WebAssembly.Global() constructor

Syntax

new WebAssembly.Global(descriptor, value)

Parameters

descriptor

An object containing 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.
• funcref
• externref

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 if descriptor.value is i32, i64, f32, or f64
• a reference to undefined is used if descriptor.value is externref
• null is used if descriptor.value is funcref

Exceptions

TypeError

Thrown if any of these conditions is met:

• descriptor is not an object.
• descriptor.value is missing or not a valid value type (such as "v128").
• value is specified but cannot be converted to the WebAssembly type named by descriptor.value.

Examples

Creating a new Global instance

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}\n`
      : `FAIL!\nGot: ${got}\nExpected: ${expected}\n`;
  output.innerText += `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);
  },
);

Specifications

Browser compatibility

See also

• global definition
• WebAssembly overview
• WebAssembly concepts
• Using the WebAssembly JavaScript API