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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`][source-map] package's `SourceMapConsumer`. This
provides the same `originalPositionFor` API, without requires WASM.
## Installation
```sh
npm install @jridgewell/trace-mapping
```
## Usage
```typescript
import { TraceMap, originalPositionFor } 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',
});
```
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 { originalPositionFor } 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
`trace-mapping` is the fastest source map tracing library, by a factor of 4-10x when
constructing/parsing source maps and another 6-10x when using `originalPositionFor` on an already
constructed instance.
```
node v16.13.2
trace-mapping: decoded JSON input x 7,224 ops/sec ±0.24% (99 runs sampled)
trace-mapping: encoded JSON input x 22,539 ops/sec ±0.17% (98 runs sampled)
trace-mapping: decoded Object input x 161,786 ops/sec ±0.11% (101 runs sampled)
trace-mapping: encoded Object input x 24,485 ops/sec ±0.10% (100 runs sampled)
source-map-js: encoded Object input x 6,195 ops/sec ±0.36% (100 runs sampled)
source-map: encoded Object input x 2,602 ops/sec ±0.16% (100 runs sampled)
Fastest is trace-mapping: decoded Object input
trace-mapping: decoded originalPositionFor x 19,860 ops/sec ±0.11% (101 runs sampled)
trace-mapping: encoded originalPositionFor x 19,250 ops/sec ±0.23% (100 runs sampled)
source-map-js: encoded originalPositionFor x 2,897 ops/sec ±0.09% (100 runs sampled)
source-map: encoded originalPositionFor x 1,571 ops/sec ±0.10% (100 runs sampled)
Fastest is trace-mapping: decoded originalPositionFor
```
[source-map]: https://www.npmjs.com/package/source-map
|
