What is WAT?

WAT (WebAssembly Text Format) is the human-readable form of WebAssembly.

Where .wasm files contain compact binary bytecode, .wat files express the same instructions as plain text you can read and edit directly.

(module
  ;; import log
  (import "env" "log" (func $log (param i32)))
  ;; export the import
  (export "log" (func $log))
)

WebAssembly in brief

WebAssembly (Wasm) is a portable compilation target — a low-level language that cleanly maps to most hardware architectures without significant performance loss.

Languages like C, C++, Rust, and Go can compile to Wasm. Runtimes exist that can execute it in browsers, servers, and embedded environments.

Wasm uses a deny-by-default security model: modules cannot access anything (files, network, DOM) unless the host explicitly provides it via imports. This makes it well-suited for running untrusted code safely.

How WAT relates to Wasm

WAT and Wasm are two representations of the same thing.

The conversion between them is almost 1:1 — you can compile WAT to Wasm and decompile it back with barely any loss of information. WAT exists so humans can inspect, debug, and hand-write WebAssembly without staring at hex dumps.

A small example:

(module
  (func (export "add") (param $a i32) (param $b i32) (result i32)
    (i32.add (local.get $a) (local.get $b)))
)

binary output

00000000: 0061 736d 0100 0000 0107 0160 027f 7f01  .asm.......`....
00000010: 7f03 0201 0007 0701 0361 6464 0000 0a09  .........add....
00000020: 0107 0020 0020 016a 0b                   ... . .j.

This module exports a single function add that takes two 32-bit integers and returns their sum. The parenthesized syntax (S-expressions) may look unusual, but every construct maps directly to a binary instruction.

Why learn WAT?

• Understand what compilers produce. When Rust or C compiles to Wasm, WAT lets you see exactly what ended up in the binary.
• Debug at the lowest level. Browser devtools can show you WAT when stepping through Wasm modules.
• Learn how Wasm actually works. Writing WAT by hand is the most direct way to understand the stack machine, type system, and module structure.