[WASM] Create and Run a Native WebAssembly Function

In this introduction, we show a simple WebAssembly function that returns the square root of a number. To create this function we load up WebAssembly Explorer (https://mbebenita.github.io/WasmExplorer/), writing the native WAST code to create and export the function. We compile and download the resulting WebAssembly binary, loading this with the Fetch API and WebAssembly JavaScript API to call the function in the browser.

Demo Repo: https://github.com/guybedford/wasm-intro

Every WebAssembly starts with Module:

(module

)

Define a function inside the module:

(module
 (func $sqrt

 )   
)

Now we defined a empty function,

To add input and output we can do:

(module
  (func $sqrt
     (param $num f32) # Input: param is the keyword, $num is the param name, f32 is the type
     (result f32) # Output: result is the keyword, f32 is the type 
  )
)

Now we can define function body:

(module
  (func $sqrt 
    (param $num f32)
    (result f32)
    
    (f32.sqrt (get_local $num))  # call the function sqrt on f32, pass in the params $num though get_local function
  )
)

The calculation value will be the return value.

If we want to use the fucntion in Javascript, we need to export the function:

(module
  (export "sqrt" (func $sqrt)) # export the function call "sqrt" refer to $sqrt function we defined below
  (func $sqrt 
    (param $num f32)
    (result f32)
    
    (f32.sqrt (get_local $num))
  )
)

After "Assemble" it and "Download" the file, we can load in the project:

<!doctype>
<html>
    <header>
        <title>
            WASM
        </title>
        <script>
            fetch('./test.wasm')
                .then((res) => {
                    if(res.ok){
                        return res.arrayBuffer();
                    }
                    throw new Error('Unable to fetch WASM')
                })
                .then((bytes)=> {
                    return WebAssembly.compile(bytes);
                })
                .then(module => {
                    return WebAssembly.instantiate(module);
                })
                .then(instance => {
                    window.wasmSqrt =instance.exports.sqrt;
                })
        </script>
    </header>
</html>

Open the console, we can type:

wasmSqrt(25) //5