oxc-project__oxc/crates/oxc_minifier/docs/OPTIMIZATIONS.md

Optimization Catalog

Complete list of optimizations for maximum size reduction.

Many optimizations rely on oxc_ecmascript for ECMAScript operations like constant evaluation, type conversion, and side effect analysis.

Current Optimizations (18)

Constant Folding

Module: fold_constants.rs Size Impact: High Description: Evaluates expressions at compile time using oxc_ecmascript::constant_evaluation

// Before
2 + 3;
"a" + "b";

// After
5;
"ab";

Dead Code Elimination

Module: remove_dead_code.rs Size Impact: Very High Description: Removes unreachable code

// Before
if (false) {
  console.log("never runs");
}

// After
// (removed entirely)

Control Flow Optimization

Module: minimize_conditions.rs, minimize_if_statement.rs Size Impact: Medium-High Description: Simplifies conditional logic and if statements

// Before
if (x) return true;
else return false;

// After
return !!x;

// Before
if (a) b();

// After
a && b();

Expression Simplification

Module: minimize_logical_expression.rs, minimize_not_expression.rs Size Impact: Medium Description: Simplifies boolean, logical, and negation expressions

// Before
!(!x || !y);

// After
x && y;

// Before
!!true;

// After
true;

Syntax Substitution

Module: substitute_alternate_syntax.rs Size Impact: High Description: Uses shorter equivalent syntax leveraging oxc_ecmascript type conversions

// Before
true;
false;
undefined;

// After
!0;
!1;
void 0;

Property Access Optimization

Module: convert_to_dotted_properties.rs Size Impact: Medium Description: Converts bracket notation to dot notation when safe

// Before
obj["property"];

// After
obj.property;

Template Literal Optimization

Module: inline.rs Size Impact: Medium Description: Simplifies template literals

// Before
`hello ${"world"}`;

// After
"hello world";

Built-in Method Replacement

Module: replace_known_methods.rs Size Impact: High Description: Optimizes known built-in method calls

// Before
"test".indexOf("e");
Math.pow(2, 3);

// After
"test".indexOf("e"); // or optimized form
2 ** 3;

For Statement Optimization

Module: minimize_for_statement.rs Size Impact: Low-Medium Description: Optimizes for loops

// Before
for (;;) {}

// After
for (;;);

Statement-Level Optimizations

Module: minimize_statements.rs Size Impact: Medium Description: Combines and simplifies statements

// Before
a();
b();

// After (with sequences option)
a(), b();

Unused Code Removal

Module: remove_unused_declaration.rs, remove_unused_expression.rs Size Impact: High Description: Removes unused variables and simplifies unused expressions

// Before
let unused = 5;
console.log("hello");

// After
console.log("hello");

// Before
[1, 2, fn()]; // unused array with side effects

// After
fn(); // keep only side effects

Normalization

Module: normalize.rs Size Impact: Enables other optimizations Description: Converts code to canonical form

// Example: while -> for conversion
// Before
while (true) {}

// After
for (;;) {}

Conditional Expression Optimization

Module: minimize_conditional_expression.rs Size Impact: Medium Description: Optimizes ternary operators

// Before
x ? true : false;

// After
!!x;

Boolean Context Optimization

Module: minimize_expression_in_boolean_context.rs Size Impact: Low-Medium Description: Simplifies expressions in boolean contexts

// Before
if (x === true) {
}

// After
if (x) {
}

Variable Inlining

Module: inline.rs Size Impact: Medium Description: Inlines single-use variables

// Before
const x = 5;
console.log(x);

// After
console.log(5);

Planned Optimizations

From Closure Compiler

• Cross-module constant propagation: Track constants across files
• Advanced function inlining: Inline functions when beneficial
• Enum unboxing: Replace enum objects with primitives
• Property collapsing: Flatten nested property access
• Method devirtualization: Direct method calls when possible

From Terser

• Switch statement optimization: Simplify switch statements
• Advanced array/object patterns: Recognize and optimize patterns
• String optimizations: Join strings, optimize concatenation
• RegExp optimizations: Simplify regular expressions

From esbuild

• CSS-in-JS optimization: Optimize styled components
• Import optimization: Remove unused imports
• Enum inlining: Replace enum access with values

Size Optimization Techniques

1. Multiple passes: Run optimizations until fixed-point
2. Optimization ordering: Some optimizations enable others
3. Pattern matching: Recognize common code patterns
4. Syntactic substitution: Always prefer shorter forms
5. Exhaustive application: Apply everywhere possible

How Optimizations Interact

Some optimizations enable others:

• Normalization → enables better pattern matching
• Constant folding → enables dead code elimination
• Inlining → enables further constant folding
• Dead code removal → enables more inlining

The fixed-point iteration ensures all optimization opportunities are found.