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.
Syntax
new WebAssembly.Global(descriptor, value);
Parameters
- descriptor
- A
GlobalDescriptor
dictionary object, which contains two properties:value
: AUSVString
representing the data type of the global. This can be one ofi32
,i64
,f32
, andf64
. USVString corresponds to the set of all possible sequences of unicode scalar values. USVString maps to a String when returned in JavaScript; it's generally only used for APIs that perform text processing and need a string of unicode scalar values to operate on. USVString is equivalent to DOMString except for not allowing unpaired surrogate codepoints. Unpaired surrogate codepoints present in USVString are converted by the browser to Unicode 'replacement character' U+FFFD, (�).mutable
: A boolean value that determines whether the global is mutable or not. By default, this isfalse
.
- 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.
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) {
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 |
---|
WebAssembly JavaScript Interface The definition of 'WebAssembly.Global() constructor' in that specification. |
Browser compatibility
BCD tables only load in the browser