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Source map format specification

This Ecma Standard defines the Source map format, used for mapping transpiled source code back to the original sources.

The source map format has the following goals:

• Support source-level debugging allowing bidirectional mapping
• Support server-side stack trace deobfuscation

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 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).

The source map format does not have version numbers anymore, and it is instead hard-coded to always be "3".

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.

Asumu Takikawa, Nicolò Ribaudo, Jon Kuperman
ECMA-426, 1^st edition, Project Editors

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.

A conforming source map document is a JSON document that conforms to the structure detailed in this specification.

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).

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 .

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.

ECMA-262, ECMAScript® Language Specification.
https://tc39.es/ecma262/

ECMA-404, The JSON Data Interchange Format.
https://www.ecma-international.org/publications-and-standards/standards/ecma-404/

IETF RFC 4648, The Base16, Base32, and Base64 Data Encodings.
https://datatracker.ietf.org/doc/html/rfc4648

WebAssembly Core Specification.
https://www.w3.org/TR/wasm-core-2/

WHATWG Encoding.
https://encoding.spec.whatwg.org/

WHATWG Fetch .
https://fetch.spec.whatwg.org/

WHATWG Infra.
https://infra.spec.whatwg.org/

WHATWG URL .
https://url.spec.whatwg.org/

This specification follows the same notational conventions as defined by ECMA-262 (Notational conventions), with the extensions defined in this section.

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

4.1.1.1 GetTheAnswer ( input )

The abstract operation GetTheAnswer takes argument input (an integer ) and returns an integer .

is equivalent to:

4.1.1.2 GetTheAnswer2 ( input )

The abstract operation GetTheAnswer2 takes argument input (an integer ) and returns either a normal completion containing an integer or a throw completion .

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:

1. Let result be GetTheAnswer (value).
2. Let second be Completion ( GetTheAnswer (value)).

is equivalent to:

1. Let result be ReturnIfAbrupt ( GetTheAnswer (value)).
2. Let second be Completion ( GetTheAnswer (value)).

Whenever an algorithm is to optionally report an error, an implementation may choose one of the following behaviours:

• Continue executing the rest of the algorithm.
• Report an error to the user (for example, in the browser console), and continue executing the rest of the algorithm.
• Return a ThrowCompletion .

An implementation can choose different behaviours for different optional errors.

This specification follows the same grammar notation convention as defined by ECMA-262 (Grammar Notation), with the following caveats:

• The terminal symbols of grammars defined in this specification are individual code points. This is similar to ECMA-262's lexical grammar, rather than to ECMA-262's syntactic grammar.
• This specification does not use grammatical parameters or lookahead restrictions, to reduce the grammar definitions complexity.

For the purposes of this document, the following terms and definitions apply.

generated code

code which is generated by the compiler or transpiler.

original source

source code which has not been passed through a compiler or transpiler.

source map URL

URL referencing the location of a source map from the generated code .

column

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.

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.

A base64 VLQ is a base64 -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.

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.

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.

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.

A base64 VLQ adheres to the following lexical grammar:

The syntax-directed operation VLQSignedValue takes no arguments and returns an integer . It is defined piecewise over the following productions:

1. Let unsigned be the VLQUnsignedValue of VlqDigitList .
2. If unsigned modulo 2 = 1, let sign be -1.
3. Else, let sign be 1.
4. Let value be floor (unsigned / 2).
5. If value is 0 and sign is -1, return -2³¹.
6. If value is ≥ 2³¹, throw an error.
7. Return sign × value.

The check in step

is needed because

is the

of

, not of

.

The syntax-directed operation VLQUnsignedValue takes no arguments and returns an non-negative integer . It is defined piecewise over the following productions:

1. Let value be the VLQUnsignedValue of VlqDigitList .
2. If value is ≥ 2³², throw an error.
3. Return value.

1. Let left be the VLQUnsignedValue of ContinuationDigit .
2. Let right be the VLQUnsignedValue of VlqDigitList .
3. Return left + right × 2⁵.

1. Let digit be the character matched by this production.
2. Let value be the integer corresponding to digit, according to the base64 encoding as defined by IETF RFC 4648.
3. Assert : value < 32.
4. Return value.

1. Let digit be the character matched by this production.
2. Let value be the integer corresponding to digit, according to the base64 encoding as defined by IETF RFC 4648.
3. Assert : 32 ≤ value < 64.
4. Return value - 32.

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.

A JSON value is either a JSON object , a JSON array , a String , a Number , a Boolean , or null .

A JSON object is an Object such that each of its properties:

• is a data property ,
• has a String key,
• has JSON value value.

A JSON array is a JSON object such that:

• it has a property whose key is "length" and whose value is a Number ,
• all its other properties' keys are integer indices .

The abstract operation ParseJSON takes argument string (a String) and returns a JSON value . It performs the following steps when called:

1. Let result be Call ( %JSON.parse% , null , « string »).
2. Assert : result is a JSON value .
3. Return result.

This abstract operation is in the process of being exposed by ECMA-262 itself, at

.

The abstract operation JSONObjectGet takes arguments object (a JSON object ) and key (a String) and returns a JSON value or missing . It returns the value associated with the specified key in object. It performs the following steps when called:

1. If object does not have an own property with key key, return missing .
2. Let prop be object's own property whose key is key.
3. Return prop's [[Value]] attribute.

The abstract operation JSONArrayIterate takes argument array (a JSON array ) and returns a List of JSON values . It returns a List containing all elements of array, so that it can be iterated by algorithms using "For each". It performs the following steps when called:

1. Let length be JSONObjectGet (array, "length" ).
2. Assert : length is a non-negative integral Number .
3. Let list be a new empty List .
4. Let i be 0.
5. Repeat, while i < ℝ (length),
   1. Let value be JSONObjectGet (array, ToString ( 𝔽 (i))).
   2. Assert : value is not missing .
   3. Append value to list.
   4. Set i to i + 1.
6. Return list.

The abstract operation StringSplit takes arguments string (a String) and separators (a List of String) and returns a List of Strings. 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. It performs the following steps when called:

1. Let parts be a new empty List .
2. Let strLen be the length of string.
3. Let lastStart be 0.
4. Let i be 0.
5. Repeat, while i < strLen,
   1. Let matched be false .
   2. For each String sep of separators, do
      1. Let sepLen be the length of sep.
      2. Let candidate be the substring of string from i to min (i + sepLen, strLen).
      3. If candidate = sep and matched is false , then
         1. Let chunk be the substring of string from lastStart to i.
         2. Append chunk to parts.
         3. Set lastStart to i + sepLen.
         4. Set i to i + sepLen.
         5. Set matched to true .
   3. If matched is false , set i to i + 1.
6. Let chunk be the substring of string from lastStart to strLen.
7. Append chunk to parts.
8. Return parts.

A Position Record is a tuple of a non-negative line and non-negative column number:

A Original Position Record 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.

The abstract operation ComparePositions takes arguments first (a Position Record or a Original Position Record ) and second (a Position Record or a Original Position Record ) and returns lesser , equal or greater . 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. It performs the following steps when called:

1. If first.[[Line]] < second.[[Line]], return lesser .
2. If first.[[Line]] > second.[[Line]], return greater .
3. Assert : first.[[Line]] is equal to second.[[Line]].
4. If first.[[Column]] < second.[[Column]], return lesser .
5. If first.[[Column]] > second.[[Column]], return greater .
6. Return equal .

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

{
  "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]
}

• The version field shall always be the number 3 as an integer . The source map may be rejected if the field has any other value.
• The file field 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.
• The sourceRoot field 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 .
• The sources field 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.
• The sourcesContent field 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 original sources should be retrieved by name.
• The names field is an optional list of symbol names which may be used by the mappings field .
• The mappings field is a string with the encoded mapping data (see section 9.2 ).
• The ignoreList field is an optional list of indices of files that should be considered third party code, such as framework code or bundler- generated code . 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.

A Decoded Source Map Record has the following fields:

A Decoded Source Record has the following fields:

The abstract operation ParseSourceMap takes arguments string (a String) and baseURL (an URL ) and returns a Decoded Source Map Record . It performs the following steps when called:

1. Let json be ParseJSON (string).
2. If json is not a JSON object , throw an error.
3. If JSONObjectGet (json, "sections" ) is not missing , then
   1. Return DecodeIndexSourceMap (json, baseURL).
4. Return DecodeSourceMap (json, baseURL).

The abstract operation DecodeSourceMap takes arguments json (a JSON object ) and baseURL (an URL ) and returns a Decoded Source Map Record . It performs the following steps when called:

1. If JSONObjectGet (json, "version" ) is not 3 _𝔽, optionally report an error .
2. Let mappingsField be JSONObjectGet (json, "mappings" ).
3. If mappingsField is not a String , throw an error.
4. If JSONObjectGet (json, "sources" ) is not a JSON array , throw an error.
5. Let fileField be GetOptionalString (json, "file" ).
6. Let sourceRootField be GetOptionalString (json, "sourceRoot" ).
7. Let sourcesField be GetOptionalListOfOptionalStrings (json, "sources" ).
8. Let sourcesContentField be GetOptionalListOfOptionalStrings (json, "sourcesContent" ).
9. Let ignoreListField be GetOptionalListOfArrayIndexes (json, "ignoreList" ).
10. Let sources be DecodeSourceMapSources (baseURL, sourceRootField, sourcesField, sourcesContentField, ignoreListField).
11. Let namesField be GetOptionalListOfStrings (json, "names" ).
12. Let mappings be DecodeMappings (mappingsField, namesField, sources).
13. Return the Decoded Source Map Record { [[File]]: fileField, [[Sources]]: sources, [[Mappings]]: mappings }.

The abstract operation GetOptionalString takes arguments object (a JSON object ) and key (a String) and returns a String or null . It performs the following steps when called:

1. Let value be JSONObjectGet (object, key).
2. If value is a String , return value.
3. If value is not missing , optionally report an error .
4. Return null .

The abstract operation GetOptionalListOfStrings takes arguments object (a JSON object ) and key (a String) and returns a List of Strings. It performs the following steps when called:

1. Let list be a new empty List .
2. Let values be JSONObjectGet (object, key).
3. If values is missing , return list.
4. If values is not a JSON array , then
   1. Optionally report an error .
   2. Return list.
5. For each element item of JSONArrayIterate (values), do
   1. If item is a String , then
      1. Append item to list.
   2. Else,
      1. Optionally report an error .
      2. Append "" to list.
6. Return list.

The abstract operation GetOptionalListOfOptionalStrings takes arguments object (a JSON object ) and key (a String) and returns a List of either Strings or null . It performs the following steps when called:

1. Let list be a new empty List .
2. Let values be JSONObjectGet (object, key).
3. If values is missing , return list.
4. If values is not a JSON array , then
   1. Optionally report an error .
   2. Return list.
5. For each element item of JSONArrayIterate (values), do
   1. If item is a String , then
      1. Append item to list.
   2. Else,
      1. If item ≠ null , optionally report an error .
      2. Append null to list.
6. Return list.

The abstract operation GetOptionalListOfArrayIndexes takes arguments object (an Object) and key (a String) and returns a List of non-negative integers . It performs the following steps when called:

1. Let list be a new empty List .
2. Let values be JSONObjectGet (object, key).
3. If values is missing , return list.
4. If values is not a JSON array , then
   1. Optionally report an error .
   2. Return list.
5. For each element item of JSONArrayIterate (values), do
   1. If item is an integral Number , item ≠ +0 _𝔽, and item ≥ +0 _𝔽, then
      1. Append ℝ (item) to list.
   2. Else,
      1. If item ≠ null , optionally report an error .
      2. Append null to list.
6. Return list.

The mappings field 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 from the subsequent fields below because the previous value is reset after every generated line.
2. 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.
3. 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.
4. 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.
5. 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.

The purpose of this encoding is to reduce the source map size. VLQ encoding reduced source maps by 50% relative to the

in tests performed using Google Calendar.

Segments with one field are intended to represent

that is unmapped because there is no corresponding

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.

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.

A Decoded Mapping Record has the following fields:

The mappings String must adhere to the following grammar:

A Decode Mapping State Record has the following fields:

The syntax-directed operation DecodeMappingsField takes arguments state (a Decode Mapping State Record ), mappings (a List of Decoded Mapping Records ), names (a List of Strings), and sources (a List of Decoded Source Records ). It is defined piecewise over the following productions:

1. Perform DecodeMappingsField of Line with arguments state, mappings, names and sources.
2. Set state.[[GeneratedLine]] to state.[[GeneratedLine]] + 1.
3. Set state.[[GeneratedColumn]] to 0.
4. Perform DecodeMappingsField of LineList with arguments state, mappings, names and sources.

1. Perform DecodeMappingsField of Mapping with arguments state, mappings, names and sources.
2. Perform DecodeMappingsField of MappingList with arguments state, mappings, names and sources.

1. Perform DecodeMappingsField of GeneratedColumn with arguments state, mappings, names and sources.
2. If state.[[GeneratedColumn]] < 0, then
   1. Optionally report an error .
   2. Return.
3. Let position be a new Position Record { [[Line]]: state.[[GeneratedLine]], [[Column]]: state.[[GeneratedColumn]] }.
4. Let decodedMapping be a new DecodedMappingRecord { [[GeneratedPosition]]: position, [[OriginalPosition]]: null , [[Name]]: null  }.
5. Append decodedMapping to mappings.

1. Perform DecodeMappingsField of GeneratedColumn with arguments state, mappings, names and sources.
2. If state.[[GeneratedColumn]] < 0, then
   1. Optionally report an error .
   2. Return.
3. Let generatedPosition be a new Position Record { [[Line]]: state.[[GeneratedLine]], [[Column]]: state.[[GeneratedColumn]] }.
4. Perform DecodeMappingsField of OriginalSource with arguments state, mappings, names and sources.
5. Perform DecodeMappingsField of OriginalLine with arguments state, mappings, names and sources.
6. Perform DecodeMappingsField of OriginalColumn with arguments state, mappings, names and sources.
7. 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 .
   2. Let originalPosition be null .
8. Else,
   1. Let originalPosition be a new Original Position Record { [[Source]]: sources[state.[[SourceIndex]]], [[Line]]: state.[[OriginalLine]], [[Column]]: state.[[OriginalColumn]] }.
9. Let name be null .
10. If Name is present, then
    1. Perform DecodeMappingsField of Name with arguments state, mappings, names and sources.
    2. If state.[[NameIndex]] < 0 or state.[[NameIndex]] ≥ the number of elements of names, optionally report an error .
    3. Else, set name to names[state.[[NameIndex]]].
11. Let decodedMapping be a new DecodedMappingRecord { [[GeneratedPosition]]: generatedPosition, [[OriginalPosition]]: originalPosition, [[Name]]: name }.
12. Append decodedMapping to mappings.

1. Let relativeColumn be the VLQSignedValue of Vlq .
2. Set state.[[GeneratedColumn]] to state.[[GeneratedColumn]] + relativeColumn.

1. Let relativeSourceIndex be the VLQSignedValue of Vlq .
2. Set state.[[SourceIndex]] to state.[[SourceIndex]] + relativeSourceIndex.

1. Let relativeLine be the VLQSignedValue of Vlq .
2. Set state.[[OriginalLine]] to state.[[OriginalLine]] + relativeLine.

1. Let relativeColumn be the VLQSignedValue of Vlq .
2. Set state.[[OriginalColumn]] to state.[[OriginalColumn]] + relativeColumn.

1. Let relativeName be the VLQSignedValue of Vlq .
2. Set state.[[NameIndex]] to state.[[NameIndex]] + relativeName.

The abstract operation DecodeMappings takes arguments rawMappings (a String), names (a List of Strings), and sources (a List of Decoded Source Records ) and returns a List of Decoded Mapping Record . It performs the following steps when called:

1. Let mappings be a new empty List .
2. Let mappingsNode be the root Parse Node when parsing rawMappings using MappingsField as the goal symbol .
3. If parsing failed, then
   1. Optionally report an error .
   2. Return mappings.
4. Let state be a new Decode Mapping State Record with all fields set to 0.
5. Perform DecodeMappingsField of mappingsNode with arguments state, mappings, names and sources.
6. Return mappings.

Generated code positions that may have mapping entries are defined in terms of input elements, as per the ECMAScript Lexical Grammar. Mapping entries shall point to either:

• the first code point of the source text matched by IdentifierName , PrivateIdentifier , Punctuator , DivPunctuator , RightBracePunctuator , NumericLiteral and RegularExpressionLiteral .
• any code point of the source text matched by Comment , HashbangComment , StringLiteral , Template , TemplateSubstitutionTail , WhiteSpace and LineTerminator .

Source map generators should create a mapping entry with a [[Name]] field for a JavaScript token, if:

• The original source language construct maps semantically to the generated JavaScript code.
• The original source language construct has a name.

Then the [[Name]] of the mapping entry should be the name of the original source language construct. A mapping with a non-null [[Name]] is called a named mapping.

A minifier renaming functions and variables or removing function names from immediately invoked function expressions.

The following enumeration lists productions of the ECMAScript Syntactic Grammar and the respective token or non-terminal (on the right-hand side of the production) for which source map generators should emit a named mapping. The mapping entry created for such tokens shall follow section 9.2.3 .

The enumeration should be understood as the "minimum". In general, source map generators are free to emit any additional named mappings.

The enumeration also lists tokens where generators "may" emit named mappings in addition to the tokens where they "should". These reflect the reality where existing tooling emits or expects named mappings. The duplicated named mapping is comparably cheap: Indices into names are encoded relative to each other so subsequent mappings to the same name are encoded as 0 (A).

• The BindingIdentifier (s) for LexicalDeclaration , VariableStatement and FormalParameterList .

• The BindingIdentifier for FunctionDeclaration , FunctionExpression , AsyncFunctionDeclaration , AsyncFunctionExpression , GeneratorDeclaration , GeneratorExpression , AsyncGeneratorDeclaration , and AsyncGeneratorExpression if it exists, or the opening parenthesis ( preceding the FormalParameters otherwise.

  Source map generators may chose to emit a named mapping on the opening parenthesis regardless of the presence of the BindingIdentifier .

• For an ArrowFunction or AsyncArrowFunction :

  • The => token where ArrowFunction is produced with a single BindingIdentifier for ArrowParameters or AsyncArrowFunction is produced with an AsyncArrowBindingIdentifier .

    Note 3

    This describes the case of (async) arrow functions with a single parameter, where that single parameter is not wrapped in parenthesis.

  • The opening parenthesis ( where ArrowFunction or AsyncArrowFunction is produced with ArrowFormalParameters .

    Source map generators may chose to additionally emit a named mapping on the => token for consistency with the previous case.

• The ClassElementName for MethodDefinition . This includes generators, async methods, async generators and accessors. For MethodDefinition where ClassElementName is "constructor" , the [[Name]] should be the original class name if applicable.

  Source map generators may chose to additionally emit a named mapping on the opening parenthesis (.

• Source map generators may emit named mapping for IdentifierReference in Expression .

If the sources are not absolute URLs after prepending the sourceRoot, the sources are resolved relative to the source map (like resolving the script src attribute in an HTML document).

The abstract operation DecodeSourceMapSources takes arguments baseURL (an URL ), sourceRoot (a String or null ), sources (a List of either Strings or null ), sourcesContent (a List of either Strings or null ), and ignoreList (a List of non-negative integers ) and returns a Decoded Source Record . It performs the following steps when called:

1. Let decodedSources be a new empty List .
2. Let sourcesContentCount be the the number of elements in sourcesContent.
3. Let sourceUrlPrefix be "" .
4. If sourceRoot ≠ null , then
   1. If sourceRoot ends with the code point U+002F (SOLIDUS), then
      1. Set sourceUrlPrefix to sourceRoot.
   2. Else,
      1. Set sourceUrlPrefix to the string-concatenation of sourceRoot and "/" .
5. Let index be 0.
6. Repeat, while index < sources' length,
   1. Let source be sources[index].
   2. Let decodedSource be the Decoded Source Record { [[URL]]: null , [[Content]]: null , [[Ignored]]: false  }.
   3. If source ≠ null , then
      1. Set source to the string-concatenation of sourceUrlPrefix and source.
      2. Let sourceURL be the result of URL parsing source with baseURL.
      3. If sourceURL is failure , optionally report an error .
      4. Else, set decodedSource.[[URL]] to sourceURL.
   4. If ignoreList contains index, set decodedSource.[[Ignored]] to true .
   5. If sourcesContentCount > index, set decodedSource.[[Content]] to sourcesContent[index].
   6. Append decodedSource to decodedSources.
7. Return decodedSources.

Implementations that support showing source contents but do not support showing multiple sources with the same

and different content will arbitrarily choose one of the various contents corresponding to the given

.

Source map consumers shall ignore any additional unrecognized properties, rather than causing the source map to be rejected, so that additional features can be added to this format without breaking existing users.

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

{
  "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 .

The sections field is an array of objects with the following fields:

• offset field is an object with two fields, line and column, that represent the offset into generated code that the referenced source map represents.
• map field is an embedded complete source map object. An embedded map does not inherit any values from the containing index map.

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

The abstract operation DecodeIndexSourceMap takes arguments json (an Object) and baseURL (an URL ) and returns a Decoded Source Map Record . It performs the following steps when called:

1. Let sectionsField be JSONObjectGet (json, "sections" ).
2. Assert : sectionsField is not missing .
3. If sectionsField is not a JSON array , throw an error.
4. If JSONObjectGet (json, "version" ) is not 3 _𝔽, optionally report an error .
5. Let fileField be GetOptionalString (json, "file" ).
6. Let sourceMap be the Decoded Source Map Record { [[File]]: fileField, [[Sources]]: « », [[Mappings]]: « » }.
7. Let previousOffsetPosition be null .
8. Let previousLastMapping be null .
9. For each JSON value section of JSONArrayIterate (sectionsField), do
   1. If section is not a JSON object , then
      1. Optionally report an error .
   2. Else,
      1. Let offset be JSONObjectGet (section, "offset" ).
      2. If offset is not a JSON object , throw an error.
      3. Let offsetLine be JSONObjectGet (offset, "line" ).
      4. Let offsetColumn be JSONObjectGet (offset, "column" ).
      5. If offsetLine is not an integral Number , then
         1. Optionally report an error .
         2. Set offsetLine to +0 _𝔽.
      6. If offsetColumn is not an integral Number , then
         1. Optionally report an error .
         2. Set offsetColumn to +0 _𝔽.
      7. Let offsetPosition be a new Position Record { [[Line]]: offsetLine, [[Column]]: offsetColumn }.
      8. If previousOffsetPosition ≠ null , then
         1. If ComparePositions (offsetPosition, previousOffsetPosition) is lesser , optionally report an error .
      9. If previousLastMapping ≠ null , then
         1. If ComparePositions (offsetPosition, previousLastMapping.[[GeneratedPosition]]) is lesser , optionally report an error .
         2. NOTE: This part of the decoding algorithm checks that entries of the sections field of index source maps are ordered and do not overlap. While it is expected that generators should not produce index source maps with overlapping sections, source map consumers may, for example, only check the simpler condition that the section offsets are ordered.
      10. Let mapField be JSONObjectGet (section, "map" ).
      11. If mapField is not a JSON object , throw an error.
      12. Let decodedSectionCompletion be Completion ( DecodeSourceMap (json, baseURL)).
      13. If decodedSectionCompletion is a throw completion , then
          1. Optionally report an error .
      14. Else,
          1. Let decodedSection be decodedSectionCompletion.[[Value]].
          2. For each Decoded Source Record additionalSource of decodedSection.[[Sources]], do
             1. If sourceMap.[[Sources]] does not contain additionalSource, then
                1. Append additionalSource to sourceMap.[[Sources]].
          3. Let offsetMappings be a new empty List .
          4. For each Decoded Mapping Record mapping of decodedSection.[[Mappings]], do
             1. If mapping.[[GeneratedPosition]].[[Line]] = 0, then
                1. Set mapping.[[GeneratedPosition]].[[Column]] to mapping.[[GeneratedPosition]].[[Column]] + offsetColumn.
             2. Set mapping.[[GeneratedPosition]].[[Line]] to mapping.[[GeneratedPosition]].[[Line]] + offsetLine.
             3. Append mapping to offsetMappings.
          5. Set sourceMap.[[Mappings]] to the list-concatenation of sourceMap.[[Mappings]] and offsetMappings.
          6. Set previousOffsetPosition to offsetPosition.
          7. Let sortedMappings be a copy of offsetMappings, sorted in ascending order, with a Decoded Mapping Record a being less than a Decoded Mapping Record b if ComparePositions (a.[[GeneratedPosition]], b.[[GeneratedPosition]]) is lesser .
          8. If sortedMappings is not empty, set previousLastMapping to the last element of sortedMappings.
   3. Return sourceMap.

Implementations may choose to represent index source map sections without appending the mappings together, for example, by storing each section separately and conducting a binary search.

While the source map format is intended to be language and platform agnostic, it is useful to define how to reference to them for the expected use-case of web server-hosted JavaScript.

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

Source maps are linked through URLs as defined in WHATWG URL ; in particular, characters outside the set permitted to appear in URIs shall 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.

The HTTP sourcemap header has precedence over a source annotation, and if both are present, the header URL should be used to resolve the source map file.

Regardless of the method used to retrieve the source map URL the same process is used to resolve it, which is as follows.

When the source map URL is not absolute, then it is relative to the generated code 's source origin. 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.

The generated code should include a comment, or the equivalent construct depending on its language or format, named sourceMappingURL and that contains the URL of the source map. This specification defines how the comment should look like for JavaScript, CSS, and WebAssembly. Other languages should follow a similar convention.

For a given language there can be multiple ways of detecting the sourceMappingURL comment, to allow for different implementations to choose what is less complex for them. The generated code unambiguously links to a source map if the result of all the extraction methods is the same.

If a tool consumes one or more source files that unambiguously links to a source map and it produces an output file that links to a source map, it shall do so unambiguously .

The following JavaScript code links to a source map, but it does not do so unambiguously :

let a = `
//# sourceMappingURL=foo.js.map
//`

Extracting a source map URL from it through parsing gives null , while without parsing gives foo.js.map.

Having multiple ways to extract a source map URL , that can lead to different results, can have negative security and privacy implications. Implementations that need to detect which source maps are potentially going to be loaded are strongly encouraged to always apply both algorithms, rather than just assuming that they will give the same result.

A fix to this problem is being worked on, and is expected to be included in a future version of the standard. It will likely involve early returning from the below algorithms whenever there is a comment (or comment-like) that contains the characters U+0060 (`), U+0022 ("), or U+0027 ('), or the the sequence U+002A U+002F (*/).

1. Let module be module_decode (bytes).
2. If module is WebAssembly error , return null .
3. For each custom section customSection of module, do
   1. Let name be the name of customSection.
   2. If CodePointsToString (name) is "sourceMappingURL" , then
      1. Let value be the bytes of customSection.
      2. Return CodePointsToString (value).
4. Return null .

Since WebAssembly is not a textual format and it does not support comments, it supports a single unambiguous extraction method. The URL is encoded as a WebAssembly name , and it's placed as the content of the custom section . It is invalid for tools that generate WebAssembly code to generate two or more custom sections with the sourceMappingURL name.

The abstract operation FetchSourceMap takes argument url (an URL ) and returns a Promise. It performs the following steps when called:

1. Let promiseCapability be NewPromiseCapability ( %Promise% ).
2. Let request be a new request whose request URL is url.
3. Let processResponseConsumeBody be a new Abstract Closure with parameters (response, bodyBytes) that captures promiseCapability and url, and performs the following steps when called:
   1. If bodyBytes is null or failure , then
      1. Perform Call (promiseCapability.[[Reject]], undefined , « a new TypeError  »).
      2. Return.
   2. If url's scheme is an HTTP(S) scheme and the byte sequence `)]}'` is a byte-sequence-prefix of bodyBytes, then
      1. Repeat, while bodyBytes's byte-sequence-length ≠ 0 and bodyBytes[0] is not an HTTP newline byte ,
         1. Remove the 0th element from bodyBytes.
   3. Let bodyString be Completion ( UTF-8 decode of bodyBytes).
   4. IfAbruptRejectPromise (bodyString, promiseCapability).
   5. Let jsonValue be Completion ( ParseJSON (bodyString)).
   6. IfAbruptRejectPromise (jsonValue, promiseCapability).
   7. Perform Call (promiseCapability.[[Resolve]], undefined , « jsonValue »).
4. Perform fetch request with processResponseConsumeBody set to processResponseConsumeBody.
5. Return promiseCapability.[[Promise]].

For historic reasons, when delivering source maps over HTTP(S), servers may prepend a line starting with the string )]}' to the source map.

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

is interpreted as

{"version": 3, ...}

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

Commonly, 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.

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

It is described in Give your eval a name with //@ sourceURL .

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

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.

This section lists all terms and algorithms used by this document defined by external specifications other than ECMA-262.

WebAssembly Core Specification <https://www.w3.org/TR/wasm-core-2/>

custom section, module_decode, WebAssembly error, WebAssembly names

WHATWG Encoding <https://encoding.spec.whatwg.org/>

UTF-8 decode

WHATWG Fetch <https://fetch.spec.whatwg.org/>

fetch, HTTP newline byte, processResponseConsumeBody, request, request URL

WHATWG Infra <https://infra.spec.whatwg.org/>

byte sequence, byte-sequence-prefix, byte-sequence-length,

WHATWG URL <https://url.spec.whatwg.org/>

HTTP(S) scheme, scheme, URL, URL parsing

1. IETF RFC 4648, The Base16, Base32, and Base64 Data Encodings, available at <https://datatracker.ietf.org/doc/html/rfc4648>
2. ECMA-262, ECMAScript® Language Specification, available at <https://tc39.es/ecma262/>
3. ECMA-404, The JSON Data Interchange Format, available at <https://www.ecma-international.org/publications-and-standards/standards/ecma-404/>
4. WebAssembly Core Specification, available at <https://www.w3.org/TR/wasm-core-2/>
5. WHATWG Encoding, available at <https://encoding.spec.whatwg.org/>
6. WHATWG Fetch , available at <https://fetch.spec.whatwg.org/>
7. WHATWG Infra, available at <https://infra.spec.whatwg.org/>
8. WHATWG URL , available at <https://url.spec.whatwg.org/>
9. Give your eval a name with //@ sourceURL, Firebug (2009), available at <http://blog.getfirebug.com/2009/08/11/give-your-eval-a-name-with-sourceurl/>
10. Source Map Revision 2 Proposal, John Lenz (2010), available at <https://docs.google.com/document/d/1xi12LrcqjqIHTtZzrzZKmQ3lbTv9mKrN076UB-j3UZQ/>
11. Variable-length quantity, Wikipedia, available at <https://en.wikipedia.org/wiki/Variable-length_quantity>

This specification is authored on GitHub in a plaintext source format called Ecmarkup. Ecmarkup is an HTML and Markdown dialect that provides a framework and toolset for authoring ECMAScript specifications in plaintext and processing the specification into a full-featured HTML rendering that follows the editorial conventions for this document. Ecmarkup builds on and integrates a number of other formats and technologies including Grammarkdown for defining syntax and Ecmarkdown for authoring algorithm steps. PDF renderings of this specification are produced by printing the HTML rendering to a PDF.

The first edition of this specification was authored using Bikeshed, a different plaintext source format based on HTML and Markdown.

Pre-standard versions of this document were authored using Google Docs.

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