The @jridgewell/trace-mapping package is designed to work with source maps, which are files that provide a mapping between the transformed, bundled, or minified code and the original source files. This package allows users to trace the source location of a piece of code through a source map, which is particularly useful for debugging purposes when working with compiled or minified JavaScript.

What are @jridgewell/trace-mapping's main functionalities?

Trace Source Location

This feature allows you to trace the original source location of a specific line and column in the transformed file. You can use the source map to find out where in the original source code a piece of the transformed code came from.

{"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,MAAM,K"}

Generate Source Map

This feature allows you to generate a new source map by adding mappings between the original and the transformed code. This is useful when you are creating a new tool that transforms code and you want to provide source maps for debugging.

const { SourceMapGenerator } = require('@jridgewell/trace-mapping');
const generator = new SourceMapGenerator({ file: 'min.js' });
generator.addMapping({
  generated: { line: 1, column: 0 },
  source: 'one.js',
  original: { line: 1, column: 0 }
});
const map = generator.toJSON();

Other packages similar to @jridgewell/trace-mapping

Readme

Source

@jridgewell/trace-mapping

Trace the original position through a source map

trace-mapping allows you to take the line and column of an output file and trace it to the original location in the source file through a source map.

You may already be familiar with the source-map package's SourceMapConsumer. This provides the same originalPositionFor and generatedPositionFor API, without requiring WASM.

Installation

npm install @jridgewell/trace-mapping

Usage

import { TraceMap, originalPositionFor, generatedPositionFor } from '@jridgewell/trace-mapping';

const tracer = new TraceMap({
  version: 3,
  sources: ['input.js'],
  names: ['foo'],
  mappings: 'KAyCIA',
});

// Lines start at line 1, columns at column 0.
const traced = originalPositionFor(tracer, { line: 1, column: 5 });
assert.deepEqual(traced, {
  source: 'input.js',
  line: 42,
  column: 4,
  name: 'foo',
});

const generated = generatedPositionFor(tracer, {
  source: 'input.js',
  line: 42,
  column: 4,
});
assert.deepEqual(generated, {
  line: 1,
  column: 5,
});

We also provide a lower level API to get the actual segment that matches our line and column. Unlike originalPositionFor, traceSegment uses a 0-base for line:

import { traceSegment } from '@jridgewell/trace-mapping';

// line is 0-base.
const traced = traceSegment(tracer, /* line */ 0, /* column */ 5);

// Segments are [outputColumn, sourcesIndex, sourceLine, sourceColumn, namesIndex]
// Again, line is 0-base and so is sourceLine
assert.deepEqual(traced, [5, 0, 41, 4, 0]);

SectionedSourceMaps

The sourcemap spec defines a special sections field that's designed to handle concatenation of output code with associated sourcemaps. This type of sourcemap is rarely used (no major build tool produces it), but if you are hand coding a concatenation you may need it. We provide an AnyMap helper that can receive either a regular sourcemap or a SectionedSourceMap and returns a TraceMap instance:

import { AnyMap } from '@jridgewell/trace-mapping';
const fooOutput = 'foo';
const barOutput = 'bar';
const output = [fooOutput, barOutput].join('\n');

const sectioned = new AnyMap({
  version: 3,
  sections: [
    {
      // 0-base line and column
      offset: { line: 0, column: 0 },
      // fooOutput's sourcemap
      map: {
        version: 3,
        sources: ['foo.js'],
        names: ['foo'],
        mappings: 'AAAAA',
      },
    },
    {
      // barOutput's sourcemap will not affect the first line, only the second
      offset: { line: 1, column: 0 },
      map: {
        version: 3,
        sources: ['bar.js'],
        names: ['bar'],
        mappings: 'AAAAA',
      },
    },
  ],
});

const traced = originalPositionFor(sectioned, {
  line: 2,
  column: 0,
});

assert.deepEqual(traced, {
  source: 'bar.js',
  line: 1,
  column: 0,
  name: 'bar',
});

Benchmarks

node v18.0.0

amp.js.map
trace-mapping:    decoded JSON input x 183 ops/sec ±0.41% (87 runs sampled)
trace-mapping:    encoded JSON input x 384 ops/sec ±0.89% (89 runs sampled)
trace-mapping:    decoded Object input x 3,085 ops/sec ±0.24% (100 runs sampled)
trace-mapping:    encoded Object input x 452 ops/sec ±0.80% (84 runs sampled)
source-map-js:    encoded Object input x 88.82 ops/sec ±0.45% (77 runs sampled)
source-map-0.6.1: encoded Object input x 38.39 ops/sec ±1.88% (52 runs sampled)
Fastest is trace-mapping:    decoded Object input

trace-mapping:    decoded originalPositionFor x 4,025,347 ops/sec ±0.15% (97 runs sampled)
trace-mapping:    encoded originalPositionFor x 3,333,136 ops/sec ±1.26% (90 runs sampled)
source-map-js:    encoded originalPositionFor x 824,978 ops/sec ±1.06% (94 runs sampled)
source-map-0.6.1: encoded originalPositionFor x 741,300 ops/sec ±0.93% (92 runs sampled)
source-map-0.8.0: encoded originalPositionFor x 2,587,603 ops/sec ±0.75% (97 runs sampled)
Fastest is trace-mapping:    decoded originalPositionFor

***

babel.min.js.map
trace-mapping:    decoded JSON input x 17.43 ops/sec ±8.81% (33 runs sampled)
trace-mapping:    encoded JSON input x 34.18 ops/sec ±4.67% (50 runs sampled)
trace-mapping:    decoded Object input x 1,010 ops/sec ±0.41% (98 runs sampled)
trace-mapping:    encoded Object input x 39.45 ops/sec ±4.01% (52 runs sampled)
source-map-js:    encoded Object input x 6.57 ops/sec ±3.04% (21 runs sampled)
source-map-0.6.1: encoded Object input x 4.23 ops/sec ±2.93% (15 runs sampled)
Fastest is trace-mapping:    decoded Object input

trace-mapping:    decoded originalPositionFor x 7,576,265 ops/sec ±0.74% (96 runs sampled)
trace-mapping:    encoded originalPositionFor x 5,019,743 ops/sec ±0.74% (94 runs sampled)
source-map-js:    encoded originalPositionFor x 3,396,137 ops/sec ±42.32% (95 runs sampled)
source-map-0.6.1: encoded originalPositionFor x 3,753,176 ops/sec ±0.72% (95 runs sampled)
source-map-0.8.0: encoded originalPositionFor x 6,423,633 ops/sec ±0.74% (95 runs sampled)
Fastest is trace-mapping:    decoded originalPositionFor

***

preact.js.map
trace-mapping:    decoded JSON input x 3,499 ops/sec ±0.18% (98 runs sampled)
trace-mapping:    encoded JSON input x 6,078 ops/sec ±0.25% (99 runs sampled)
trace-mapping:    decoded Object input x 254,788 ops/sec ±0.13% (100 runs sampled)
trace-mapping:    encoded Object input x 14,063 ops/sec ±0.27% (94 runs sampled)
source-map-js:    encoded Object input x 2,465 ops/sec ±0.25% (98 runs sampled)
source-map-0.6.1: encoded Object input x 1,174 ops/sec ±1.90% (95 runs sampled)
Fastest is trace-mapping:    decoded Object input

trace-mapping:    decoded originalPositionFor x 7,720,171 ops/sec ±0.14% (97 runs sampled)
trace-mapping:    encoded originalPositionFor x 6,864,485 ops/sec ±0.16% (101 runs sampled)
source-map-js:    encoded originalPositionFor x 2,387,219 ops/sec ±0.28% (98 runs sampled)
source-map-0.6.1: encoded originalPositionFor x 1,565,339 ops/sec ±0.32% (101 runs sampled)
source-map-0.8.0: encoded originalPositionFor x 3,819,732 ops/sec ±0.38% (98 runs sampled)
Fastest is trace-mapping:    decoded originalPositionFor

***

react.js.map
trace-mapping:    decoded JSON input x 1,719 ops/sec ±0.19% (99 runs sampled)
trace-mapping:    encoded JSON input x 4,284 ops/sec ±0.51% (99 runs sampled)
trace-mapping:    decoded Object input x 94,668 ops/sec ±0.08% (99 runs sampled)
trace-mapping:    encoded Object input x 5,287 ops/sec ±0.24% (99 runs sampled)
source-map-js:    encoded Object input x 814 ops/sec ±0.20% (98 runs sampled)
source-map-0.6.1: encoded Object input x 429 ops/sec ±0.24% (94 runs sampled)
Fastest is trace-mapping:    decoded Object input

trace-mapping:    decoded originalPositionFor x 28,927,989 ops/sec ±0.61% (94 runs sampled)
trace-mapping:    encoded originalPositionFor x 27,394,475 ops/sec ±0.55% (97 runs sampled)
source-map-js:    encoded originalPositionFor x 16,856,730 ops/sec ±0.45% (96 runs sampled)
source-map-0.6.1: encoded originalPositionFor x 12,258,950 ops/sec ±0.41% (97 runs sampled)
source-map-0.8.0: encoded originalPositionFor x 22,272,990 ops/sec ±0.58% (95 runs sampled)
Fastest is trace-mapping:    decoded originalPositionFor

Keywords

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Last updated on 20 Apr 2022

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