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source-map-explorer

Analyze and debug JavaScript (or Sass or LESS) code bloat through source maps.

The source map explorer determines which file each byte in your minified code came from. It shows you a treemap visualization to help you debug where all the code is coming from. Check out this Chrome Developer video (3:25) for a demo of the tool in action.

Install:

npm install -g source-map-explorer

Use (you can specify filenames or use glob pattern):

source-map-explorer bundle.min.js
source-map-explorer bundle.min.js bundle.min.js.map
source-map-explorer bundle.min.js*
source-map-explorer *.js

This will open up a visualization of how the space is used in your minified bundle:

Here's a demo with a more complex bundle.

Here's another demo where you can see a bug: there are two copies of React in the bundle (perhaps because of out-of-date dependencies).

Options

Default behavior - write HTML to a temp file and open it in your browser

source-map-explorer foo.min.js

Write output in specific formats to stdout

source-map-explorer foo.min.js --html
source-map-explorer foo.min.js --json
source-map-explorer foo.min.js --tsv

Write output in specific formats to a file

source-map-explorer foo.min.js --html result.html
source-map-explorer foo.min.js --json result.json
source-map-explorer foo.min.js --tsv result.tsv

  • --json: output JSON instead of displaying a visualization:

    source-map-explorer foo.min.js --json
{
  "results": [
    {
      "bundleName": "tests/data/foo.min.js",
      "totalBytes": 697,
      "unmappedBytes": 0,
      "files": {
        "node_modules/browserify/node_modules/browser-pack/_prelude.js": 463,
        "dist/bar.js": 97,
        "dist/foo.js": 137,
        "<unmapped>": 0
      }
    }
  ]
}

  • --tsv: output tab-delimited values instead of displaying a visualization:

    source-map-explorer foo.min.js --tsv
Source  Size
node_modules/browserify/node_modules/browser-pack/_prelude.js  463
dist/foo.js  137
dist/bar.js  97
<unmapped>  0

    If you just want a list of files, you can do source-map-explorer foo.min.js --tsv | sed 1d | cut -f1.

  • --html: output HTML to stdout. If you want to save the output (e.g. to share), specify filename after --html:

    source-map-explorer foo.min.js --html tree.html

  • -m, --only-mapped: exclude "unmapped" bytes from the output. This will result in total counts less than the file size.

  • --replace, --with: The paths in source maps sometimes have artifacts that are difficult to get rid of. These flags let you do simple find & replaces on the paths. For example:

    source-map-explorer foo.min.js --replace 'dist/' --with ''

    You can specify these flags multiple times. Be aware that the find/replace is done after eliminating shared prefixes between paths.

    These are regular expressions.

  • --no-root: By default, source-map-explorer finds common prefixes between all source files and eliminates them, since they add complexity to the visualization with no real benefit. But if you want to disable this behavior, set the --no-root flag.

See more at wiki page

API

explore(bundlesAndFileTokens, [options])

bundlesAndFileTokens:

  • Glob: dist/js/*.*
  • Filename: dist/js/chunk.1.js
  • Bundle: { code: 'dist/js/chunk.1.js', map: 'dist/js/chunk.1.js.map' } or { code: fs.readFileSync('dist/js/chunk.2.js') }
  • Array of globs, filenames and bundles: 
    [
  'dist/js/chunk.2.*',
  'dist/js/chunk.1.js', 'dist/js/chunk.1.js.map',
  { code: 'dist/js/chunk.3.js', map: 'dist/js/chunk.3.js.map' }
]

options:

  • onlyMapped: boolean (default false) - Exclude "unmapped" bytes from the output. This will result in total counts less than the file size
  • output: Object - Output options
    • format: string - 'json', 'tsv' or 'html'
    • filename: string - Filename to save output to
  • noRoot: boolean (default false) - See --no-root option above for details
  • replaceMap: <Object<{ [from: string]: string }>> - Mapping for replacement, see --replace, --with options above for details.

Example:

import { explore } from 'source-map-explorer'
// or import explore from 'source-map-explorer'

explore('tests/data/foo.min.js', { output: { format: 'html' } }).then()

// Returns
{
  bundles: [{
    bundleName: 'tests/data/foo.min.js',
    totalBytes: 697,
    unmappedBytes: 0,
    files: {
      'node_modules/browserify/node_modules/browser-pack/_prelude.js': 463,
      'dist/bar.js': 97,
      'dist/foo.js': 137,
      '<unmapped>': 0
    }
  }],
  output: '<!doctype html>...',
  errors: []
}

See more at wiki page

Generating source maps

For source-map-explorer to be useful, you need to generate a source map which maps positions in your minified file all the way back to the files from which they came.

If you use browserify, you can generate a JavaScript file with an inline source map using the --debug flag:

browserify -r .:foo --debug -o foo.bundle.js
source-map-explorer foo.bundle.js

If you subsequently minify your JavaScript, you'll need to ensure that the final source map goes all the way back to the original files. For example, using browserify, uglify and exorcist:

browserify -r .:foo --debug -o foo.bundle.js
# foo.bundle.js has an inline source map
cat foo.bundle.js | exorcist foo.bundle.js.map > /dev/null
# foo.bundle.js.map is an external source map for foo.bundle.js
uglifyjs -c -m \
  --in-source-map foo.bundle.js.map \
  --source-map foo.min.js.map \
  -o foo.min.js \
  foo.bundle.js
# foo.min.js has an external source map in foo.min.js.map
source-map-explorer foo.min.js

Types of source maps

There are two types of source maps: inline and external.

If your JS file has an inline source map, then its last line will look something like this:

//# sourceMappingURL=data:application/json;base64,eyJ2ZXJzaW9uIjozLCJm...

This encodes the sourcemap as a base64 data URL. If your file has an inline source map, the source-map-explorer should have no trouble understanding it.

If your last line instead looks like this:

//# sourceMappingURL=foo.min.js.map

Then the source map lives in an external .map file. The source-map-explorer will try to find this file, but this often fails because it's unclear what the URL is relative to.

If this happens, just pass in the source map explicitly, e.g. (in bash or zsh):

source-map-explorer path/to/foo.min.js{,.map}