# @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
![[ICO]](/icons/blank.gif){kind=icon}
![[PARENTDIR]](/icons/back.gif){kind=icon}
![[DIR]](/icons/folder.gif){kind=icon}
![[ ]](/icons/unknown.gif){kind=icon}
![[TXT]](/icons/text.gif){kind=icon}