with arguments _state_, _mappings_, _names_ and _sources_.
1. Set _state_.[[GeneratedLine]] to _state_.[[GeneratedLine]] + 1.
1. Set _state_.[[GeneratedColumn]] to 0.
1. Perform DecodeMappingsField of |LineList| with arguments _state_, _mappings_, _names_ and _sources_.
</emu-alg>
<emu-grammar>
MappingList :
Mapping `,` MappingList
</emu-grammar>
<emu-alg>
1. Perform DecodeMappingsField of |Mapping| with arguments _state_, _mappings_, _names_ and _sources_.
1. Perform DecodeMappingsField of |MappingList| with arguments _state_, _mappings_, _names_ and _sources_.
</emu-alg>
<emu-grammar>
Mapping :
GeneratedColumn
</emu-grammar>
<emu-alg>
1. Perform DecodeMappingsField of |GeneratedColumn| with arguments _state_, _mappings_, _names_ and _sources_.
1. If _state_.[[GeneratedColumn]] < 0, then
1. Optionally report an error.
1. Return.
1. Let _position_ be a new Position Record { [[Line]]: _state_.[[GeneratedLine]], [[Column]]: _state_.[[GeneratedColumn]] }.
1. Let _decodedMapping_ be a new DecodedMappingRecord { [[GeneratedPosition]]: _position_, [[OriginalPosition]]: *null*, [[Name]]: *null* }.
1. Append _decodedMapping_ to _mappings_.
</emu-alg>
<emu-grammar>
Mapping :
GeneratedColumn OriginalSource OriginalLine OriginalColumn Name?
</emu-grammar>
<emu-alg>
1. Perform DecodeMappingsField of |GeneratedColumn| with arguments _state_, _mappings_, _names_ and _sources_.
1. If _state_.[[GeneratedColumn]] < 0, then
1. Optionally report an error.
1. Return.
1. Let _generatedPosition_ be a new Position Record { [[Line]]: _state_.[[GeneratedLine]], [[Column]]: _state_.[[GeneratedColumn]] }.
1. Perform DecodeMappingsField of |OriginalSource| with arguments _state_, _mappings_, _names_ and _sources_.
1. Perform DecodeMappingsField of |OriginalLine| with arguments _state_, _mappings_, _names_ and _sources_.
1. Perform DecodeMappingsField of |OriginalColumn| with arguments _state_, _mappings_, _names_ and _sources_.
1. If _state_.[[SourceIndex]] < 0 or _state_.[[SourceIndex]] ≥ the number of elements of _sources_ or _state_.[[OriginalLine]] < 0 or _state_.[[OriginalColumn]] < 0, then
1. Optionally report an error.
1. Let _originalPosition_ be *null*.
1. Else,
1. Let _originalPosition_ be a new Original Position Record { [[Source]]: _sources_[_state_.[[SourceIndex]]], [[Line]]: _state_.[[OriginalLine]], [[Column]]: _state_.[[OriginalColumn]] }.
1. Let _name_ be *null*.
1. If |Name| is present, then
1. Perform DecodeMappingsField of |Name| with arguments _state_, _mappings_, _names_ and _sources_.
1. If _state_.[[NameIndex]] < 0 or _state_.[[NameIndex]] ≥ the number of elements of _sources_, optionally report an error.
1. Else, set _name_ to _names_[_state_.[[NameIndex]]].
1. Let _decodedMapping_ be a new DecodedMappingRecord { [[GeneratedPosition]]: _generatedPosition_, [[OriginalPosition]]: _originalPosition_, [[Name]]: _name_ }.
1. Append _decodedMapping_ to _mappings_.
</emu-alg>
<emu-grammar>
GeneratedColumn :
Vlq
</emu-grammar>
<emu-alg>
1. Let _relativeColumn_ be the VLQSignedValue of |Vlq|.
1. Set _state_.[[GeneratedColumn]] to _state_.[[GeneratedColumn]] + _relativeColumn_.
</emu-alg>
<emu-grammar>
OriginalSource :
Vlq
</emu-grammar>
<emu-alg>
1. Let _relativeSourceIndex_ be the VLQSignedValue of |Vlq|.
1. Set _state_.[[SourceIndex]] to _state_.[[SourceIndex]] + _relativeSourceIndex_.
</emu-alg>