A WebAssembly.Global() constructor creates a new Global object representing 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.


new WebAssembly.Global(descriptor, value);


A GlobalDescriptor dictionary object, which contains two properties:
  • value: A USVString representing the data type of the global. This can be one of i32, i64, f32, and f64.
  • mutable: A boolean value that determines whether the global is mutable or not. By default, this is false.
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, as specified by the DefaultValue algorithm.


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) {
    output.innerHTML += `Testing ${msg}: `;
    if (got !== expected)
        output.innerHTML += `FAIL!<br>Got: ${got}<br>Expected: ${expected}<br>`;
        output.innerHTML += `SUCCESS! Got: ${got}<br>`;

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);
    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.


Browser compatibilityUpdate compatibility data on GitHub

Chrome Edge Firefox Internet Explorer Opera Safari
Global() constructor 69 No 62 No No 13.1
Android webview Chrome for Android Firefox for Android Opera for Android Safari on iOS Samsung Internet
Global() constructor 69 69 62 No 13.4 10.0
Global() constructor No

See also

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