Building a JavaScript Compiler with WebAssembly

WebAssembly (Wasm) has opened a world of possibilities for web developers, allowing them to run code on the web at near-native speed. This has significant implications for performance-intensive applications, including games, music streaming, and even compilers. In this tutorial, we will delve into the creation of a JavaScript compiler that targets WebAssembly. We’ll start with an understanding of compilers and WebAssembly before jumping into the practical steps.

Understanding Compilers and JavaScript Interpretation

A compiler is a program that translates code written in one programming language into another. In the context of JavaScript, most modern browsers have a Just-In-Time (JIT) compiler that compiles JavaScript to optimized machine code at runtime.

To build a JavaScript compiler, we must understand the structure of the language, parsing, syntax trees, and ultimately, code generation. JavaScript is an interpreted language, meaning it is typically parsed and executed by a JavaScript engine (like V8 in Chrome or SpiderMonkey in Firefox) at runtime.

WebAssembly Basics

WebAssembly is a binary instruction format that serves as a compilation target for languages like C, C++, and Rust, allowing them to run on the web. It is designed to be fast, safe, and portable. Unlike JavaScript, WebAssembly is compiled and executed at near-native speed by leveraging common hardware capabilities.

Writing a Simple Compiler in JavaScript

Define the Language Subset or DSL

Before we begin, decide on the subset of JavaScript or a Domain-Specific Language (DSL) that you wish to compile. For simplicity, let’s consider a DSL that includes variables, arithmetic operations, and basic function calls.


Parsing is the process of analyzing a string of symbols, either in natural language or computer languages, according to the rules of a formal grammar. For our compiler, we’ll need a parser that can read our DSL and produce an Abstract Syntax Tree (AST).

// Example of a simple parser using a hypothetical parse function
const code = 'let x = (2 + 3) * 5;';
const ast = parse(code);

The Abstract Syntax Tree (AST)

The AST is a tree representation of the syntactic structure of the source code. Each node of the tree denotes a construct occurring in the source code.

"type": "Program",
"body": [
"type": "VariableDeclaration",
"declarations": [
"type": "VariableDeclarator",
"id": {
"type": "Identifier",
"name": "x"
"init": {
"type": "BinaryExpression",
"operator": "*",
"left": {
"type": "BinaryExpression",
"operator": "+",
"left": {
"type": "Literal",
"value": 2
"right": {
"type": "Literal",
"value": 3
"right": {
"type": "Literal",
"value": 5
"kind": "let"

Code Generation

Code generation involves traversing the AST and generating the corresponding WebAssembly code. WebAssembly text format (WAT) is a human-readable format that can be written directly or generated from an AST.

// Example of code generation function
function generateWAT(ast) {
// Code to traverse the AST and generate WAT
// ...
return watCode;

const watCode = generateWAT(ast);

Compilation to WebAssembly

Once we have the WAT, we can use the WebAssembly JavaScript API to compile it into executable code.

const wasmModule = new WebAssembly.Module(wasmCodeBuffer);
const wasmInstance = new WebAssembly.Instance(wasmModule);
console.log(wasmInstance.exports.x); // This will log the value of x

Testing and Iteration

Finally, test your compiler thoroughly. Compiling code and executing it in various scenarios is crucial to ensure the correctness and efficiency of your compiler.

// Test your compiler with different pieces of code
const codeToCompile = '...';
const wasmCodeBuffer = compileToWasmBuffer(codeToCompile);
// Execute the compiled code and verify the output

Building a compiler is a significant undertaking that requires a deep understanding of both the source and target languages. In this tutorial, we’ve outlined the steps to create a simple compiler that can translate a subset of JavaScript or a DSL to WebAssembly. Remember that compiler design is an iterative process, often requiring multiple passes to refine and optimize the output.

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