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Here's another demo where you can see a bug: there are two copies of React in the bundle (perhaps because of out-of-date dependencies).

Requirements

Node 10 or later

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": 718, "mappedBytes": 681, "unmappedBytes": 1, "eolBytes": 1, "sourceMapCommentBytes": 35, "files": { "node_modules/browser-pack/_prelude.js": { "size": 480 }, "src/bar.js": { "size": 104 }, "src/foo.js": { "size": 97 }, "[sourceMappingURL]": { "size": 35 }, "[unmapped]": { "size": 1 }, "[EOLs]": { "size": 1 } } } ] }

--tsv : output tab-delimited values instead of displaying a visualization: source-map-explorer foo.min.js --tsv Source Size node_modules/browser-pack/_prelude.js 480 src/bar.js 104 src/foo.js 97 [sourceMappingURL] 35 [unmapped] 1 [EOLs] 1 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.

--exclude-source-map : exclude source map comment size from 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.

--no-border-checks : Disable invalid mapping column/line checks. By default, when a source map references column/line with bigger index than available in the source source-map-explorers throws an error indicating that specified source map might be wrong for the source.

--coverage : If the path to a valid a chrome code coverage JSON export is supplied, the tree map will be colorized according to which percentage of the modules code was executed

--gzip : Calculate gzip size. It also sets onlyMapped flag

--sort : Sort filenames

Examples Get help source-map-explorer -h Analyze and debug space usage through source maps. Usage: source-map-explorer script.js [script.js.map] [--json [result.json] | --html [result.html] | --tsv [result.csv]] [-m | --only-mapped] [--exclude-source-map] [--no-border-checks] [--gzip] [--sort] [--replace=BEFORE_1 BEFORE_2 --with=AFTER_1 AFTER_2] [--no-root] [--coverage coverage.json] [--version] [--help | -h] Output: --json If filename specified save output as JSON to specified file otherwise output to stdout. [string] --tsv If filename specified save output as TSV to specified file otherwise output to stdout. [string] --html If filename specified save output as HTML to specified file otherwise output to stdout rather than opening a browser. [string] Replace: --replace Apply a simple find/replace on source file names. This can be used to fix some oddities with paths that appear in the source map generation process. Accepts regular expressions. [array] --with See --replace. [array] Options: --version Show version number [boolean] --only-mapped, -m Exclude "unmapped" bytes from the output. This will result in total counts less than the file size [boolean] --exclude-source-map Exclude source map comment size from output [boolean] --no-root To simplify the visualization, source-map-explorer will remove any prefix shared by all sources. If you wish to disable this behavior, set --no-root. [boolean] --no-border-checks Disable invalid mapping column/line checks. [boolean] --coverage If the path to a valid a chrome code coverage JSON export is supplied, the tree map will be colorized according to which percentage of the modules code was executed [string] --gzip Calculate gzip size. It also sets onlyMapped flag [boolean] --sort Sort filenames [boolean] -h, --help Show help [boolean] Examples: source-map-explorer script.js script.js.map Explore bundle source-map-explorer script.js Explore bundle with inline source map source-map-explorer dist/js/*.* Explore all bundles inside dist/js folder source-map-explorer script.js --tsv Explore and output result as TSV to stdout source-map-explorer script.js --json result.json Explore and save result as JSON to the file Explore bundle and view result as an interactive map source-map-explorer script.js script.js.map Will open an HTML file containing explore result as a tree data map Explore and output result as JSON source-map-explorer script.js script.js.map --json { "results": [ { "bundleName": "script.js", "totalBytes": 718, "unmappedBytes": 1, "eolBytes": 1, "sourceMapCommentBytes": 35, "files": { "node_modules/browser-pack/_prelude.js": { "size": 480 } "src/bar.js": { "size": 104 } "src/foo.js": { "size": 97 }, "[sourceMappingURL]": { "size": 35 } "[unmapped]": { "size": 1 } } } ] } Explore and output result as TSV source-map-explorer script1.js script2.js --tsv Source Size node_modules/browser-pack/_prelude.js 480 src/bar.js 104 src/foo.js 97 [sourceMappingURL] 35 [unmapped] 1 [sourceMappingURL] 2308 node_modules/browser-pack/_prelude.js 480 src/bar.js 104 src/foo.js 97 [unmapped] 1 Explore and output result as HTML source-map-explorer script.js --html <!doctype html> <html lang="en"> <head> ... selectBundle(selectedBundle); </script> <html> Explore and save result as HTML file source-map-explorer script.js --html ./sme/result.html Replace substring in filenames source-map-explorer script.js --tsv --replace dist node_modules --with gist modules Source Size gist/bar.js 2854 modules/browserify/modules/browser-pack/_prelude.js 463 gist/foo.js 137 [unmapped] 0 Remove [unmapped] from result files source-map-explorer script.js --tsv --only-mapped Source Size [sourceMappingURL] 2308 node_modules/browser-pack/_prelude.js 480 src/bar.js 104 src/foo.js 97 Remove [sourceMappingURL] from result files source-map-explorer script.js --tsv --exclude-source-map Source Size node_modules/browser-pack/_prelude.js 480 src/bar.js 104 src/foo.js 97 [unmapped] 1 Do not remove common path prefix source-map-explorer script.js --tsv --no-root On error Errors will be displayed only if no output flags specified source-map-explore with-unmapped.js no-map-comment.js no-map-comment.js Unable to find a source map. See https://github.com/danvk/source-map-explorer/blob/master/README.md#generating-source-maps with-unmapped.js Unable to map 274/1335 bytes (20.52%)

API

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') }

or 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 - Exclude "unmapped" bytes from the output. This will result in total counts less than the file size

: boolean - Exclude "unmapped" bytes from the output. This will result in total counts less than the file size excludeSourceMapComment : boolean - Exclude source map comment size from output. This will result in total counts less than the file size.

: boolean - Exclude source map comment size from 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

: Object - Output options noRoot : boolean - See --no-root option above for details

: boolean - See option above for details noBorderChecks : boolean - Disable invalid mapping column/line checks. See --no-border-checks above.

: boolean - Disable invalid mapping column/line checks. See above. replaceMap : <Object<{ [from: string]: string }>> - Mapping for replacement, see --replace , --with options above for details.

: <Object<{ [from: string]: string }>> - Mapping for replacement, see , options above for details. coverage : string - If the path to a valid a chrome code coverage JSON export is supplied, the tree map will be colorized according to which percentage of the modules code was executed

: string - If the path to a valid a chrome code coverage JSON export is supplied, the tree map will be colorized according to which percentage of the modules code was executed gzip : boolean - Calculate gzip size. It also sets onlyMapped flag

Example:

import { explore } from ' source-map-explorer ' explore ( ' tests/data/foo.min.js ' , { output : { format : ' html ' } } ) . then ( ) { bundles : [ { bundleName : ' tests/data/foo.min.js ' , totalBytes : 718 , unmappedBytes : 1 , mappedBytes : 681 , eolBytes : 1 , sourceMapCommentBytes : 35 , files : { ' node_modules/browserify/node_modules/browser-pack/_prelude.js ' : { size : 480 } , ' dist/bar.js ' : { size : 104 } , ' dist/foo.js ' : { size : 97 } , ' [sourceMappingURL] ' : { size : 35 } , ' [unmapped] ' : { size : 1 } , ' [EOLs] ' : { size : 1 } } } ] , output : ' <!doctype html>... ' , errors : [ ] }

See more at wiki page

More details explore(bundlesAndFileTokens, [options]) Returns Promise that is resolved to an object with properties: bundles : array - List of bundle explore result objects bundleName : string - Path associated with the bundle totalBytes : number - Size of the provided file unmappedBytes : number | undefined eolBytes : number - Bytes taken by end of line characters sourceMapCommentBytes : number - sourceMappingURL comment bytes files : { [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 .

: array - List of bundle explore result objects output : string - Result as a string if output.format options specified. If output='html' it contains self-packed HTML that can be opened in the browser

: string - Result as a string if options specified. If it contains self-packed HTML that can be opened in the browser errors : array - List of bundle explore error objects bundleName : string - Path associated with the bundle code : string - Error code message : string - User friendly message error : Error isWarning : boolean - Whether error isn't fatal

: array - List of bundle explore error objects The promise is rejected when there is a fatal error or all bundles explore failed. Reject reason is either explore result object (the same one returned when promise resolved) or Error object with code property specified. Possible error codes are: Unknown

NoBundles - Empty array is passed to explore

- Empty array is passed to OneSourceSourceMap - Bundle source map only contains one source

- Bundle source map only contains one source UnmappedBytes (warning) - There are unmapped bytes

(warning) - There are unmapped bytes InvalidMappingLine - Source map refers to the generated line beyond source last line

- Source map refers to the generated line beyond source last line InvalidMappingColumn - Source map refers to generated column beyond source last column at the line

- Source map refers to generated column beyond source last column at the line CannotSaveFile - Error saving HTML to file when file option specified

- Error saving HTML to file when option specified CannotCreateTempFile - Temporary HTLM file with visualization cannot be created. Check temporary folder access.

- Temporary HTLM file with visualization cannot be created. Check temporary folder access. CannotOpenTempFile - Temporary HTLM file cannot be opened. Check if default browser can openen html files.

- Temporary HTLM file cannot be opened. Check if default browser can openen html files. CannotOpenCoverageFile - Unable to open/parse coverage file

- Unable to open/parse coverage file NoCoverageMatches - No matched bundles found for coverages.

- No matched bundles found for coverages. Node.js error code (e.g. 'ENOENT') Examples Full explore('js/*.*', { file: './sme-results/2019-04-27.html', output: { format: 'html', filename: './sme-result.html' }, noRoot: true, onlyMapped: true, replaceMap: { dist: '' } }) .then((result: ExploreResult) => { result.errors.forEach((error: ExploreErrorResult) => { if (error.isWarning) { console.log(`Issue during '${error.bundleName}; explore`, error.message); } else { console.log(`Failed to explore '${error.bundleName}'`, error.message); } }); result.bundles.forEach((bundle: ExploreBundleResult) => { console.log(bundle.bundleName); console.log(JSON.stringify(bundle.files)); }); }) .catch(error => { console.log('Failed to explore'); if (error.errors) { error.errors.forEach((exploreError: ExploreErrorResult) => { console.log(exploreError.bundleName); console.log(exploreError.message); }); } else { console.log(error); } }); Inline or referenced map explore('with-inline-map.js'); Separate map explore(['foo.min.js', 'foo.min.js.map']); Glob pattern explore('js/*.*'); Multiple globs explore(['js/foo.1*.js', 'js/foo.mi?.js']); Specify bundles explicitly const bundle: Bundle = { code: 'foo.min.js', map: 'foo.min.js.map' }; explore(bundle); explore([{ code: 'foo.min.js', map: 'foo.min.js.map' }, { code: 'with-inline-map.js' }]); Pass buffer explore({ code: fs.readFileSync('js/foo.min.js'), map: fs.readFileSync('js/foo.min.js.map') });

gzip size

When gzip option (or --gzip parameter) is specified result size calculated as gzip size. Due to the nature of compression a gzip file size is inaccurate. It means that removing a 1k gzipped file in a bundle may reduce the bundle size by less than 1k. Also it's impossible to calculate unmapped bytes because the sum of spans' gzip sizes isn't equal to gzip size of the source file.

Code coverage heat map

In Google Chrome, you can collect code coverage stats. source-map-explorer accepts path to via --coverage argument (or coverage API option) and attempts to color code the heat map. This allows you to find the code that is not strictly needed for the initial page load and helps to identify the ideal ways to code split.

Red boxes correspond to code that would only be executed if the user took some action, or if some condition was met. For example, it may be a component inside of a dropdown the user never interacted with, or components that are only needed if the user opens a modal. In cases where the parent is green but the boxes inside are red, that means maybe some "initialization" logic ran, but the inner code never ran. Maybe we mounted a button, but not the other components in that module that are only needed if and when the user clicks the button, in that case, I would have the button trigger the rest of the code to load.

The heat map feature helps you identify the code that is needed for a fast initial page load (green), as well as helps to identify the code that can be (potentially) deferred because it doesn't run until the user interacts with some feature (red).

What might contribute to a generated file size

In addition to mapped generated code a file may contain:

sourceMappingURL comment - A comment containing source map or referencing the file with source map. Represented by [sourceMappingURL] in explore result.

comment - A comment containing source map or referencing the file with source map. Represented by in explore result. Mapped code without source. It might be code generated by a bundler (e.g. webpack). Represented by [no source] in explore result.

in explore result. Unmapped code - code that is not referenced within the source map. Represented by [unmapped] in explore result. For example webpack keeps on-demand chunk's content unmapped.

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 cat foo.bundle.js | exorcist foo.bundle.js.map > /dev/null uglifyjs -c -m \ -- in -source-map foo.bundle.js.map \ --source-map foo.min.js.map \ -o foo.min.js \ foo.bundle.js 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}

Other source map tools

source-map-visualization

bundle-wizard - Easier analysis of webapp entry points (uses source-map-explorer under the hood)

Learn about source maps