Minification

Assumptions

SWC Minifier makes small amount of assumptions to make the minification efficient. This is similar to other minifiers.

• .toString() and .valueOf() don’t have side effects, and for built-in objects they have not been overridden.

• undefined, NaN and Infinity have not been externally redefined.

• arguments.callee, arguments.caller and Function.prototype.caller are not used.

These are not valid in strict mode.

• The code doesn’t expect the contents of Function.prototype.toString() or Error.prototype.stack to be anything in particular.

• Getting and setting properties on a plain object does not cause other side effects (using .watch() or Proxy).

• Object properties can be added, removed and modified (not prevented with Object.defineProperty(), Object.defineProperties(), Object.freeze(), Object.preventExtensions() or Object.seal()).

• document.all is not null.

• Assigning properties to a class doesn’t have side effects and does not throw.

• Acesssing declared top-level identifiers do not have side effects.

If you declare a variable on globalThis using a getter with side-effects, SWC Minifier will likely break it.

• TDZ violation does not exist.

TDZ violation will be ignored and not preserved.

• Arithmetic expression may not have side effects.

This assumption means that you can’t rely on the exception thrown by the JS engine when adding a bigint and a number.

const a = 1n;
const b = 1;
const c = a + b;

If you execute this code, the JS engine will throw an exception. But SWC Minifier will ignore it.

Related: terser - compiler assumptions 

Starting with v1.2.67, you can configure SWC to minify your code by enabling minify in your .swcrc file:

{
  // Enable minification
  "minify": true,
  // Optional, configure minification options
  "jsc": {
    "minify": {
      "compress": {
        "unused": true
      },
      "mangle": true
    }
  }
}

Configuration

Note about comments

If you set jsc.minify.compress to true or {}, SWC will remove all comments starting from v1.11.11. Previous versions will preserve only license comments. If you don’t want this, modify jsc.minify.format.

jsc.minify.compress

Type: boolean | object.

Similar to the compress option  of terser.

{
  "jsc": {
    "minify": {
      "compress": true // equivalent to {}
    }
  }
}

• arguments, defaults to false.
• arrows, defaults to true.
• booleans, defaults to true.
• booleans_as_integers, defaults to false.
• collapse_vars, defaults to true.
• comparisons, defaults to true.
• computed_props, defaults to true.
• conditionals, defaults to true.
• dead_code, defaults to true.
• defaults, defaults to true.
• directives, defaults to true.
• drop_console, defaults to false.
• drop_debugger, defaults to true.
• ecma, defaults to 5.
• evaluate, defaults to true.
• global_defs, defaults to {}.
• hoist_funs, defaults to false.
• hoist_props, defaults to true.
• hoist_vars, defaults to false.
• ie8, Ignored.
• if_return, defaults to true.
• inline, defaults to true.
• join_vars, defaults to true.
• keep_classnames, defaults to false.
• keep_fargs, defaults to false.
• keep_infinity, defaults to false.
• loops, defaults to true.
• negate_iife, defaults to true.
• passes, defaults to 0, which means no limit.
• properties, defaults to true.
• pure_getters, defaults to “.
• pure_funcs, defaults to []. Type is an array of string.
• reduce_funcs, defaults to false.
• reduce_vars, defaults to true.
• sequences, defaults to true.
• side_effects, defaults to true.
• switches, defaults to true.
• top_retain, defaults to “.
• toplevel, defaults to true.
• typeofs, defaults to true.
• unsafe, defaults to false.
• unsafe_arrows, defaults to false.
• unsafe_comps, defaults to false.
• unsafe_Function, defaults to false.
• unsafe_math, defaults to false.
• unsafe_symbols, defaults to false.
• unsafe_methods, defaults to false.
• unsafe_proto, defaults to false.
• unsafe_regexp, defaults to false.
• unsafe_undefined, defaults to false.
• unused, defaults to true.
• module, Ignored. Currently, all files are treated as module.

jsc.minify.mangle

Type: boolean | object.

Similar to the mangle option  of terser.

{
  "jsc": {
    "minify": {
      "mangle": true // equivalent to {}
    }
  }
}

• props, Defaults to false, and true is identical to {}.
• topLevel, Defaults to true. Aliased as toplevel for compatibility with terser.
• keepClassNames, Defaults to false. Aliased as keep_classnames for compatibility with terser.
• keepFnNames, Defaults to false.
• keepPrivateProps, Defaults to false. Aliased as keep_private_props for compatibility with terser.
• reserved, Defaults to []
• ie8, Ignored.
• safari10, Defaults to false.

jsc.minify.mangle.properties

Type: object.

Similar to the mangle properties option  of terser.

{
  "jsc": {
    "minify": {
      "mangle":{
        "properties":{
          "reserved": ["foo", "bar"],
          "undeclared":false,
          "regex":"rust regex"
        }
      }
    }
  }
}

• reserved: Don’t use these names as properties.

• undeclared: Mangle properties even if it’s not declared.

• regex: Mangle properties only if it matches this regex

jsc.minify.format

These properties are mostly not implemented yet, but it exists to support passing terser config to swc minify without modification.

• asciiOnly, Defaults to false. Implemented as v1.2.184 and aliased as ascii_only for compatibility with terser.
• beautify, Defaults to false. Currently noop.
• braces, Defaults to false. Currently noop.
• comments, Defaults to some.
  • false removes all comments
  • 'some' preserves some comments
  • 'all' preserves all comments
• ecma, Defaults to 5. Currently noop.
• indentLevel, Currently noop and aliases as indent_level for compatibility with terser.
• indentStart, Currently noop and aliases as indent_start for compatibility with terser.
• inlineScript, Aliases as inline_script for compatibility with terser and requires @swc/core@1.9.2 or later.
• keepNumbers, Currently noop and aliases as keep_numbers for compatibility with terser.
• keepQuotedProps, Currently noop and aliases as keep_quoted_props for compatibility with terser.
• maxLineLen, Currently noop, and aliases as max_line_len for compatibility with terser.
• preamble, Supported since v1.3.66.
• quoteKeys, Currently noop and aliases as quote_keys for compatibility with terser.
• quoteStyle, Currently noop and aliases as quote_style for compatibility with terser.
• preserveAnnotations, Currently noop and aliases as preserve_annotations for compatibility with terser.
• safari10, Currently noop.
• semicolons, Currently noop.
• shebang, Currently noop.
• webkit, Currently noop.
• wrapIife, Currently noop and aliases as wrap_iife for compatibility with terser.
• wrapFuncArgs, Currently noop and aliases as wrap_func_args for compatibility with terser.

@swc/core Usage

swc.minify(code, options)

This API is asynchronous and all of parsing, minification, and code generation will be done in background thread. The options argument is same as jsc.minify object. For example:

import swc from "@swc/core";
 
const { code, map } = await swc.minify(
  "import foo from '@src/app'; console.log(foo)",
  {
    compress: false,
    mangle: true,
  }
);
 
expect(code).toMatchInlineSnapshot(`"import a from'@src/app';console.log(a);"`);

Returns Promise<{ code: string, map: string }>.

swc.minifySync(code, options)

This API exists on @swc/core, @swc/wasm, @swc/wasm-web.

import swc from "@swc/core";
 
const { code, map } = swc.minifySync(
  "import foo from '@src/app'; console.log(foo)",
  {
    compress: false,
    mangle: true,
    module: true
  }
);
 
expect(code).toMatchInlineSnapshot(`"import a from'@src/app';console.log(a);"`);

Returns { code: string, map: string }.

APIs for WebAssembly

Replacing Terser

You can reduce build time and override Terser without needing a library to update their dependencies through yarn resolutions . Example package.json would include:

{
  "resolutions": { "terser": "npm:@swc/core" }
}

This will use the SWC minifier instead of Terser for all nested dependencies. Ensure you remove your lockfile and re-install your dependencies.

$ rm -rf node_modules yarn.lock
$ yarn