Building a Simple WebAssembly Interpreter (WAInterp) in TypeScript

This article presents a step‑by‑step guide to creating WAInterp, a simple WebAssembly interpreter written in TypeScript. It begins with an overview of WebAssembly module lifecycles and explains why implementing a custom interpreter is the most effective way to master the technology.

The core of the guide details the decoding process: parsing the binary format, handling each section (type, import, function, table, memory, global, export, start, element, data, and custom), and constructing in‑memory representations using TypeScript interfaces. Code snippets illustrate how binary structures are mapped to TypeScript types and how each section is parsed.

Next, the article describes module instantiation. It explains how to create a runtime Store , allocate instances for functions, tables, memories, and globals, and link imported symbols. The process of initializing tables and memories from element and data sections, as well as exporting module members, is covered with concrete examples.

The execution engine is then introduced. Using a switch‑dispatch loop, the interpreter processes WebAssembly instructions, managing an operand stack and a control stack. The article walks through the implementation of parametric, variable, memory, numeric, and control instructions, showing how each operation manipulates the stacks and interacts with the runtime store.

Finally, a complete example program is provided. The source code defines an imported printstr function, an iadd helper, and a main function that branches and calls iadd . The article demonstrates how to run the compiled WebAssembly binary with the WAInterp CLI, displaying the interpreter’s trace and the program’s output.

The conclusion summarizes the achievements: a functional WebAssembly decoder, instantiator, and interpreter that together form a minimal yet extensible virtual machine, offering a solid foundation for further exploration of WebAssembly features.