How can I return a JavaScript string from a WebAssembly function -

how can return javascript string webassembly function?

can following module written in c(++) ?

export function foo() {   return 'hello world!'; } 

also: can pass js engine garbage collected?

webassembly doesn't natively support string type, rather supports i32 / i64 / f32 / f64 value types i8 / i16 storage.

you can interact webassembly instance using:

• exports, javascript call webassembly, , webassembly returns single value type.
• imports webassembly calls javascript, many value types want (note: count must known @ module compilation time, isn't array , isn't variadic).
• memory.buffer, arraybuffer can indexed using (among others) uint8array.

it depends on want do, seems accessing buffer directly easiest:

const bin = ...; // webassembly binary, assume below imports memory module "imports", field "memory". const module = new webassembly.module(bin); const memory = new webassembly.memory({ initial: 2 }); // size in pages. const instance = new webassembly.instance(module, { imports: { memory: memory } }); const arraybuffer = memory.buffer; const buffer = new uint8array(arraybuffer); 

if module had start function got executed @ instantiation time. otherwise you'll have export call, e.g. instance.exports.doit().

once that's done, need string size + index in memory, expose through export:

const size = instance.exports.mystringsize(); const index = instance.exports.mystringindex(); 

you'd read out of buffer:

let s = ""; (let = index; < index + size; ++i)   s += string.fromcharcode(buffer[i]); 

note i'm reading 8-bit values buffer, i'm therefore assuming strings ascii. that's std::string give (index in memory .c_str() returns), expose else such utf-8 you'd need use c++ library supporting utf-8, , read utf-8 javascript, obtain codepoints, , use string.fromcodepoint.

you rely on string being null-terminated, didn't here.

you use textdecoder api once it's available more in browsers creating arraybufferview webassembly.memory's buffer (which arraybuffer).

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if, instead, you're doing logging webassembly javascript, can expose memory above, , webassembly declare import calls javascript size + position. instantiate 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 = "";             (let = index; < index + size; ++i)                 s += string.fromcharcode(buffer[i]);             console.log(s);     } }); 

this has caveat if ever grow memory (either through javascript using memory.prototype.grow, or using grow_memory opcode) arraybuffer gets neutered , need create anew.

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on garbage collection: webassembly.module / webassembly.instance / webassembly.memory garbage collected javascript engine, that's pretty big hammer. want gc strings, , that's not possible objects live inside webassembly.memory. we've discussed adding gc support in future.