wasm-add-two-from-scratch.js

// --- WASM COMMANDS

const WASM_BINARY_MAGIC = [0x00, 0x61, 0x73, 0x6d]
const WASM_BINARY_VERSION = [0x01, 0x00, 0x00, 0x00]
const HEADER = WASM_BINARY_MAGIC.concat(WASM_BINARY_VERSION)
const I32 = 0x7F
const I32_ADD = 0x6a
const SECTION_CODE_TYPE = 0x01
const SECTION_CODE_FUNCTION = 0x03
const SECTION_CODE_EXPORT = 0x07
const SECTION_CODE_CODE = 0x0a
const KEYWORD_FUNC = 0x60
const KEYWORD_END = 0x0b
const KEYWORD_LOCAL_GET = 0x20

// --- HELPERS
const getSectionSize = (items) => (
  items.map((i) => i.length).reduce((a, b) => a + b, 1)
)

const flatten = (items) => items.reduce((a, b) => a.concat(b), [])

// --- PROGRAM

const functionTypes = [
  [
    KEYWORD_FUNC,
    0x02, // Two parameters
    I32,
    I32,
    0x01, // One result (only option for now)
    I32,
  ]
]

const typeSection = [
  SECTION_CODE_TYPE,
  getSectionSize(functionTypes),
  functionTypes.length,
  ...flatten(functionTypes),
]

const functions = [
  [
    0x00,  // function type index, TODO link dynamically 
  ]
]

const functionSection = [
  SECTION_CODE_FUNCTION,
  getSectionSize(functions),
  functions.length,
  ...flatten(functions),
]

const moduleExports = [
  [
    'addTwo'.length,
    ...'addTwo'.split('').map((char) => char.codePointAt(0)),
    0x00, // "export kind" -- always zero from the examples I saw
    0x00, // export fucntion index, TODO link dynamically
  ]
]

const exportSection = [
  SECTION_CODE_EXPORT,
  getSectionSize(moduleExports),
  moduleExports.length,
  ...flatten(moduleExports),
]

const functionBodies = [
  [
    0x00, // "local decl count" -- always zero from the examples I saw
    KEYWORD_LOCAL_GET,
    0x00, // first parameter
    KEYWORD_LOCAL_GET,
    0x01, // second parameter
    I32_ADD,
    KEYWORD_END
  ]
]

const functionBodiesWithSize = functionBodies.map(
  (functionBody) => [functionBody.length, ...functionBody]
)

const codeSection = [
  SECTION_CODE_CODE,
  getSectionSize(functionBodiesWithSize),
  functionBodiesWithSize.length,
  ...flatten(functionBodiesWithSize)
]

const fullModule = [
  ...HEADER,
  ...typeSection,
  ...functionSection,
  ...exportSection,
  ...codeSection,
]

console.log({
  HEADER,
  typeSection,
  functionSection,
  exportSection,
  codeSection,
  fullModule,
  length: fullModule.length,
})

const buf = Buffer.from(fullModule)


console.log(buf)

const compiled = new WebAssembly.Module(buf)
const instance = new WebAssembly.Instance(compiled, {})

const { addTwo } = instance.exports

console.log(addTwo(2, 3))