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name wasm
description Modern WebAssembly (Wasm) development expertise covering modern Wasm features and optimization techniques. Use this skill when working with WebAssembly modules or optimizing Wasm performance.
compatibility WebAssembly v3.0 and later

WebAssembly Development Skill

WAT Syntax

Use folded (S-expression) syntax for readability:

;; Folded syntax (preferred)
(i32.add (local.get $x) (local.get $y))

;; Flat syntax (avoid)
local.get $x
local.get $y
i32.add

WebAssembly Features

Memory64 (64-bit Address Space)

  • Memories and tables use i64 as address type
  • Expands addressable space from 4GB to 16 exabytes
  • Syntax: (memory i64 1) instead of (memory 1)

Multiple Memories

(module
  (memory $main 1)
  (memory $scratch 1))

Tail Call Optimization

  • Efficient recursion via return_call and return_call_indirect
  • Prevents stack overflow for tail-recursive functions
(func $factorial (param $n i64) (param $acc i64) (result i64)
  (if (result i64) (i64.eqz (local.get $n))
    (then (local.get $acc))
    (else (return_call $factorial
      (i64.sub (local.get $n) (i64.const 1))
      (i64.mul (local.get $n) (local.get $acc))))))

Exception Handling

  • Native try/catch/throw semantics
  • Interoperates with JavaScript exceptions
(tag $error (param i32))
(func $may_throw
  (throw $error (i32.const 42)))

Relaxed SIMD

  • Hardware-dependent SIMD optimizations beyond fixed-width 128-bit
  • i8x16.relaxed_swizzle, f32x4.relaxed_madd, etc.

WasmGC

  • Native garbage-collected types: struct, array
  • Instructions: array.new, array.get, array.set, struct.new, struct.get
  • Reference types: (ref $type), (ref null $type)

externref

  • Opaque reference to host (JS) objects
  • Cannot be inspected or modified in Wasm, only passed around
  • Used with js-string-builtins for efficient string handling

js-string-builtins

  • Import "wasm:js-string" for direct JS string operations
  • Functions: length, charCodeAt, fromCharCodeArray, intoCharCodeArray
  • Avoids costly JS↔Wasm boundary crossings for string processing

SIMD Example

;; Process 16 bytes at a time
(v128.store (local.get $dst)
  (i8x16.add
    (v128.load (local.get $src1))
    (v128.load (local.get $src2))))

Toolchain (Binaryen)

Task Command
Assemble WAT to Wasm wasm-as module.wat -o module.wasm
Disassemble Wasm to WAT wasm-dis module.wasm -o module.wat
Optimize for size wasm-opt -Oz in.wasm -o out.wasm
Optimize for speed wasm-opt -O3 in.wasm -o out.wasm

Resources