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The fast, future-friendly minifier
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The fast, future-friendly minifier. Try before you buy at

Warning: this is alpha software. Test thoroughly before using in production! Consider using the check option. Please report any bugs you find!


Butternut is significantly faster than other JavaScript minifiers, and works with the latest version of JavaScript (ES2015, aka ES6, and beyond). It's typically around 3x faster than UglifyJS with default minify options, and 10-15x faster than Babili.

The compression is better than Babili and closure-compiler-js (in standard compilation mode — you can get better results with Closure in advanced mode, but only by writing your code in a very particular way). It's almost as good as Uglify in its current version.

You can test out the different tools with npm run bench.

Note: UglifyJS supports ES2015+ as of very recently — see uglify-es.


The traditional approach to minification is this: parse your source code into an abstract syntax tree (AST) using something like Acorn, manipulate the AST, and finally generate code from it.

Butternut takes a different approach. It uses Acorn to generate an AST, but instead of steps 2 and 3 it then edits the code in place using magic-string — which is much less costly than AST manipulation and code generation.


The easiest way to use Butternut is to plug it into your existing build process:

Alternatively, you can use it directly via the CLI or the JavaScript API:

Command Line Interface

Install Butternut globally, then use the squash command:

npm install --global butternut # or npm i -g butternut
squash app.js > app.min.js

Run squash --help to see the available options.

JavaScript API

Install Butternut to your project...

npm install --save-dev butternut # or npm i -D butternut

...then use it like so:

const butternut = require('butternut');
const { code, map } = butternut.squash(source, options);

The options argument, if supplied, is an object that can have the following properties:

Option CLI equivalent Default value Description
check --check false Parse output. See below
allowDangerousEval n/a false Whether to allow direct eval calls
sourceMap -m, --sourcemap true Whether to create a sourcemap. Set to inline to append to the output (not recommended)
file n/a (automatic) null The output filename, used in sourcemap generation
source n/a (automatic) null The source filename, used in sourcemap generation
includeContent n/a true Whether to include the source file in the sourcesContent property of the generated sourcemap

The check option

Since Butternut is a new project, it hasn't yet been battle-tested. It may generate code that you don't expect. If you pass check: true (or use the --check flag, if using the CLI), Butternut will parse the generated output to verify that it is valid JavaScript. If not, it means it's messed something up, in which case it will try to help you find the code that it failed to minify correctly.

If you find bugs while using Butternut, please raise an issue!



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