index.html

<!DOCTYPE html>
<html>
<head>
	<meta charset="utf-8" />
</head>
<body>
<h1>500!</h1>
<p id="factorial500"></p>

<h1>string_to_int('42')</h1>
<p id="stringToInt"></p>

<h1>Control</h1>
<p>
	<button onclick="wasm.instance.exports.debug_dump_memory();">
		Dump memory
	</button>
</p>
<script>
let wasm;

function get_memory() {
	return new Uint8Array(wasm.instance.exports.memory.buffer);
}

const decoder = new TextDecoder("utf-8");
const encoder = new TextEncoder("utf-8");

function charPtrToString(str) {
	const memory = get_memory();
	let length=0;
	for (; memory[str + length] !== 0 ;++length) {}
	return decoder.decode(memory.subarray(str, str + length));
}

function stringToCharPtr(str) {
	const data = encoder.encode(str + "\x00");
	const ptr = wasm.instance.exports.get_memory_for_string(data.length + 1);
	const memory = get_memory();
	memory.subarray(ptr).set(data);
	return ptr;
}

function freeCharPtr(ptr) {
	wasm.instance.exports.free_memory_for_string(ptr);
}

// If the Webassemby module had been linked with
//
//   --import-memory
//
// its memory will be provided with
//
//   const wasmMemory = new WebAssembly.Memory({initial:10, maximum:100});
//   const memory = new Uint8Array(wasmMemory.buffer);
//
// But this module is providing its own memory and exporting it.

let printString = function(str) {
	console.log(str);
};

const importObject = {
	env: {
		print_string: function(str) {
			printString(charPtrToString(str));
		}
	}
};

WebAssembly.instantiateStreaming(fetch('library.wasm'), importObject)
	.then(function(obj) {
		wasm = obj;

		const s = stringToCharPtr('42');
		document.getElementById("stringToInt").innerHTML = "" + wasm.instance.exports.string_to_int(s);
		freeCharPtr(s);

		wasm.instance.exports.factorial(50);
		wasm.instance.exports.factorial(100);
		wasm.instance.exports.factorial(200);

		const oldPrintString = printString;
		printString = function(str) {
			document.getElementById("factorial500").innerHTML += str;
		}
		wasm.instance.exports.factorial(500);
		printString = oldPrintString;
	});
</script>
</body>
</html>