Compile WebAssembly text to binary in JavaScript

Performance

A single walk over the parsed tree emits the bytes: no IR, no runtime, nothing to instantiate.

API

The tagged template compiles, instantiates and imports interpolated values in one step. TypeScript types included.

import watr from 'watr'

const { test } = watr`(func (export "test") (call ${console.log} (i32.const 42)))`
test()  // 42

parse('(i32.const 42)')
// ['i32.const', '42']

print(src)
// indented, one op per line

print(src, { indent: false, newline: false })
// (module(func(result i32)(i32.const 42)))

import polyfill from 'watr/polyfill'
compile(polyfill(src))

Options, AST format and the rest of the surface: API reference.

Optimizer

Folds constants, treeshakes, inlines, coalesces and outlines.

21-module corpus: smaller than -O3 on every module, ties or beats -Oz on 18 of 20 files.

Passes and options: optimize docs.

Conformance

CI runs the WebAssembly spec testsuite, pinned at 2026-08-12. Feature coverage, including stringref, rounding mode control and wide arithmetic, is tracked on webassembly.org. Errors report line and column.

> compile('(func (export "x"')
Error: Unclosed parenthesis at 1:18

watr compiles WebAssembly text only: C and Rust need LLVM or Emscripten, and decoding .wasm back to text is wabt's wasm2wat.

CLI

npx watr input.wat              # input.wasm
npx watr input.wat -o out.wasm  # custom output
npx watr input.wat --print      # pretty-print
npx watr input.wat --minify     # minify
npx watr input.wat --optimize   # fold, treeshake, inline, coalesce
npx watr input.wat --polyfill   # newer features to MVP

Used by

• a JS to WASM compiler.