jarle/Bun Minifier

Reduce bundle sizes with Bun's JavaScript and TypeScript minifier

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Bun includes a fast JavaScript and TypeScript minifier that can reduce bundle sizes by 80% or more (depending on the codebase) and make output code run faster. The minifier performs dozens of optimizations including constant folding, dead code elimination, and syntax transformations. Unlike other minifiers, Bun's minifier makes bun build run faster since there's less code to print.

CLI Usage

Enable all minification

Use the --minify flag to enable all minification modes:

bun build ./index.ts --minify --outfile=out.js

The --minify flag automatically enables:

• Whitespace minification
• Syntax minification
• Identifier minification

Production mode

The --production flag automatically enables minification:

bun build ./index.ts --production --outfile=out.js

The --production flag also:

• Sets process.env.NODE_ENV to production
• Enables the production-mode JSX import & transform

Granular control

You can enable specific minification modes individually:

# Only remove whitespace
bun build ./index.ts --minify-whitespace --outfile=out.js

# Only minify syntax
bun build ./index.ts --minify-syntax --outfile=out.js

# Only minify identifiers
bun build ./index.ts --minify-identifiers --outfile=out.js

# Combine specific modes
bun build ./index.ts --minify-whitespace --minify-syntax --outfile=out.js

JavaScript API

When using Bun's bundler programmatically, configure minification through the minify option:

await Bun.build({
  entrypoints: ["./index.ts"],
  outdir: "./out",
  minify: true, // Enable all minification modes
});

For granular control, pass an object:

await Bun.build({
  entrypoints: ["./index.ts"],
  outdir: "./out",
  minify: {
    whitespace: true,
    syntax: true,
    identifiers: true,
  },
});

Minification Modes

Bun's minifier has three independent modes that can be enabled separately or combined.

Whitespace minification (--minify-whitespace)

Removes all unnecessary whitespace, newlines, and formatting from the output.

Syntax minification (--minify-syntax)

Rewrites JavaScript syntax to shorter equivalent forms and performs constant folding, dead code elimination, and other optimizations.

Identifier minification (--minify-identifiers)

Renames local variables and function names to shorter identifiers using frequency-based optimization.

All Transformations

Boolean literal shortening

Mode: --minify-syntax

Converts boolean literals to shorter expressions.

true
false

!0
!1

Boolean algebra optimizations

Mode: --minify-syntax

Simplifies boolean expressions using logical rules.

!!x
x === true
x && true
x || false
!true
!false

x
x
x
x
!1
!0

Undefined shortening

Mode: --minify-syntax

Replaces undefined with shorter equivalent.

undefined
let x = undefined;

void 0
let x=void 0;

Undefined equality optimization

Mode: --minify-syntax

Optimizes loose equality checks with undefined.

x == undefined
x != undefined

x == null
x != null

Infinity shortening

Mode: --minify-syntax

Converts Infinity to mathematical expressions.

Infinity
-Infinity

1/0
-1/0

Typeof optimizations

Mode: --minify-syntax

Optimizes typeof comparisons and evaluates constant typeof expressions.

typeof x === 'undefined'
typeof x !== 'undefined'
typeof require
typeof null
typeof true
typeof 123
typeof "str"
typeof 123n

typeof x>'u'
typeof x<'u'
"function"
"object"
"boolean"
"number"
"string"
"bigint"

Number formatting

Mode: --minify-syntax

Formats numbers in the most compact representation.

10000
100000
1000000
1.0
-42.0

1e4
1e5
1e6
1
-42

Arithmetic constant folding

Mode: --minify-syntax

Evaluates arithmetic operations at compile time.

1 + 2
10 - 5
3 * 4
10 / 2
10 % 3
2 ** 3

3
5
12
5
1
8

Bitwise constant folding

Mode: --minify-syntax

Evaluates bitwise operations at compile time.

5 & 3
5 | 3
5 ^ 3
8 << 2
32 >> 2
~5

1
7
6
32
8
-6

String concatenation

Mode: --minify-syntax

Combines string literals at compile time.

"a" + "b"
"x" + 123
"foo" + "bar" + "baz"

"ab"
"x123"
"foobarbaz"

String indexing

Mode: --minify-syntax

Evaluates string character access at compile time.

"foo"[2]
"hello"[0]

"o"
"h"

Template literal folding

Mode: --minify-syntax

Evaluates template literals with constant expressions.

`a${123}b`
`result: ${5 + 10}`

"a123b"
"result: 15"

Template literal to string conversion

Mode: --minify-syntax

Converts simple template literals to regular strings.

`Hello World`
`Line 1
Line 2`

"Hello World"
"Line 1\nLine 2"

String quote optimization

Mode: --minify-syntax

Chooses the optimal quote character to minimize escapes.

"It's a string"
'He said "hello"'
`Simple string`

"It's a string"
'He said "hello"'
"Simple string"

Array spread inlining

Mode: --minify-syntax

Inlines array spread operations with constant arrays.

[1, ...[2, 3], 4]
[...[a, b]]

[1,2,3,4]
[a,b]

Array indexing

Mode: --minify-syntax

Evaluates constant array access at compile time.

[x][0]
['a', 'b', 'c'][1]
['a', , 'c'][1]

x
'b'
void 0

Property access optimization

Mode: --minify-syntax

Converts bracket notation to dot notation when possible.

obj["property"]
obj["validName"]
obj["123"]
obj["invalid-name"]

obj.property
obj.validName
obj["123"]
obj["invalid-name"]

Comparison folding

Mode: --minify-syntax

Evaluates constant comparisons at compile time.

3 < 5
5 > 3
3 <= 3
5 >= 6
"a" < "b"

!0
!0
!0
!1
!0

Logical operation folding

Mode: --minify-syntax

Simplifies logical operations with constant values.

true && x
false && x
true || x
false || x

x
!1
!0
x

Nullish coalescing folding

Mode: --minify-syntax

Evaluates nullish coalescing with known values.

null ?? x
undefined ?? x
42 ?? x

x
x
42

Comma expression simplification

Mode: --minify-syntax

Removes side-effect-free expressions from comma sequences.

(0, x)
(123, "str", x)

x
x

Ternary conditional folding

Mode: --minify-syntax

Evaluates conditional expressions with constant conditions.

true ? a : b
false ? a : b
x ? true : false
x ? false : true

a
b
x ? !0 : !1
x ? !1 : !0

Unary expression folding

Mode: --minify-syntax

Simplifies unary operations.

+123
+"123"
-(-x)
~~x
!!x

123
123
123
123
x
~~x
!!x
x

Double negation removal

Mode: --minify-syntax

Removes unnecessary double negations.

!!x
!!!x

x
!x

If statement optimization

Mode: --minify-syntax

Optimizes if statements with constant conditions.

if (true) x;
if (false) x;
if (x) { a; }
if (x) {} else y;

x;
// removed
if(x)a;
if(!x)y;

Dead code elimination

Mode: --minify-syntax

Removes unreachable code and code without side effects.

if (false) {
  unreachable();
}
function foo() {
  return x;
  deadCode();
}

function foo(){return x}

Unreachable branch removal

Mode: --minify-syntax

Removes branches that can never execute.

while (false) {
  neverRuns();
}

// removed entirely

Empty block removal

Mode: --minify-syntax

Removes empty blocks and unnecessary braces.

{ }
if (x) { }

;
// removed

Single statement block unwrapping

Mode: --minify-syntax

Removes unnecessary braces around single statements.

if (condition) {
  doSomething();
}

if(condition)doSomething();

TypeScript enum inlining

Mode: --minify-syntax

Inlines TypeScript enum values at compile time.

enum Color { Red, Green, Blue }
const x = Color.Red;

const x=0;

Pure annotation support

Mode: Always active

Respects /*@__PURE__*/ annotations for tree shaking.

const x = /*@__PURE__*/ expensive();
// If x is unused...

// removed entirely

Identifier renaming

Mode: --minify-identifiers

Renames local variables to shorter names based on usage frequency.

function calculateSum(firstNumber, secondNumber) {
  const result = firstNumber + secondNumber;
  return result;
}

function a(b,c){const d=b+c;return d}

Naming strategy:

• Most frequently used identifiers get the shortest names (a, b, c...)
• Single letters: a-z (26 names)
• Double letters: aa-zz (676 names)
• Triple letters and beyond as needed

Preserved identifiers:

• JavaScript keywords and reserved words
• Global identifiers
• Named exports (to maintain API)
• CommonJS names: exports, module

Whitespace removal

Mode: --minify-whitespace

Removes all unnecessary whitespace.

function add(a, b) {
    return a + b;
}
let x = 10;

function add(a,b){return a+b;}let x=10;

Semicolon optimization

Mode: --minify-whitespace

Inserts semicolons only when necessary.

let a = 1;
let b = 2;
return a + b;

let a=1;let b=2;return a+b

Operator spacing removal

Mode: --minify-whitespace

Removes spaces around operators.

a + b
x = y * z
foo && bar || baz

a+b
x=y*z
foo&&bar||baz

Comment removal

Mode: --minify-whitespace

Removes comments except important license comments.

// This comment is removed
/* So is this */
/*! But this license comment is kept */
function test() { /* inline comment */ }

/*! But this license comment is kept */
function test(){}

Object and array formatting

Mode: --minify-whitespace

Removes whitespace in object and array literals.

const obj = {
    name: "John",
    age: 30
};
const arr = [1, 2, 3];

const obj={name:"John",age:30};const arr=[1,2,3];

Control flow formatting

Mode: --minify-whitespace

Removes whitespace in control structures.

if (condition) {
    doSomething();
}
for (let i = 0; i < 10; i++) {
    console.log(i);
}

if(condition)doSomething();for(let i=0;i<10;i++)console.log(i);

Function formatting

Mode: --minify-whitespace

Removes whitespace in function declarations.

function myFunction(param1, param2) {
    return param1 + param2;
}
const arrow = (a, b) => a + b;

function myFunction(a,b){return a+b}const arrow=(a,b)=>a+b;

Parentheses minimization

Mode: Always active

Only adds parentheses when necessary for operator precedence.

(a + b) * c
a + (b * c)
((x))

(a+b)*c
a+b*c
x

Property mangling

Mode: --minify-identifiers (with configuration)

Renames object properties to shorter names when configured.

obj.longPropertyName

obj.a

Template literal value folding

Mode: --minify-syntax

Converts non-string interpolated values to strings and folds them into the template.

`hello ${123}`
`value: ${true}`
`result: ${null}`
`status: ${undefined}`
`big: ${10n}`

"hello 123"
"value: true"
"result: null"
"status: undefined"
"big: 10"

String length constant folding

Mode: --minify-syntax

Evaluates .length property on string literals at compile time.

"hello world".length
"test".length

11
4

Constructor call simplification

Mode: --minify-syntax

Simplifies constructor calls for built-in types.

new Object()
new Object(null)
new Object({a: 1})
new Array()
new Array(x, y)

{}
{}
{a:1}
[]
[x,y]

Single property object inlining

Mode: --minify-syntax

Inlines property access for objects with a single property.

({fn: () => console.log('hi')}).fn()

(() => console.log('hi'))()

String charCodeAt constant folding

Mode: Always active

Evaluates charCodeAt() on string literals for ASCII characters.

"hello".charCodeAt(1)
"A".charCodeAt(0)

101
65

Void 0 equality to null equality

Mode: --minify-syntax

Converts loose equality checks with void 0 to null since they're equivalent.

x == void 0
x != void 0

x == null
x != null

Negation operator optimization

Mode: --minify-syntax

Moves negation operator through comma expressions.

-(a, b)
-(x, y, z)

a,-b
x,y,-z

Import.meta property inlining

Mode: Bundle mode

Inlines import.meta properties at build time when values are known.

import.meta.dir
import.meta.file
import.meta.path
import.meta.url

"/path/to/directory"
"filename.js"
"/full/path/to/file.js"
"file:///full/path/to/file.js"

Variable declaration merging

Mode: --minify-syntax

Merges adjacent variable declarations of the same type.

let a = 1;
let b = 2;
const c = 3;
const d = 4;

let a=1,b=2;
const c=3,d=4;

Expression statement merging

Mode: --minify-syntax

Merges adjacent expression statements using comma operator.

console.log(1);
console.log(2);
console.log(3);

console.log(1),console.log(2),console.log(3);

Return statement merging

Mode: --minify-syntax

Merges expressions before return with comma operator.

console.log(x);
return y;

return console.log(x),y;

Throw statement merging

Mode: --minify-syntax

Merges expressions before throw with comma operator.

console.log(x);
throw new Error();

throw(console.log(x),new Error());

TypeScript enum cross-module inlining

Mode: --minify-syntax (bundle mode)

Inlines enum values across module boundaries.

// lib.ts
export enum Color { Red, Green, Blue }

// Input (main.ts)
import { Color } from './lib';
const x = Color.Red;

const x=0;

Computed property enum inlining

Mode: --minify-syntax

Inlines enum values used as computed object properties.

enum Keys { FOO = 'foo' }
const obj = { [Keys.FOO]: value }

const obj={foo:value}

String number to numeric index

Mode: --minify-syntax

Converts string numeric property access to numeric index.

obj["0"]
arr["5"]

obj[0]
arr[5]

Arrow function body shortening

Mode: Always active

Uses expression body syntax when an arrow function only returns a value.

() => { return x; }
(a) => { return a + 1; }

() => x
a => a + 1

Object property shorthand

Mode: Always active

Uses shorthand syntax when property name and value identifier match.

{ x: x, y: y }
{ name: name, age: age }

{ x, y }
{ name, age }

Method shorthand

Mode: Always active

Uses method shorthand syntax in object literals.

{
  foo: function() {},
  bar: async function() {}
}

{
  foo() {},
  async bar() {}
}

Drop debugger statements

Mode: --drop=debugger

Removes debugger statements from code.

function test() {
  debugger;
  return x;
}

function test(){return x}

Drop console calls

Mode: --drop=console

Removes all console.* method calls from code.

console.log("debug");
console.warn("warning");
x = console.error("error");

void 0;
void 0;
x=void 0;

Drop custom function calls

Mode: --drop=<name>

Removes calls to specified global functions or methods.

assert(condition);
obj.assert(test);

void 0;
void 0;

Keep Names

When minifying identifiers, you may want to preserve original function and class names for debugging purposes. Use the --keep-names flag:

bun build ./index.ts --minify --keep-names --outfile=out.js

Or in the JavaScript API:

await Bun.build({
  entrypoints: ["./index.ts"],
  outdir: "./out",
  minify: {
    identifiers: true,
    keepNames: true,
  },
});

This preserves the .name property on functions and classes while still minifying the actual identifier names in the code.

Combined Example

Using all three minification modes together:

const myVariable = 42;

const myFunction = () => {
  const isValid = true;
  const result = undefined;
  return isValid ? myVariable : result;
};

const output = myFunction();

// Output with --minify (49 bytes, 69% reduction)
const a=42,b=()=>{const c=!0,d=void 0;return c?a:d},e=b();

When to Use Minification

Use --minify for:

• Production bundles
• Reducing CDN bandwidth costs
• Improving page load times

Use individual modes for:

• --minify-whitespace: Quick size reduction without semantic changes
• --minify-syntax: Smaller output while keeping readable identifiers for debugging
• --minify-identifiers: Maximum size reduction (combine with --keep-names for better stack traces)

Avoid minification for:

• Development builds (harder to debug)
• When you need readable error messages
• Libraries where consumers may read the source