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Wasm By Example




In our Hello World Example, we called a function exported from WebAssembly, in our Javascript. However, you can export both functions and constants to Javascript. Let's dive a little deeper into exports and how they are used.


So first, let's create our exports.ts AssemblyScript file:

// This exports an add function.
// It takes in two 32-bit integer values
// And returns a 32-bit integer value.
export function callMeFromJavascript(a: i32, b: i32): i32 {
  return addIntegerWithConstant(a, b);

// This exports a 32-bit integer constant
export const GET_THIS_CONSTANT_FROM_JAVASCRIPT: i32 = 2424;

// A NOT exported function
// It takes in two 32-bit integer values
// And returns a 32-bit integer value.
function addIntegerWithConstant(a: i32, b: i32): i32 {
  return a + b + ADD_CONSTANT;

// A NOT export constant
// a 32-bit integer constants
const ADD_CONSTANT: i32 = 1;

Then, let's compile that into a wasm module, using the AssemblyScript Compiler, which will output a export-function.wasm:

asc exports.ts -b exports.wasm

Next, Let's load / instantiate the wasm module, export-function.wasm in a new export-function.js file. Looking at the WebAssembly Module Documentation, We see that after instantiation of our module, our Module has a .instance property, which is a WebAssembly Module Instance. An example of instantiating a module can be found in the Hello World Example. Lastly, we see that our instance has an .exports property. The .exports property is an object that contains all of the exported functions / constants, that be called / accessed synchronously from Javascript. Functions can be called with normal function syntax, whereas constants require accessing them with .valueOf() depending on if they are exports as WebAssembly Globals. See the snippet below:

const runWasm = async () => {
  // Instantiate our wasm module
  const wasmModule = await wasmBrowserInstantiate("./exports.wasm");

  // Get our exports object, with all of our exported Wasm Properties
  const exports = wasmModule.instance.exports;

  console.log(exports.callMeFromJavascript(24, 24)); // Logs 49

  // Since our constant is a global we use `.valueOf()`.
  // Though, in some cases this could simply be: exports.GET_THIS_CONSTANT_FROM_JAVASCRIPT
  console.log(exports.GET_THIS_CONSTANT_FROM_JAVASCRIPT.valueOf()); // Logs 2424

  // Trying to access a property we did NOT export
  console.log(exports.addIntegerWithConstant); // Logs undefined

Lastly, lets load our ES6 Module, exports.js Javascript file in our index.html. And you should get something similar to the demo (Source Code) below!


Next let's take a look at WebAssembly Linear Memory.