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@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]);

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