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.


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

Returns the function that created this object's instance. By default this is the WebAssembly.Global() constructor.
The initial value of the @@toStringTag property is the String value "WebAssembly.Global".
The value contained inside the global variable — this can be used to directly set and get the global's value.

Instance methods

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


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

See also

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