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@jridgewell/sourcemap-codec

Encode/decode the mappings property of a sourcemap.

Why?

Sourcemaps are difficult to generate and manipulate, because the mappings property the part that actually links the generated code back to the original source is encoded using an obscure method called Variable-length quantity. On top of that, each segment in the mapping contains offsets rather than absolute indices, which means that you can't look at a segment in isolation you have to understand the whole sourcemap.

This package makes the process slightly easier.

Installation

npm install @jridgewell/sourcemap-codec

Usage

import { encode, decode } from '@jridgewell/sourcemap-codec';

var decoded = decode( ';EAEEA,EAAE,EAAC,CAAE;ECQY,UACC' );

assert.deepEqual( decoded, [
	// the first line (of the generated code) has no mappings,
	// as shown by the starting semi-colon (which separates lines)
	[],

	// the second line contains four (comma-separated) segments
	[
		// segments are encoded as you'd expect:
		// [ generatedCodeColumn, sourceIndex, sourceCodeLine, sourceCodeColumn, nameIndex ]

		// i.e. the first segment begins at column 2, and maps back to the second column
		// of the second line (both zero-based) of the 0th source, and uses the 0th
		// name in the `map.names` array
		[ 2, 0, 2, 2, 0 ],

		// the remaining segments are 4-length rather than 5-length,
		// because they don't map a name
		[ 4, 0, 2, 4 ],
		[ 6, 0, 2, 5 ],
		[ 7, 0, 2, 7 ]
	],

	// the final line contains two segments
	[
		[ 2, 1, 10, 19 ],
		[ 12, 1, 11, 20 ]
	]
]);

var encoded = encode( decoded );
assert.equal( encoded, ';EAEEA,EAAE,EAAC,CAAE;ECQY,UACC' );

Benchmarks

node v18.0.0

amp.js.map - 45120 segments

Decode Memory Usage:
@jridgewell/sourcemap-codec       5479160 bytes
sourcemap-codec                   5659336 bytes
source-map-0.6.1                 17144440 bytes
source-map-0.8.0                  6867424 bytes
Smallest memory usage is @jridgewell/sourcemap-codec

Decode speed:
decode: @jridgewell/sourcemap-codec x 502 ops/sec ±1.03% (90 runs sampled)
decode: sourcemap-codec x 445 ops/sec ±0.97% (92 runs sampled)
decode: source-map-0.6.1 x 36.01 ops/sec ±1.64% (49 runs sampled)
decode: source-map-0.8.0 x 367 ops/sec ±0.04% (95 runs sampled)
Fastest is decode: @jridgewell/sourcemap-codec

Encode Memory Usage:
@jridgewell/sourcemap-codec       1261620 bytes
sourcemap-codec                   9119248 bytes
source-map-0.6.1                  8968560 bytes
source-map-0.8.0                  8952952 bytes
Smallest memory usage is @jridgewell/sourcemap-codec

Encode speed:
encode: @jridgewell/sourcemap-codec x 738 ops/sec ±0.42% (98 runs sampled)
encode: sourcemap-codec x 238 ops/sec ±0.73% (88 runs sampled)
encode: source-map-0.6.1 x 162 ops/sec ±0.43% (84 runs sampled)
encode: source-map-0.8.0 x 191 ops/sec ±0.34% (90 runs sampled)
Fastest is encode: @jridgewell/sourcemap-codec


***


babel.min.js.map - 347793 segments

Decode Memory Usage:
@jridgewell/sourcemap-codec      35338184 bytes
sourcemap-codec                  35922736 bytes
source-map-0.6.1                 62366360 bytes
source-map-0.8.0                 44337416 bytes
Smallest memory usage is @jridgewell/sourcemap-codec

Decode speed:
decode: @jridgewell/sourcemap-codec x 40.35 ops/sec ±4.47% (54 runs sampled)
decode: sourcemap-codec x 36.76 ops/sec ±3.67% (51 runs sampled)
decode: source-map-0.6.1 x 4.44 ops/sec ±2.15% (16 runs sampled)
decode: source-map-0.8.0 x 59.35 ops/sec ±0.05% (78 runs sampled)
Fastest is decode: source-map-0.8.0

Encode Memory Usage:
@jridgewell/sourcemap-codec       7212604 bytes
sourcemap-codec                  21421456 bytes
source-map-0.6.1                 25286888 bytes
source-map-0.8.0                 25498744 bytes
Smallest memory usage is @jridgewell/sourcemap-codec

Encode speed:
encode: @jridgewell/sourcemap-codec x 112 ops/sec ±0.13% (84 runs sampled)
encode: sourcemap-codec x 30.23 ops/sec ±2.76% (53 runs sampled)
encode: source-map-0.6.1 x 19.43 ops/sec ±3.70% (37 runs sampled)
encode: source-map-0.8.0 x 19.40 ops/sec ±3.26% (37 runs sampled)
Fastest is encode: @jridgewell/sourcemap-codec


***


preact.js.map - 1992 segments

Decode Memory Usage:
@jridgewell/sourcemap-codec        500272 bytes
sourcemap-codec                    516864 bytes
source-map-0.6.1                  1596672 bytes
source-map-0.8.0                   517272 bytes
Smallest memory usage is @jridgewell/sourcemap-codec

Decode speed:
decode: @jridgewell/sourcemap-codec x 16,137 ops/sec ±0.17% (99 runs sampled)
decode: sourcemap-codec x 12,139 ops/sec ±0.13% (99 runs sampled)
decode: source-map-0.6.1 x 1,264 ops/sec ±0.12% (100 runs sampled)
decode: source-map-0.8.0 x 9,894 ops/sec ±0.08% (101 runs sampled)
Fastest is decode: @jridgewell/sourcemap-codec

Encode Memory Usage:
@jridgewell/sourcemap-codec        321026 bytes
sourcemap-codec                    830832 bytes
source-map-0.6.1                   586608 bytes
source-map-0.8.0                   586680 bytes
Smallest memory usage is @jridgewell/sourcemap-codec

Encode speed:
encode: @jridgewell/sourcemap-codec x 19,876 ops/sec ±0.78% (95 runs sampled)
encode: sourcemap-codec x 6,983 ops/sec ±0.15% (100 runs sampled)
encode: source-map-0.6.1 x 5,070 ops/sec ±0.12% (102 runs sampled)
encode: source-map-0.8.0 x 5,641 ops/sec ±0.17% (100 runs sampled)
Fastest is encode: @jridgewell/sourcemap-codec


***


react.js.map - 5726 segments

Decode Memory Usage:
@jridgewell/sourcemap-codec        734848 bytes
sourcemap-codec                    954200 bytes
source-map-0.6.1                  2276432 bytes
source-map-0.8.0                   955488 bytes
Smallest memory usage is @jridgewell/sourcemap-codec

Decode speed:
decode: @jridgewell/sourcemap-codec x 5,723 ops/sec ±0.12% (98 runs sampled)
decode: sourcemap-codec x 4,555 ops/sec ±0.09% (101 runs sampled)
decode: source-map-0.6.1 x 437 ops/sec ±0.11% (93 runs sampled)
decode: source-map-0.8.0 x 3,441 ops/sec ±0.15% (100 runs sampled)
Fastest is decode: @jridgewell/sourcemap-codec

Encode Memory Usage:
@jridgewell/sourcemap-codec        638672 bytes
sourcemap-codec                   1109840 bytes
source-map-0.6.1                  1321224 bytes
source-map-0.8.0                  1324448 bytes
Smallest memory usage is @jridgewell/sourcemap-codec

Encode speed:
encode: @jridgewell/sourcemap-codec x 6,801 ops/sec ±0.48% (98 runs sampled)
encode: sourcemap-codec x 2,533 ops/sec ±0.13% (101 runs sampled)
encode: source-map-0.6.1 x 2,248 ops/sec ±0.08% (100 runs sampled)
encode: source-map-0.8.0 x 2,303 ops/sec ±0.15% (100 runs sampled)
Fastest is encode: @jridgewell/sourcemap-codec

License

MIT