WASM Binary Inspector

Every valid WebAssembly binary starts with the 4-byte magic sequence 0x00 0x61 0x73 0x6D, which spells '\0asm' in ASCII. Following the magic bytes are 4 version bytes (0x01 0x00 0x00 0x00 = version 1, the MVP). If these 8 bytes are incorrect, the runtime will reject the file immediately before parsing any sections.

A WASM binary is divided into sections identified by a 1-byte section ID: Type (1) defines function signatures, Import (2) lists imported functions/memories/tables/globals, Function (3) maps functions to type signatures, Table (4) defines function reference tables, Memory (5) defines linear memory, Global (6) declares global variables, Export (7) lists exported symbols, Start (8) specifies an entry point, Element (9) and Data (11) initialize tables and memory, Code (10) contains function bodies, and Custom (0) holds arbitrary metadata like debug info or DWARF.

LEB128 (Little Endian Base 128) is a variable-length integer encoding used throughout WASM binaries. Each byte uses 7 bits for data and 1 bit (the MSB) as a continuation flag. If the MSB is 1, another byte follows. This allows small integers to occupy just 1 byte while supporting arbitrarily large values. WASM uses unsigned LEB128 for sizes and indices, and signed LEB128 for integer constants.

You can produce WASM from multiple sources: (1) Compile Rust with 'cargo build --target wasm32-unknown-unknown', (2) Compile C/C++ with Emscripten ('emcc main.c -o out.wasm'), (3) Compile WAT text format with 'wat2wasm module.wat -o module.wasm' from the wabt toolkit, (4) Use AssemblyScript which compiles TypeScript to WASM. The wabt toolkit also includes wasm-objdump and wasm-validate for inspection.

Imports are symbols a WASM module needs from the host environment — your JavaScript must provide these when instantiating the module via the imports object: WebAssembly.instantiate(bytes, { 'env': { 'memory': mem, 'log': console.log } }). Exports are symbols the WASM module exposes to the host — after instantiation, these appear as instance.exports.functionName(). A module can import/export functions, tables, memories, and globals.