How can I return a JavaScript string from a WebAssembly function

WebAssembly doesn’t natively support a string type, it rather supports i32 / i64 / f32 / f64 value types as well as i8 / i16 for storage.

You can interact with a WebAssembly instance using:

• exports, where from JavaScript you call into WebAssembly, and WebAssembly returns a single value type.
• imports where WebAssembly calls into JavaScript, with as many value types as you want (note: the count must be known at Module compilation time, this isn’t an array and isn’t variadic).
• Memory.buffer, which is an ArrayBuffer that can be indexed using (among others) Uint8Array.

It depends on what you want to do, but it seems like accessing the buffer directly is the easiest:

const bin = ...; // WebAssembly binary, I assume below that it imports a memory from module "imports", field "memory".
const module = new WebAssembly.Module(bin);
const memory = new WebAssembly.Memory({ initial: 2 }); // Size is in pages.
const instance = new WebAssembly.Instance(module, { imports: { memory: memory } });
const arrayBuffer = memory.buffer;
const buffer = new Uint8Array(arrayBuffer);

If your module had a start function then it got executed at instantiation time. Otherwise you’ll likely have an export which you call, e.g. instance.exports.doIt().

Once that’s done, you need to get string size + index in memory, which you would also expose through an export:

const size = instance.exports.myStringSize();
const index = instance.exports.myStringIndex();

You’d then read it out of the buffer:

let s = "";
for (let i = index; i < index + size; ++i)
  s += String.fromCharCode(buffer[i]);

Note that I’m reading 8-bit values from the buffer, I’m therefore assuming the strings were ASCII. That’s what std::string would give you (index in memory would be what .c_str() returns), but to expose something else such as UTF-8 you’d need to use a C++ library supporting UTF-8, and then read UTF-8 yourself from JavaScript, obtain the codepoints, and use String.fromCodePoint.

You could also rely on the string being null-terminated, which I didn’t do here.

You could also use the TextDecoder API once it’s available more widely in browsers by creating an ArrayBufferView into the WebAssembly.Memory‘s buffer (which is an ArrayBuffer).

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If, instead, you’re doing something like logging from WebAssembly to JavaScript, then you can expose the Memory as above, and then from WebAssembly declare an import which calls JavaScript with size + position. You could instantiate your module as:

const memory = new WebAssembly.Memory({ initial: 2 });
const arrayBuffer = memory.buffer;
const buffer = new Uint8Array(arrayBuffer);
const instance = new WebAssembly.Instance(module, {
    imports: {
        memory: memory,
        logString: (size, index) => {
            let s = "";
            for (let i = index; i < index + size; ++i)
                s += String.fromCharCode(buffer[i]);
            console.log(s);
        }
    }
});

This has the caveat that if you ever grow the memory (either through JavaScript using Memory.prototype.grow, or using the grow_memory opcode) then the ArrayBuffer gets neutered and you need to create it anew.

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On garbage collection: WebAssembly.Module / WebAssembly.Instance / WebAssembly.Memory are all garbage collected by the JavaScript engine, but that’s a pretty big hammer. You likely want to GC strings, and that’s currently not possible for objects which live inside a WebAssembly.Memory. We’ve discussed adding GC support in the future.