Web Assembly

A powerful low-level language that is meant to be a compile target of higher-level languages.

You are not meant to write web assembly directly.

It is a secure, sandbox area that can operate at near-native speed.


  1. OpCodes
  2. Interactive OpCode table

Why a web assembly and not web worker?

Web assembly can be cached, whereas a web worker needs to be spun up everytime.

Web assembly is also not targeted to JavaScript engineers.

Files types

Web assembly has two different types of files.

File typeDoes
.wasmActual assembly code in binary
.watHuman-readable textual representation of code

The .wat files are only used for editing/debugging and also compiles to WASM.


The fundamental unit of code is a module. Within a module, we create functions to export which can be called by JavaScript.

Func params are known as locals and we access them with either get_local or local.get:

(module (func $name (param $param i32) (result i32) ;; Body ) )

Web Assembly module is a tree-based structure known as an S-expression.

Web Assmebly Studio can be used to write the first hello world.

Our function will take a 32-bit integer and return one:

;; main.wat (module (func $helloworld (param $num1 i32) (result i32) get_local $num1 ) (export "helloworld" (func $helloworld)) ;; export helloworld out )

Once written, we run build to output a main.wasm file.

Don't fret too much yet about what is happening in the JS file. It needs to be update to call helloworld:

fetch("../out/main.wasm") .then((response) => response.arrayBuffer()) .then((bytes) => WebAssembly.instantiate(bytes)) .then((results) => { instance = results.instance document.getElementById( "container" ).textContent = instance.exports.helloworld(42) }) .catch(console.error)

Stack & OpCode


WebAssembly is a stack machine. It is either pushed or popped off the stack.

This is a machine stack, not what you are used to in JS land.

Code like get_local that we wrote will push the value onto the stack.

There is no needs to allocate/deallocate with this simple machine stack.


Operation codes are readable computer instructions representing machine language instructions.

Terms that we used map to these hexadecimal codes:

OpCodeHex Representation

A full list of codes can be found here.

OpCodes are specific to the data type.

Instruction Stack

All WA instructions read and write from the stack.

get_local 0 ;; push first parameter onto the stack get_local 1 ;; push second parameter onto the stack i32.mul ;; pop both values and execute operation

To push a value onto the stack use the i32.const instruction

i32.const 99 ;; push 99 onto the stack

Minusone exercise

Creating a function that takes an i32 and substracts 1:

(module (export "minusone" (func $minusone)) (func $minusone (param $x i32) (result i32) get_local $x i32.const 1 i32.sub ) )

"More complex" web assembly

The following JS:

function example(n) { if (n === 2) { return n * 2 } if (n === 3) { return n * 3 } return n * n }

Converts into this:

(func $example (param $0 i32) (result i32) get_local $0 i32.const 2 i32.eq if get_local $0 i32.const 2 i32.mul return end get_local $0 i32.const 3 i32.eq if get_local $0 i32.const 3 i32.mul return end get_local $0 get_local $0 i32.mul )

This is not so intuitive to know, so for more complicated use-cases we can use AssemblyScript.