descriptor

An object, which contains two properties:

• value: A string representing the data type of the global. This can be any one of:
  • i32: A 32-bit integer.
  • i64: A 64-bit integer. (In JavaScript, this is represented as a BigInt)
  • f32: A 32-bit floating point number.
  • f64: A 64-bit floating point number.
  • v128: A 128-bit vector.
  • externref: A host reference.
  • anyfunc: A function reference.
• mutable: A boolean value that determines whether the global is mutable or not. By default, this is false.

value

The value the variable contains. This can be any value, as long as its type matches the variable's data type. If no value is specified, a typed 0 value is used where the value of descriptor.value is one of i32, i64, f32, or f64, and null is used if descriptor.value is externref or anyfunc (as specified by the DefaultValue algorithm).

The following example shows a new global instance being created using the WebAssembly.Global() constructor. It is being defined as a mutable i32 type, with a value of 0.

The value of the global is then changed, first to 42 using the Global.value property, and then to 43 using the incGlobal() function exported out of the global.wasm module (this adds 1 to whatever value is given to it and then returns the new value).

const output = document.getElementById("output");

function assertEq(msg, got, expected) {
  const result = got === expected ? `SUCCESS! Got: ${got}<br>` : `FAIL!<br>Got: ${got}<br>Expected: ${expected}<br>`;
  output.innerHTML += `Testing ${msg}: ${result}`;
}

assertEq("WebAssembly.Global exists", typeof WebAssembly.Global, "function");

const global = new WebAssembly.Global({ value: "i32", mutable: true }, 0);

WebAssembly.instantiateStreaming(fetch("global.wasm"), { js: { global } }).then(
  ({ instance }) => {
    assertEq(
      "getting initial value from wasm",
      instance.exports.getGlobal(),
      0,
    );
    global.value = 42;
    assertEq(
      "getting JS-updated value from wasm",
      instance.exports.getGlobal(),
      42,
    );
    instance.exports.incGlobal();
    assertEq("getting wasm-updated value from JS", global.value, 43);
  }
);

• WebAssembly overview page
• WebAssembly concepts
• Using the WebAssembly JavaScript API
• Import/Export mutable globals proposal