# @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`][source-map] package's `SourceMapConsumer`. This
provides the same `originalPositionFor` and `generatedPositionFor` API, without requiring WASM.

## Installation

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

## Usage

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

const tracer = new TraceMap({
  version: 3,
  sources: ['input.js'],
  sourcesContent: ['content of 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 content = sourceContentFor(tracer, traced.source);
assert.strictEqual(content, 'content for input.js');

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`:

```typescript
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:

```typescript
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 - 45120 segments

Memory Usage:
trace-mapping decoded         562400 bytes
trace-mapping encoded        5706544 bytes
source-map-js               10717664 bytes
source-map-0.6.1            17446384 bytes
source-map-0.8.0             9701757 bytes
Smallest memory usage is trace-mapping decoded

Init speed:
trace-mapping:    decoded JSON input x 180 ops/sec ±0.34% (85 runs sampled)
trace-mapping:    encoded JSON input x 364 ops/sec ±1.77% (89 runs sampled)
trace-mapping:    decoded Object input x 3,116 ops/sec ±0.50% (96 runs sampled)
trace-mapping:    encoded Object input x 410 ops/sec ±2.62% (85 runs sampled)
source-map-js:    encoded Object input x 84.23 ops/sec ±0.91% (73 runs sampled)
source-map-0.6.1: encoded Object input x 37.21 ops/sec ±2.08% (51 runs sampled)
Fastest is trace-mapping:    decoded Object input

Trace speed:
trace-mapping:    decoded originalPositionFor x 3,952,212 ops/sec ±0.17% (98 runs sampled)
trace-mapping:    encoded originalPositionFor x 3,487,468 ops/sec ±1.58% (90 runs sampled)
source-map-js:    encoded originalPositionFor x 827,730 ops/sec ±0.78% (97 runs sampled)
source-map-0.6.1: encoded originalPositionFor x 748,991 ops/sec ±0.53% (94 runs sampled)
source-map-0.8.0: encoded originalPositionFor x 2,532,894 ops/sec ±0.57% (95 runs sampled)
Fastest is trace-mapping:    decoded originalPositionFor


***


babel.min.js.map - 347793 segments

Memory Usage:
trace-mapping decoded          89832 bytes
trace-mapping encoded       35474640 bytes
source-map-js               51257176 bytes
source-map-0.6.1            63515664 bytes
source-map-0.8.0            42933752 bytes
Smallest memory usage is trace-mapping decoded

Init speed:
trace-mapping:    decoded JSON input x 15.41 ops/sec ±8.65% (34 runs sampled)
trace-mapping:    encoded JSON input x 28.20 ops/sec ±12.87% (42 runs sampled)
trace-mapping:    decoded Object input x 964 ops/sec ±0.36% (99 runs sampled)
trace-mapping:    encoded Object input x 31.77 ops/sec ±13.79% (45 runs sampled)
source-map-js:    encoded Object input x 6.45 ops/sec ±5.16% (21 runs sampled)
source-map-0.6.1: encoded Object input x 4.07 ops/sec ±5.24% (15 runs sampled)
Fastest is trace-mapping:    decoded Object input

Trace speed:
trace-mapping:    decoded originalPositionFor x 7,183,038 ops/sec ±0.58% (95 runs sampled)
trace-mapping:    encoded originalPositionFor x 5,192,185 ops/sec ±0.41% (100 runs sampled)
source-map-js:    encoded originalPositionFor x 4,259,489 ops/sec ±0.79% (94 runs sampled)
source-map-0.6.1: encoded originalPositionFor x 3,742,629 ops/sec ±0.71% (95 runs sampled)
source-map-0.8.0: encoded originalPositionFor x 6,270,211 ops/sec ±0.64% (94 runs sampled)
Fastest is trace-mapping:    decoded originalPositionFor


***


preact.js.map - 1992 segments

Memory Usage:
trace-mapping decoded          37128 bytes
trace-mapping encoded         247280 bytes
source-map-js                1143536 bytes
source-map-0.6.1             1290992 bytes
source-map-0.8.0               96544 bytes
Smallest memory usage is trace-mapping decoded

Init speed:
trace-mapping:    decoded JSON input x 3,483 ops/sec ±0.30% (98 runs sampled)
trace-mapping:    encoded JSON input x 6,092 ops/sec ±0.18% (97 runs sampled)
trace-mapping:    decoded Object input x 249,076 ops/sec ±0.24% (98 runs sampled)
trace-mapping:    encoded Object input x 14,555 ops/sec ±0.48% (100 runs sampled)
source-map-js:    encoded Object input x 2,447 ops/sec ±0.36% (99 runs sampled)
source-map-0.6.1: encoded Object input x 1,201 ops/sec ±0.57% (96 runs sampled)
Fastest is trace-mapping:    decoded Object input

Trace speed:
trace-mapping:    decoded originalPositionFor x 7,620,192 ops/sec ±0.09% (99 runs sampled)
trace-mapping:    encoded originalPositionFor x 6,872,554 ops/sec ±0.30% (97 runs sampled)
source-map-js:    encoded originalPositionFor x 2,489,570 ops/sec ±0.35% (94 runs sampled)
source-map-0.6.1: encoded originalPositionFor x 1,698,633 ops/sec ±0.28% (98 runs sampled)
source-map-0.8.0: encoded originalPositionFor x 4,015,644 ops/sec ±0.22% (98 runs sampled)
Fastest is trace-mapping:    decoded originalPositionFor


***


react.js.map - 5726 segments

Memory Usage:
trace-mapping decoded          16176 bytes
trace-mapping encoded         681552 bytes
source-map-js                2418352 bytes
source-map-0.6.1             2443672 bytes
source-map-0.8.0              111768 bytes
Smallest memory usage is trace-mapping decoded

Init speed:
trace-mapping:    decoded JSON input x 1,720 ops/sec ±0.34% (98 runs sampled)
trace-mapping:    encoded JSON input x 4,406 ops/sec ±0.35% (100 runs sampled)
trace-mapping:    decoded Object input x 92,122 ops/sec ±0.10% (99 runs sampled)
trace-mapping:    encoded Object input x 5,385 ops/sec ±0.37% (99 runs sampled)
source-map-js:    encoded Object input x 794 ops/sec ±0.40% (98 runs sampled)
source-map-0.6.1: encoded Object input x 416 ops/sec ±0.54% (91 runs sampled)
Fastest is trace-mapping:    decoded Object input

Trace speed:
trace-mapping:    decoded originalPositionFor x 32,759,519 ops/sec ±0.33% (100 runs sampled)
trace-mapping:    encoded originalPositionFor x 31,116,306 ops/sec ±0.33% (97 runs sampled)
source-map-js:    encoded originalPositionFor x 17,458,435 ops/sec ±0.44% (97 runs sampled)
source-map-0.6.1: encoded originalPositionFor x 12,687,097 ops/sec ±0.43% (95 runs sampled)
source-map-0.8.0: encoded originalPositionFor x 23,538,275 ops/sec ±0.38% (95 runs sampled)
Fastest is trace-mapping:    decoded originalPositionFor
```

[source-map]: https://www.npmjs.com/package/source-map