WebAssembly.Global() constructor
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
-
An object, which contains two properties:
-
value
: A string representing the data type of the global. This can be any one of:i32
: A 32-bit integer.i64
: A 64-bit integer. (In JavaScript, this is represented as aBigInt
)f32
: A 32-bit floating point number.f64
: A 64-bit floating point number.v128
: A 128-bit vector.externref
: A host reference.anyfunc
: A function reference.
-
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 where the value of
descriptor.value
is one ofi32
,i64
,f32
, orf64
, andnull
is used ifdescriptor.value
isexternref
oranyfunc
(as specified by theDefaultValue
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) {
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 # dom-global-global |
Browser compatibility
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