WebAssembly.Global

Baseline Widely available

This feature is well established and works across many devices and browser versions. It’s been available across browsers since October 2017.

A WebAssembly.Global object represents a global variable instance, accessible from both JavaScript and importable/exportable across one or more WebAssembly.Module instances. This allows dynamic linking of multiple modules.

Constructor

WebAssembly.Global()

Creates a new Global object.

Global instances

All Global instances inherit from the Global() constructor's prototype object — this can be modified to affect all Global instances.

Instance properties

Global.prototype.constructor

Returns the function that created this object's instance. By default this is the WebAssembly.Global() constructor.

Global.prototype[Symbol.toStringTag]

The initial value of the [Symbol.toStringTag] property is the String value "WebAssembly.Global".

Global.prototype.value

The value contained inside the global variable — this can be used to directly set and get the global's value.

Instance methods

Global.prototype.valueOf()

Old-style method that returns the value contained inside the global variable.

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).

js
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);
  },
);

Note: You can see the example running live on GitHub; see also the source code.

Specifications

Specification
WebAssembly JavaScript Interface
# globals

Browser compatibility

Report problems with this compatibility data on GitHub
desktopmobileserver
Chrome
Edge
Firefox
Opera
Safari
Chrome Android
Firefox for Android
Opera Android
Safari on iOS
Samsung Internet
WebView Android
WebView on iOS
Deno
Node.js
Global
Global() constructor
value
valueOf

Legend

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Full support
Full support
See implementation notes.

See also