@jridgewell/gen-mapping

What is @jridgewell/gen-mapping?

The @jridgewell/gen-mapping package is a library for generating source maps, which are used to map the transformed source to the original source, enabling developers to debug their code more easily after it has been compiled or minified. It provides a simple API for adding mappings and generating the source map.

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

Creating a new source map generator

This feature allows you to create a new source map generator instance, specifying the output file name for the source map.

{"const { GenMapping } = require('@jridgewell/gen-mapping');

const map = new GenMapping({
  file: 'minified.js'
});
}

Adding mappings to the source map

This feature allows you to add individual mappings to the source map, linking a location in the generated file to a location in the original source file.

{"const { addMapping } = require('@jridgewell/gen-mapping');

addMapping(map, {
  generated: { line: 1, column: 5 },
  source: 'original.js',
  original: { line: 1, column: 30 }
});
}

Generating the source map

This feature generates the final source map from the added mappings, which can then be used by browsers or other tools to map the transformed source back to the original source.

{"const { toMapping } = require('@jridgewell/gen-mapping');

const sourceMap = toMapping(map);
}

Other packages similar to @jridgewell/gen-mapping

source-map

The 'source-map' package provides similar functionality for generating source maps. It offers a more comprehensive API for manipulating source maps, including parsing and merging existing source maps, which @jridgewell/gen-mapping does not provide.

source-map-js

The 'source-map-js' package is a fork of the 'source-map' library that focuses on performance improvements and bug fixes. It is similar to @jridgewell/gen-mapping in that it generates source maps, but it also includes features for parsing and manipulating source maps.

magic-string

The 'magic-string' package is designed for manipulating strings and generating source maps in the process. It is different from @jridgewell/gen-mapping in that it provides a higher-level API for string manipulation, which includes source map generation as a byproduct of those manipulations.

README

@jridgewell/gen-mapping

Generate source maps

gen-mapping allows you to generate a source map during transpilation or minification. With a source map, you're able to trace the original location in the source file, either in Chrome's DevTools or using a library like @jridgewell/trace-mapping.

You may already be familiar with the source-map package's SourceMapGenerator. This provides the same addMapping and setSourceContent API.

Installation

npm install @jridgewell/gen-mapping

Usage

import { GenMapping, addMapping, setSourceContent, toEncodedMap, toDecodedMap } from '@jridgewell/gen-mapping';

const map = new GenMapping({
  file: 'output.js',
  sourceRoot: 'https://example.com/',
});

setSourceContent(map, 'input.js', `function foo() {}`);

addMapping(map, {
  // Lines start at line 1, columns at column 0.
  generated: { line: 1, column: 0 },
  source: 'input.js',
  original: { line: 1, column: 0 },
});

addMapping(map, {
  generated: { line: 1, column: 9 },
  source: 'input.js',
  original: { line: 1, column: 9 },
  name: 'foo',
});

assert.deepEqual(toDecodedMap(map), {
  version: 3,
  file: 'output.js',
  names: ['foo'],
  sourceRoot: 'https://example.com/',
  sources: ['input.js'],
  sourcesContent: ['function foo() {}'],
  mappings: [
    [ [0, 0, 0, 0], [9, 0, 0, 9, 0] ]
  ],
});

assert.deepEqual(toEncodedMap(map), {
  version: 3,
  file: 'output.js',
  names: ['foo'],
  sourceRoot: 'https://example.com/',
  sources: ['input.js'],
  sourcesContent: ['function foo() {}'],
  mappings: 'AAAA,SAASA',
});

Smaller Sourcemaps

Not everything needs to be added to a sourcemap, and needless markings can cause signficantly larger file sizes. gen-mapping exposes maybeAddSegment/maybeAddMapping APIs that will intelligently determine if this marking adds useful information. If not, the marking will be skipped.

import { maybeAddMapping } from '@jridgewell/gen-mapping';

const map = new GenMapping();

// Adding a sourceless marking at the beginning of a line isn't useful.
maybeAddMapping(map, {
  generated: { line: 1, column: 0 },
});

// Adding a new source marking is useful.
maybeAddMapping(map, {
  generated: { line: 1, column: 0 },
  source: 'input.js',
  original: { line: 1, column: 0 },
});

// But adding another marking pointing to the exact same original location isn't, even if the
// generated column changed.
maybeAddMapping(map, {
  generated: { line: 1, column: 9 },
  source: 'input.js',
  original: { line: 1, column: 0 },
});

assert.deepEqual(toEncodedMap(map), {
  version: 3,
  names: [],
  sources: ['input.js'],
  sourcesContent: [null],
  mappings: 'AAAA',
});

Benchmarks

node v18.0.0

amp.js.map
Memory Usage:
gen-mapping: addSegment      5852872 bytes
gen-mapping: addMapping      7716042 bytes
source-map-js                6143250 bytes
source-map-0.6.1             6124102 bytes
source-map-0.8.0             6121173 bytes
Smallest memory usage is gen-mapping: addSegment

Adding speed:
gen-mapping:      addSegment x 441 ops/sec ±2.07% (90 runs sampled)
gen-mapping:      addMapping x 350 ops/sec ±2.40% (86 runs sampled)
source-map-js:    addMapping x 169 ops/sec ±2.42% (80 runs sampled)
source-map-0.6.1: addMapping x 167 ops/sec ±2.56% (80 runs sampled)
source-map-0.8.0: addMapping x 168 ops/sec ±2.52% (80 runs sampled)
Fastest is gen-mapping:      addSegment

Generate speed:
gen-mapping:      decoded output x 150,824,370 ops/sec ±0.07% (102 runs sampled)
gen-mapping:      encoded output x 663 ops/sec ±0.22% (98 runs sampled)
source-map-js:    encoded output x 197 ops/sec ±0.45% (84 runs sampled)
source-map-0.6.1: encoded output x 198 ops/sec ±0.33% (85 runs sampled)
source-map-0.8.0: encoded output x 197 ops/sec ±0.06% (93 runs sampled)
Fastest is gen-mapping:      decoded output


***


babel.min.js.map
Memory Usage:
gen-mapping: addSegment     37578063 bytes
gen-mapping: addMapping     37212897 bytes
source-map-js               47638527 bytes
source-map-0.6.1            47690503 bytes
source-map-0.8.0            47470188 bytes
Smallest memory usage is gen-mapping: addMapping

Adding speed:
gen-mapping:      addSegment x 31.05 ops/sec ±8.31% (43 runs sampled)
gen-mapping:      addMapping x 29.83 ops/sec ±7.36% (51 runs sampled)
source-map-js:    addMapping x 20.73 ops/sec ±6.22% (38 runs sampled)
source-map-0.6.1: addMapping x 20.03 ops/sec ±10.51% (38 runs sampled)
source-map-0.8.0: addMapping x 19.30 ops/sec ±8.27% (37 runs sampled)
Fastest is gen-mapping:      addSegment

Generate speed:
gen-mapping:      decoded output x 381,379,234 ops/sec ±0.29% (96 runs sampled)
gen-mapping:      encoded output x 95.15 ops/sec ±2.98% (72 runs sampled)
source-map-js:    encoded output x 15.20 ops/sec ±7.41% (33 runs sampled)
source-map-0.6.1: encoded output x 16.36 ops/sec ±10.46% (31 runs sampled)
source-map-0.8.0: encoded output x 16.06 ops/sec ±6.45% (31 runs sampled)
Fastest is gen-mapping:      decoded output


***


preact.js.map
Memory Usage:
gen-mapping: addSegment       416247 bytes
gen-mapping: addMapping       419824 bytes
source-map-js                1024619 bytes
source-map-0.6.1             1146004 bytes
source-map-0.8.0             1113250 bytes
Smallest memory usage is gen-mapping: addSegment

Adding speed:
gen-mapping:      addSegment x 13,755 ops/sec ±0.15% (98 runs sampled)
gen-mapping:      addMapping x 13,013 ops/sec ±0.11% (101 runs sampled)
source-map-js:    addMapping x 4,564 ops/sec ±0.21% (98 runs sampled)
source-map-0.6.1: addMapping x 4,562 ops/sec ±0.11% (99 runs sampled)
source-map-0.8.0: addMapping x 4,593 ops/sec ±0.11% (100 runs sampled)
Fastest is gen-mapping:      addSegment

Generate speed:
gen-mapping:      decoded output x 379,864,020 ops/sec ±0.23% (93 runs sampled)
gen-mapping:      encoded output x 14,368 ops/sec ±4.07% (82 runs sampled)
source-map-js:    encoded output x 5,261 ops/sec ±0.21% (99 runs sampled)
source-map-0.6.1: encoded output x 5,124 ops/sec ±0.58% (99 runs sampled)
source-map-0.8.0: encoded output x 5,434 ops/sec ±0.33% (96 runs sampled)
Fastest is gen-mapping:      decoded output


***


react.js.map
Memory Usage:
gen-mapping: addSegment       975096 bytes
gen-mapping: addMapping      1102981 bytes
source-map-js                2918836 bytes
source-map-0.6.1             2885435 bytes
source-map-0.8.0             2874336 bytes
Smallest memory usage is gen-mapping: addSegment

Adding speed:
gen-mapping:      addSegment x 4,772 ops/sec ±0.15% (100 runs sampled)
gen-mapping:      addMapping x 4,456 ops/sec ±0.13% (97 runs sampled)
source-map-js:    addMapping x 1,618 ops/sec ±0.24% (97 runs sampled)
source-map-0.6.1: addMapping x 1,622 ops/sec ±0.12% (99 runs sampled)
source-map-0.8.0: addMapping x 1,631 ops/sec ±0.12% (100 runs sampled)
Fastest is gen-mapping:      addSegment

Generate speed:
gen-mapping:      decoded output x 379,107,695 ops/sec ±0.07% (99 runs sampled)
gen-mapping:      encoded output x 5,421 ops/sec ±1.60% (89 runs sampled)
source-map-js:    encoded output x 2,113 ops/sec ±1.81% (98 runs sampled)
source-map-0.6.1: encoded output x 2,126 ops/sec ±0.10% (100 runs sampled)
source-map-0.8.0: encoded output x 2,176 ops/sec ±0.39% (98 runs sampled)
Fastest is gen-mapping:      decoded output