Fast Source Map

Keeping the JIT gods happy since circa 2015

When working with sourcemaps, VLQ encoding and decoding typically dominiates cost. To address this, this library provides a Fast VLQ encoder & decoder for JavaScript. By reducing allocations, minimizing transformations, and writing JIT friendly code we are able to drammatically improve performance.

How much faster? At the time this README was written, decoding scala.js.map took:

v8: 4.6.85.28
node: 5.0.0
os: 10.10.3
hw: 2.5 GHz Intel Core i7, 16gb

• source-map: ~5,000ms
• fast-source-map ~ 500ms

Usage

First we will need to have our sourceMap.mappings stored as a buffer. We have future plans to work directly on a sourceMap buffer, as that will allow for more ideal performance. Even without, the performance improvements are still well worth it.

It's also worth noting, this is considered a fairly low level API. Ergonomics can still be improved!

var buffer = /* try to leave the map inert, and in buffer form. Otherwise convert to buffer */

Typically, the following steps are required:

var map = JSON.parse(fs.readFileSync('path/to/source/to/decode.js.map'));
var toBuffer = require('string2buffer');

var buffer = toBuffer(map);

Setup the reader and decoder

// setup the reader
var reader = new VLQ.IntBufferReader(buffer, 0, buffer.length);
var decoder = new VLQ.Decoder();
var mappingsDecoder = new VLQ.MappingsDecoder(decoder);

Now for some actual decoding

mappingsDecoder.decode(reader);
decoder.lines // => is the quickly decoded