uglify-js

library3.19.3jsnpmunverified

UglifyJS is a robust JavaScript parser, minifier, compressor, and beautifier toolkit, currently at stable version 3.19.3. It is primarily designed for ECMAScript 5 (ES5) and most modern JavaScript language features, often receiving frequent bug fixes and performance improvements as seen in its recent release cadence. For newer or 'more exotic' ECMAScript features (e.g., beyond ES2015 without transpilation), developers are advised to pre-process their code with a transpiler like Babel before passing it to UglifyJS. Key differentiators for version 3 include a simplified programmatic API and command-line interface compared to its predecessor, UglifyJS 2, and a continued focus on efficient code size reduction through advanced compression and mangling techniques.

devops

npm install uglify-js

github ↗package ↗homepage ↗

INSTALL

IMPORT

SIG · UGLIFY-JS

uglify-js

devopsjavascriptv3.19.3

Install

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Import

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Disk

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Pass rate

0/ 6

Env Coverage0 / 6

glibc

18–22

musl

18–22

Install & Compatibility

Where this runs

tested against v? · npm install

Install × environment matrix

Each cell = how many times install + import succeeded across repeated harness runs. Partial = flaky.
glibc = Debian/Ubuntu slim · musl = Alpine Linux

musl

node 18–226 runs

build_error

glibc

node 18–226 runs

build_error

Code

Verified usage

Verified import paths — ran on the pinned version, not inferred.

minify

✓ import { minify } from 'uglify-js';

✗ const uglify = require('uglify-js'); const result = uglify(...);

The primary programmatic entry point is the `minify` function, often accessed directly from the default export in CommonJS or as a named export in ESM. While `require('uglify-js')` returns an object containing `minify`, direct access is common.

UglifyJS

✓ const UglifyJS = require('uglify-js');

✗ import UglifyJS from 'uglify-js';

For CommonJS environments, the entire UglifyJS toolkit is typically imported as a single object named `UglifyJS`, from which `minify` and other utilities are accessed. Native ESM default imports might not work as expected due to the package's primary CJS export.

AST_Node

✓ const { AST_Node } = require('uglify-js');

UglifyJS exposes its Abstract Syntax Tree (AST) nodes for advanced use cases like custom transformations. These are typically imported as named exports from the main package.

This quickstart demonstrates how to programmatically minify JavaScript code using `uglify-js`, including common compression and mangling options, and handling potential errors or warnings.

uglifyjs --version

Debug

Known issues

breakingUglifyJS 3 introduced a completely revised API and CLI that is not backward compatible with UglifyJS 2. Migrating from v2 requires updating import paths and usage patterns.

fix

Refer to the official UglifyJS 3 documentation for the new API and CLI usage. Specifically, `minify()` is the primary programmatic function.

affects: >=3.0.0

gotchaUglifyJS is designed primarily for ECMAScript 5 (ES5) and some later features. For code utilizing newer or 'exotic' ECMAScript features (e.g., ES2015+ syntax like optional chaining, nullish coalescing), it is critical to transpile the code down to a compatible syntax (e.g., ES5) using tools like Babel before feeding it to UglifyJS. Failure to do so can lead to `SyntaxError: Unexpected token` errors or incorrect minification.

fix

Integrate a transpilation step (e.g., Babel) into your build process to convert modern JavaScript syntax to ES5 or a compatible target before `uglify-js` processes the files.

affects: >=3.0.0

breakingSince v3.18.0, `uglify-js` processes input as an ES module by default (when using the `--module` CLI option or equivalent API setting). This can alter optimizations for existing CommonJS modules or scripts not explicitly marked as modules, potentially leading to different behavior or errors.

fix

If your input is CommonJS, ensure you explicitly disable ES module parsing by setting the appropriate option (e.g., `parse: { module: false }` in the `minify` options, or avoiding `--module` on the CLI) or verify your code's compatibility with ES module parsing.

affects: >=3.18.0

gotchaAggressive property mangling (`--mangle-props` or `mangle.properties` option) can break code that relies on specific property names, especially when interacting with external APIs, DOM properties, or libraries that use string-based property access (e.g., `obj['myProp']`).

fix

Always use the `reserved` option within `mangle.properties` to specify property names that should *never* be mangled. For DOM properties, consider `domprops: false` or carefully use the `builtins` option. Thorough testing after mangling is essential.

affects: >=3.0.0

gotchaWhen minifying code that includes IIFEs (Immediately Invoked Function Expressions) or CommonJS modules, if a `return` statement appears outside of a function, UglifyJS may throw a 'Return outside of function' error.

fix

Set the `parse.bare_returns: true` option to allow return statements outside of functions. This is particularly useful for CommonJS modules or userscripts wrapped by an external caller.

affects: >=3.0.0

breakingOlder versions of UglifyJS (prior to 2.6.0) had known Regular Expression Denial of Service (ReDoS) vulnerabilities in the `parse()` function.

fix

Upgrade `uglify-js` to version 2.6.0 or greater to mitigate ReDoS vulnerabilities. Regularly updating to the latest stable version is recommended for security patches.

affects: <2.6.0

Errors

Common errors & fixes

SyntaxError: Unexpected token: keyword 'const'

Attempting to minify JavaScript code written with modern ES2015+ features (like `const`, `let`, arrow functions, classes) without prior transpilation to a UglifyJS-compatible syntax (primarily ES5).

fix

Pre-process your JavaScript files with a transpiler such as Babel to transform modern syntax into ES5 before passing them to `uglify-js`. Ensure your build configuration (e.g., Webpack, Gulp) includes this step.

TypeError: UglifyJS.minify is not a function

Incorrectly importing or requiring the `uglify-js` package, or attempting to call `minify` directly on the `UglifyJS` object without accessing the function property. In ESM, it could be trying `import UglifyJS from 'uglify-js'` and then `UglifyJS.minify()`.

fix

For CommonJS, use `const UglifyJS = require('uglify-js');` and then `UglifyJS.minify(code, options);`. For ESM, use `import { minify } from 'uglify-js';` and then `minify(code, options);`.

Error: `return` outside of function

The parser encountered a `return` statement in the global scope or within an IIFE/CommonJS module that is not explicitly wrapped as a function that UglifyJS recognizes.

fix

Include `parse: { bare_returns: true }` in your `minify` options to instruct UglifyJS to tolerate `return` statements outside of functions, which is common in certain module patterns.

Cannot read properties of undefined (reading 'someProperty')

This often occurs after aggressive property mangling (`--mangle-props` or `mangle.properties`) when UglifyJS renames object properties that are accessed dynamically (e.g., `obj['someProperty']`) or are external interfaces (like DOM properties or third-party library APIs) that should not be renamed.

fix

Use the `reserved` option within `mangle.properties` to specify a list of property names that should never be mangled. If mangling DOM properties, set `domprops: false`. Thoroughly test your application after enabling property mangling.

Upgrade

Version history

3.19.3latest on npm

Audit

Dependencies

No dependency data recorded yet.

Agent activity

10 hits · last 30 days

node

Resources

githubhttps://github.com/mishoo/UglifyJS ↗

packagehttps://www.npmjs.com/package/uglify-js ↗

homepagehttps://lisperator.net/uglifyjs/ ↗