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Editing: README.md

# @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`][trace-mapping].

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

## Installation

```sh
npm install @jridgewell/gen-mapping
```

## Usage

```typescript
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.

```typescript
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
```

[source-map]: https://www.npmjs.com/package/source-map
[trace-mapping]: https://github.com/jridgewell/trace-mapping

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