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Bun
Bun
Optimization

Minifier

Reduce bundle sizes with Bun's JavaScript and TypeScript minifier

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 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

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 you can enable separately or together.

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.

Input
true
false
Output
!0
!1

Boolean algebra optimizations

Mode: --minify-syntax

Simplifies boolean expressions using logical rules.

Input
!!x
x === true
x && true
x || false
!true
!false
Output
x
x
x
x
!1
!0

Undefined shortening

Mode: --minify-syntax

Replaces undefined with shorter equivalent.

Input
undefined
let x = undefined;
Output
void 0
let x=void 0;

Undefined equality optimization

Mode: --minify-syntax

Optimizes loose equality checks with undefined.

Input
x == undefined
x != undefined
Output
x == null
x != null

Infinity shortening

Mode: --minify-syntax

Converts Infinity to mathematical expressions.

Input
Infinity
-Infinity
Output
1/0
-1/0

Typeof optimizations

Mode: --minify-syntax

Optimizes typeof comparisons and evaluates constant typeof expressions.

Input
typeof x === 'undefined'
typeof x !== 'undefined'
typeof require
typeof null
typeof true
typeof 123
typeof "str"
typeof 123n
Output
typeof x>'u'
typeof x<'u'
"function"
"object"
"boolean"
"number"
"string"
"bigint"

Number formatting

Mode: --minify-syntax

Formats numbers in the most compact representation.

Input
10000
100000
1000000
1.0
-42.0
Output
1e4
1e5
1e6
1
-42

Arithmetic constant folding

Mode: --minify-syntax

Evaluates arithmetic operations at compile time.

Input
1 + 2
10 - 5
3 * 4
10 / 2
10 % 3
2 ** 3
Output
3
5
12
5
1
8

Bitwise constant folding

Mode: --minify-syntax

Evaluates bitwise operations at compile time.

Input
5 & 3
5 | 3
5 ^ 3
8 << 2
32 >> 2
~5
Output
1
7
6
32
8
-6

String concatenation

Mode: --minify-syntax

Combines string literals at compile time.

Input
"a" + "b"
"x" + 123
"foo" + "bar" + "baz"
Output
"ab"
"x123"
"foobarbaz"

String indexing

Mode: --minify-syntax

Evaluates string character access at compile time.

Input
"foo"[2]
"hello"[0]
Output
"o"
"h"

Template literal folding

Mode: --minify-syntax

Evaluates template literals with constant expressions.

Input
`a${123}b`
`result: ${5 + 10}`
Output
"a123b"
"result: 15"

Template literal to string conversion

Mode: --minify-syntax

Converts template literals with no substitutions to regular strings.

Input
`Hello World`
`Line 1
Line 2`
Output
"Hello World"
"Line 1\nLine 2"

String quote optimization

Mode: --minify-syntax

Chooses the optimal quote character to minimize escapes.

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

Array spread inlining

Mode: --minify-syntax

Inlines array spread operations with constant arrays.

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

Array indexing

Mode: --minify-syntax

Evaluates constant array access at compile time.

Input
[x][0]
['a', 'b', 'c'][1]
['a', , 'c'][1]
Output
x
'b'
void 0

Property access optimization

Mode: --minify-syntax

Converts bracket notation to dot notation when possible.

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

Comparison folding

Mode: --minify-syntax

Evaluates constant comparisons at compile time.

Input
3 < 5
5 > 3
3 <= 3
5 >= 6
"a" < "b"
Output
!0
!0
!0
!1
!0

Logical operation folding

Mode: --minify-syntax

Simplifies logical operations with constant values.

Input
true && x
false && x
true || x
false || x
Output
x
!1
!0
x

Nullish coalescing folding

Mode: --minify-syntax

Evaluates nullish coalescing with known values.

Input
null ?? x
undefined ?? x
42 ?? x
Output
x
x
42

Comma expression simplification

Mode: --minify-syntax

Removes side-effect-free expressions from comma sequences.

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

Ternary conditional folding

Mode: --minify-syntax

Evaluates conditional expressions with constant conditions.

Input
true ? a : b
false ? a : b
x ? true : false
x ? false : true
Output
a
b
x ? !0 : !1
x ? !1 : !0

Unary expression folding

Mode: --minify-syntax

Simplifies unary operations.

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

Double negation removal

Mode: --minify-syntax

Removes unnecessary double negations.

Input
!!x
!!!x
Output
x
!x

If statement optimization

Mode: --minify-syntax

Optimizes if statements with constant conditions.

Input
if (true) x;
if (false) x;
if (x) { a; }
if (x) {} else y;
Output
x;
// removed
if(x)a;
if(!x)y;

Dead code elimination

Mode: --minify-syntax

Removes unreachable code and code without side effects.

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

Unreachable branch removal

Mode: --minify-syntax

Removes branches that can never execute.

Input
while (false) {
  neverRuns();
}
Output
// removed entirely

Empty block removal

Mode: --minify-syntax

Removes empty blocks and unnecessary braces.

Input
{ }
if (x) { }
Output
;
// removed

Single statement block unwrapping

Mode: --minify-syntax

Removes unnecessary braces around single statements.

Input
if (condition) {
  doSomething();
}
Output
if(condition)doSomething();

TypeScript enum inlining

Mode: --minify-syntax

Inlines TypeScript enum values at compile time.

Input
enum Color { Red, Green, Blue }
const x = Color.Red;
Output
const x=0;

Pure annotation support

Mode: Always active

Respects /*@__PURE__*/ annotations for tree shaking (removing unused code).

Input
const x = /*@__PURE__*/ expensive();
// If x is unused...
Output
// removed entirely

Identifier renaming

Mode: --minify-identifiers

Renames local variables to shorter names based on usage frequency.

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

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

Semicolon optimization

Mode: --minify-whitespace

Inserts semicolons only when necessary.

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

Operator spacing removal

Mode: --minify-whitespace

Removes spaces around operators.

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

Comment removal

Mode: --minify-whitespace

Removes comments except important license comments.

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

Object and array formatting

Mode: --minify-whitespace

Removes whitespace in object and array literals.

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

Control flow formatting

Mode: --minify-whitespace

Removes whitespace in control structures.

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

Function formatting

Mode: --minify-whitespace

Removes whitespace in function declarations.

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

Parentheses minimization

Mode: Always active

Only adds parentheses when necessary for operator precedence.

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

Template literal value folding

Mode: --minify-syntax

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

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

Input
"hello world".length
"test".length
Output
11
4

Constructor call simplification

Mode: --minify-syntax

Simplifies constructor calls for built-in types.

Input
new Object()
new Object(null)
new Object({a: 1})
new Array()
new Array(x, y)
Output
{}
{}
{a:1}
[]
[x,y]

Single property object inlining

Mode: --minify-syntax

Inlines property access for objects with a single property.

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

String charCodeAt constant folding

Mode: Always active

Evaluates charCodeAt() on string literals for ASCII characters.

Input
"hello".charCodeAt(1)
"A".charCodeAt(0)
Output
101
65

Void 0 equality to null equality

Mode: --minify-syntax

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

Input
x == void 0
x != void 0
Output
x == null
x != null

Negation operator optimization

Mode: --minify-syntax

Moves negation operator through comma expressions.

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

Import.meta property inlining

Mode: Bundle mode

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

Input
import.meta.dir
import.meta.file
import.meta.path
import.meta.url
Output
"/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.

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

Expression statement merging

Mode: --minify-syntax

Merges adjacent expression statements using comma operator.

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

Return statement merging

Mode: --minify-syntax

Merges expressions before return with comma operator.

Input
console.log(x);
return y;
Output
return console.log(x),y;

Throw statement merging

Mode: --minify-syntax

Merges expressions before throw with comma operator.

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

TypeScript enum cross-module inlining

Mode: --minify-syntax (bundle mode)

Inlines enum values across module boundaries.

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

// Input (main.ts)
import { Color } from './lib';
const x = Color.Red;
Output
const x=0;

Computed property enum inlining

Mode: --minify-syntax

Inlines enum values used as computed object properties.

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

Arrow function body shortening

Mode: --minify-syntax

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

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

Object property shorthand

Mode: Always active

Uses shorthand syntax when property name and value identifier match.

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

Drop debugger statements

Mode: --drop=debugger

Removes debugger statements from code.

Input
function test() {
  debugger;
  return x;
}
Output
function test(){return x}

Drop console calls

Mode: --drop=console

Removes all console.* method calls from code.

Input
console.log("debug");
console.warn("warning");
x = console.error("error");
Output
void 0;
void 0;
x=void 0;

Drop custom function calls

Mode: --drop=<name>

Removes calls to specified global functions and their methods.

Input
assert(condition);
assert.equal(a, b);
Output with
void 0;
void 0;

Keep Names

To keep original function and class names for debugging while minifying identifiers, 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,
  },
});

--keep-names preserves the .name property on functions and classes while still minifying the identifiers themselves.

Combined Example

Using all three minification modes together:

input.ts (158 bytes)
const myVariable = 42;

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

const output = myFunction();
output.js
// 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: 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
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On this page

CLI UsageEnable all minificationProduction modeGranular controlJavaScript APIMinification ModesWhitespace minification (--minify-whitespace)Syntax minification (--minify-syntax)Identifier minification (--minify-identifiers)All TransformationsBoolean literal shorteningBoolean algebra optimizationsUndefined shorteningUndefined equality optimizationInfinity shorteningTypeof optimizationsNumber formattingArithmetic constant foldingBitwise constant foldingString concatenationString indexingTemplate literal foldingTemplate literal to string conversionString quote optimizationArray spread inliningArray indexingProperty access optimizationComparison foldingLogical operation foldingNullish coalescing foldingComma expression simplificationTernary conditional foldingUnary expression foldingDouble negation removalIf statement optimizationDead code eliminationUnreachable branch removalEmpty block removalSingle statement block unwrappingTypeScript enum inliningPure annotation supportIdentifier renamingWhitespace removalSemicolon optimizationOperator spacing removalComment removalObject and array formattingControl flow formattingFunction formattingParentheses minimizationTemplate literal value foldingString length constant foldingConstructor call simplificationSingle property object inliningString charCodeAt constant foldingVoid 0 equality to null equalityNegation operator optimizationImport.meta property inliningVariable declaration mergingExpression statement mergingReturn statement mergingThrow statement mergingTypeScript enum cross-module inliningComputed property enum inliningArrow function body shorteningObject property shorthandDrop debugger statementsDrop console callsDrop custom function callsKeep NamesCombined ExampleWhen to Use Minification