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Import/Export of Mutable Globals

Export a mutable global from Wasm

Section titled “Export a mutable global from Wasm”
(module
  (global (export "counter") (mut i32) (i32.const 0))
)

From JS, read and write via .value:

const { instance } = await WebAssembly.instantiate(wasmBytes);
console.log(instance.exports.counter.value); // 0
instance.exports.counter.value = 5;
console.log(instance.exports.counter.value); // 5

Import a mutable global from JS

Section titled “Import a mutable global from JS”
(module
  (import "env" "g" (global $g (mut i32)))
  (func (export "inc")
    (global.set $g (i32.add (global.get $g) (i32.const 1))))
)

Create a WebAssembly.Global on the JS side and pass it as an import:

const g = new WebAssembly.Global({ value: 'i32', mutable: true }, 0);
const { instance } = await WebAssembly.instantiate(wasmBytes, { env: { g } });
instance.exports.inc();
console.log(g.value); // 1

Instruction Reference

Section titled “Instruction Reference”