How to Write a String to Memory in WebAssembly - string

Question

I started off working like this:
// example.wast
(module
(memory (export "mem") 1))
// example.js
WebAssembly.instantiateStreaming(fetch('example.wasm'), {}).then(mod => {
var exports = mod.instance.exports
var i8 = new Uint8Array(exports.mem)
var ra = 100
var rb = 500
var rc = 1000
var rd = 1200
var re = 2000
exports.copy(0, ra)
exports.copy(ra, rb)
exports.copy(rb, rc)
exports.copy(rc, rd)
exports.copy(rd, re)
console.log(ra, getString(ra, i8))
console.log(rb, getString(rb, i8))
console.log(rc, getString(rc, i8))
console.log(rd, getString(rd, i8))
console.log(re, getString(re, i8))
})
function getString(index, buffer) {
var s = ''
for (var i = index; i < index + size; i++) {
s += String.fromCharCode(buffer[i])
}
return s
}
But in order to implement that copy function, I first need to load strings into the WebAssembly memory. Wondering how I do that (using as little JS as possible, using mostly WebAssembly instead). Wondering if you can do this:
exports.loadString(index, 'Hello World', i8)
Or if not, you have to do something like this instead:
function loadString(index, string, buffer) {
for (var i = 0, n = string.length; i < n; i++) {
buffer[index + i] = string[i]
}
}
Or better yet:
function loadString(index, string, buffer) {
for (var i = 0, n = string.length; i < n; i++) {
exports.loadChar(index + i, string[i])
}
}
Wondering how to do this exports.loadChar or exports.loadString in WebAssembly, where it loads to a specific place in memory the string.
I would like to avoid using JS, i.e. buffer[index + i] = string[i]. Perhaps there is a way to dynamically load into the data segment, which would allow for exports.loadString.

Answer 1

WebAssembly does not have any of its own utility functions for reading / writing to linear memory, therefore there is no exported exports.loadString function.
WebAssembly exports a reference to linear memory itself, which you can read / write to as a typed array. You've already obtained a reference to it here:
var i8 = new Uint8Array(exports.mem)
This creates a byte array that allows you to read / write to the linear memory that was exported from your module with the name mem. You need to encode your string and write it to this array:
var encoder = new TextEncoder();
var encodedString = encoder.encode(str);
var i8 = new Uint8Array(exports.mem)
// Copy the UTF-8 encoded string into the WASM memory.
i8.set(encodedString);