Practical WebAssembly/

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Global Variables and Constants

Global Variables and Constants

Learn how to use global variables and constants in WebAsemmbly.

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The globals section is where we can import and export values in and out of WebAssembly modules. In a WebAssembly module, you can import either mutable or immutable values from JavaScript. Additionally, WebAssembly also supports wasmValue, an internal immutable value inside the WebAssembly module itself.

Let’s create a file called globals.wat and add the following contents to it:

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(module
     (global $mutableValue (import "js" "mutableGlobal") (mut i32))
     (global $immutableValue (import "js" "immutableGlobal") i32)
     (global $wasmValue i32 (i32.const 10))
     (func (export "getWasmValue") (result i32)
        (global.get $wasmValue))
     (func (export "getMutableValue") (result i32)
        (global.get $mutableValue))
	 (func (export "getImmutableValue") (result i32)
        (global.get $immutableValue))
     (func (export "setMutableValue") (param $v i32)
        (global.set $mutableValue
            (local.get $v)))
)

We created a module (module) and three global variables:

  • $mutableValue:  This value is imported from the mutableGlobal variable inside the js JavaScript module. We also define the global variable to be of the mut i32 type.

  • $immutableValue: This value is imported from the immutableGlobal variable inside the js JavaScript module. We also define the global variable to be of the i32 type.

  • $wasmValue: This is a global constant. We define the global keyword followed by the name of the global variable, $wasmValue, then the type of i32, and finally the actual value (i32.const 10).

Note: $wasmValue is immutable and cannot be exported to the external world. ...