28771f2 Jul 17, 2018
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@danvk @penx @knownasilya @Hypercubed @jakub300
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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.


npm install -g source-map-explorer


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Here's another demo where you can see a bug: there are two copies of React in the bundle (perhaps because of out-of-date dependencies).


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

    source-map-explorer --json foo.min.js
      "node_modules/browserify/node_modules/browser-pack/_prelude.js": 463,
      "bar.js": 62,
      "foo.js": 137
  • --tsv: output tab-delimited values instead of displaying a visualization:

    source-map-explorer --tsv foo.min.js
    Source	Size
    dist/bar.js	62
    dist/foo.js	137

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

  • --html: output HTML to stdout. By default, source-map-explorer writes HTML to a temporary file and opens it in your default browser. If you want to save the output (e.g. to share), pipe it to a file:

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

  • --noroot: 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 --noroot flag.


explore(filePathOrContent[, sourceMapPathOrContent][, options])

  • filePathOrContent <string|Buffer> - path to file or Buffer with contents
  • sourceMapPathOrContent <string|Buffer> - path to source map or Buffer with contents
  • options <Object> Options for generation
    • onlyMapped <boolean> (default false) See --only-mapped option above for details
    • html <boolean> (default false) When true html will be included in returned object
    • noRoot <boolean> (default false) See --noroot option above for details
    • replace <Object<{ [from: string]: string }>> Mapping for replacement, see --replace, --with options above for details.
  • returns: <Object>
    • totalBytes <number> Size of the provided file
    • unmappedBytes <number>
    • files <Object<{ [sourceFile: string]: number }>> Map containing filenames from the source map and size in bytes they take inside of provided file. Additional key <unmapped> is included if options.onlyMapped is false.
    • html <string> Contains self-packed html that can be opened in the browser, only if options.html is true


require('source-map-explorer')('testdata/foo.min.js', { html: true })

// Returns
  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
  html: '<!doctype html>...'

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 > /dev/null
# is an external source map for foo.bundle.js
uglifyjs -c -m \
  --in-source-map \
  --source-map \
  -o foo.min.js \
# foo.min.js has an external source map in
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:


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}