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# Source Map JS [](https://www.npmjs.com/package/source-map-js) Difference between original [source-map](https://github.com/mozilla/source-map): > TL,DR: it's fork of original source-map@0.6, but with perfomance optimizations. This journey starts from [source-map@0.7.0](https://github.com/mozilla/source-map/blob/master/CHANGELOG.md#070). Some part of it was rewritten to Rust and WASM and API became async. It's still a major block for many libraries like PostCSS or Sass for example because they need to migrate the whole API to the async way. This is the reason why 0.6.1 has 2x more downloads than 0.7.3 while it's faster several times.  More important that WASM version has some optimizations in JS code too. This is why [community asked to create branch for 0.6 version](https://github.com/mozilla/source-map/issues/324) and port these optimizations but, sadly, the answer was «no». A bit later I discovered [the issue](https://github.com/mozilla/source-map/issues/370) created by [Ben Rothman (@benthemonkey)](https://github.com/benthemonkey) with no response at all. [Roman Dvornov (@lahmatiy)](https://github.com/lahmatiy) wrote a [serveral posts](https://t.me/gorshochekvarit/76) (russian, only, sorry) about source-map library in his own Telegram channel. He mentioned the article [«Maybe you don't need Rust and WASM to speed up your JS»](https://mrale.ph/blog/2018/02/03/maybe-you-dont-need-rust-to-speed-up-your-js.html) written by [Vyacheslav Egorov (@mraleph)](https://github.com/mraleph). This article contains optimizations and hacks that lead to almost the same performance compare to WASM implementation. I decided to fork the original source-map and port these optimizations from the article and several others PR from the original source-map. --------- This is a library to generate and consume the source map format [described here][format]. [format]: https://docs.google.com/document/d/1U1RGAehQwRypUTovF1KRlpiOFze0b-_2gc6fAH0KY0k/edit ## Use with Node $ npm install source-map-js <!-- ## Use on the Web <script src="https://raw.githubusercontent.com/mozilla/source-map/master/dist/source-map.min.js" defer></script> --> -------------------------------------------------------------------------------- <!-- `npm run toc` to regenerate the Table of Contents --> <!-- START doctoc generated TOC please keep comment here to allow auto update --> <!-- DON'T EDIT THIS SECTION, INSTEAD RE-RUN doctoc TO UPDATE --> ## Table of Contents - [Examples](#examples) - [Consuming a source map](#consuming-a-source-map) - [Generating a source map](#generating-a-source-map) - [With SourceNode (high level API)](#with-sourcenode-high-level-api) - [With SourceMapGenerator (low level API)](#with-sourcemapgenerator-low-level-api) - [API](#api) - [SourceMapConsumer](#sourcemapconsumer) - [new SourceMapConsumer(rawSourceMap)](#new-sourcemapconsumerrawsourcemap) - [SourceMapConsumer.prototype.computeColumnSpans()](#sourcemapconsumerprototypecomputecolumnspans) - [SourceMapConsumer.prototype.originalPositionFor(generatedPosition)](#sourcemapconsumerprototypeoriginalpositionforgeneratedposition) - [SourceMapConsumer.prototype.generatedPositionFor(originalPosition)](#sourcemapconsumerprototypegeneratedpositionfororiginalposition) - [SourceMapConsumer.prototype.allGeneratedPositionsFor(originalPosition)](#sourcemapconsumerprototypeallgeneratedpositionsfororiginalposition) - [SourceMapConsumer.prototype.hasContentsOfAllSources()](#sourcemapconsumerprototypehascontentsofallsources) - [SourceMapConsumer.prototype.sourceContentFor(source[, returnNullOnMissing])](#sourcemapconsumerprototypesourcecontentforsource-returnnullonmissing) - [SourceMapConsumer.prototype.eachMapping(callback, context, order)](#sourcemapconsumerprototypeeachmappingcallback-context-order) - [SourceMapGenerator](#sourcemapgenerator) - [new SourceMapGenerator([startOfSourceMap])](#new-sourcemapgeneratorstartofsourcemap) - [SourceMapGenerator.fromSourceMap(sourceMapConsumer)](#sourcemapgeneratorfromsourcemapsourcemapconsumer) - [SourceMapGenerator.prototype.addMapping(mapping)](#sourcemapgeneratorprototypeaddmappingmapping) - [SourceMapGenerator.prototype.setSourceContent(sourceFile, sourceContent)](#sourcemapgeneratorprototypesetsourcecontentsourcefile-sourcecontent) - [SourceMapGenerator.prototype.applySourceMap(sourceMapConsumer[, sourceFile[, sourceMapPath]])](#sourcemapgeneratorprototypeapplysourcemapsourcemapconsumer-sourcefile-sourcemappath) - [SourceMapGenerator.prototype.toString()](#sourcemapgeneratorprototypetostring) - [SourceNode](#sourcenode) - [new SourceNode([line, column, source[, chunk[, name]]])](#new-sourcenodeline-column-source-chunk-name) - [SourceNode.fromStringWithSourceMap(code, sourceMapConsumer[, relativePath])](#sourcenodefromstringwithsourcemapcode-sourcemapconsumer-relativepath) - [SourceNode.prototype.add(chunk)](#sourcenodeprototypeaddchunk) - [SourceNode.prototype.prepend(chunk)](#sourcenodeprototypeprependchunk) - [SourceNode.prototype.setSourceContent(sourceFile, sourceContent)](#sourcenodeprototypesetsourcecontentsourcefile-sourcecontent) - [SourceNode.prototype.walk(fn)](#sourcenodeprototypewalkfn) - [SourceNode.prototype.walkSourceContents(fn)](#sourcenodeprototypewalksourcecontentsfn) - [SourceNode.prototype.join(sep)](#sourcenodeprototypejoinsep) - [SourceNode.prototype.replaceRight(pattern, replacement)](#sourcenodeprototypereplacerightpattern-replacement) - [SourceNode.prototype.toString()](#sourcenodeprototypetostring) - [SourceNode.prototype.toStringWithSourceMap([startOfSourceMap])](#sourcenodeprototypetostringwithsourcemapstartofsourcemap) <!-- END doctoc generated TOC please keep comment here to allow auto update --> ## Examples ### Consuming a source map ```js var rawSourceMap = { version: 3, file: 'min.js', names: ['bar', 'baz', 'n'], sources: ['one.js', 'two.js'], sourceRoot: 'http://example.com/www/js/', mappings: 'CAAC,IAAI,IAAM,SAAUA,GAClB,OAAOC,IAAID;CCDb,IAAI,IAAM,SAAUE,GAClB,OAAOA' }; var smc = new SourceMapConsumer(rawSourceMap); console.log(smc.sources); // [ 'http://example.com/www/js/one.js', // 'http://example.com/www/js/two.js' ] console.log(smc.originalPositionFor({ line: 2, column: 28 })); // { source: 'http://example.com/www/js/two.js', // line: 2, // column: 10, // name: 'n' } console.log(smc.generatedPositionFor({ source: 'http://example.com/www/js/two.js', line: 2, column: 10 })); // { line: 2, column: 28 } smc.eachMapping(function (m) { // ... }); ``` ### Generating a source map In depth guide: [**Compiling to JavaScript, and Debugging with Source Maps**](https://hacks.mozilla.org/2013/05/compiling-to-javascript-and-debugging-with-source-maps/) #### With SourceNode (high level API) ```js function compile(ast) { switch (ast.type) { case 'BinaryExpression': return new SourceNode( ast.location.line, ast.location.column, ast.location.source, [compile(ast.left), " + ", compile(ast.right)] ); case 'Literal': return new SourceNode( ast.location.line, ast.location.column, ast.location.source, String(ast.value) ); // ... default: throw new Error("Bad AST"); } } var ast = parse("40 + 2", "add.js"); console.log(compile(ast).toStringWithSourceMap({ file: 'add.js' })); // { code: '40 + 2', // map: [object SourceMapGenerator] } ``` #### With SourceMapGenerator (low level API) ```js var map = new SourceMapGenerator({ file: "source-mapped.js" }); map.addMapping({ generated: { line: 10, column: 35 }, source: "foo.js", original: { line: 33, column: 2 }, name: "christopher" }); console.log(map.toString()); // '{"version":3,"file":"source-mapped.js","sources"