watr [](https://npmjs.org/watr) [](https://github.com/dy/watr/actions/workflows/test.js.yml)

Light & fast WAT compiler

• feature & spec-complete, zero deps
• compile · polyfill · optimize · parse · prettify · minify
• instant wasm, JS interop, CLI, clear errors

docs  ·  demo

Usage

import watr, { compile, parse, print } from 'watr'

// compile to binary
const src = '(func (export "f") (result f64) (f64.const 1))'
const binary = compile(src)
const module = new WebAssembly.Module(binary)
const { f } = new WebAssembly.Instance(module).exports

// optional, heavy transforms ship as separate entries — compose them
import optimize from 'watr/optimize'   // fold constants, treeshake, eliminate dead code …
import polyfill from 'watr/polyfill'   // newer features → MVP
compile(optimize(polyfill(src)))

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

// print
print('(module(func(result i32)i32.const 42))') // (module\n  (func (result i32)\n  ...

// instant wasm function
const { add } = watr`(func (export "add") (param i32 i32) (result i32)
  (i32.add (local.get 0) (local.get 1))
)`
add(2, 3) // 5

// instant wasm: interpolate, auto-import ...
const { test } = watr`(func (export "test") (call ${console.log} (i32.const 42)))`
test() // logs 42

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 → MVP

Metrics

• watr — ~44 KB minified (~14 KB gzipped), 3,123 op/s
• spec/wast.js — 216 KB, 1,509 op/s
• wabt — 282 KB, 859 op/s
• binaryen — 1,100 KB, 473 op/s
• wat-compiler — ~152 KB (+ wabt dep), 355 op/s

Optimizer vs binaryen (wasm-opt 128)

	size (total)	time (batch)	footprint
watr/optimize	19,734 B	~110 ms in-process	149 KB min (46 KB gz)
wasm-opt -Oz	19,852 B	990 ms CLI	~1.1 MB js / native binary
wasm-opt -O3	22,302 B	—	—

^{Measured on test/example (21 modules, optimize(src) defaults vs wasm-opt -all).
Smaller than -O3 on every module and than -Oz in aggregate (19,734 vs 19,852 B); ties or beats -Oz on 18 of 20 files. On a 5.5 MB module watr's output is 1.7% SMALLER than -Oz (261,345 vs 265,977 B — cross-function outlining); binaryen's native core is faster there (0.44 s vs ~2.6 s optimize + 0.19 s compile) — watr's edge is output size, batch/in-process use with no process spawn, and a ~25× smaller footprint.}

Used by

• jz – minimal static JS subset

ॐ