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WebAssembly.global

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 Syntax

var myGlobal = new WebAssembly.Global(descriptor, value);

Parameters

descriptor
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.
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, as specified by the DefaultValue algorithm.

Function properties of the Global constructor

None.

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[@@toStringTag]
The initial value of the @@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

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>`;
    else
        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);
    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 Status Comment
WebAssembly JavaScript Interface
The definition of 'WebAssembly.Global()' in that specification.
Working Draft Initial draft definition.

Browser compatibilityUpdate compatibility data on GitHub

Desktop
Chrome Edge Firefox Internet Explorer Opera Safari
Basic support 69 No 62 No No No
value 69 No 62 No No No
valueOf 69 No 62 No No No
Mobile
Android webview Chrome for Android Edge Mobile Firefox for Android Opera for Android iOS Safari Samsung Internet
Basic support 69 69 No 62 No No No
value 69 69 No 62 No No No
valueOf 69 69 No 62 No No No
Server
Node.js
Basic support No
value No
valueOf No

See also

© 2005–2018 Mozilla Developer Network and individual contributors.
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/WebAssembly/Global