README JavaScript example fails to instantiate generated WASM module

[huangyxi]

The JavaScript snippet from the README did not work for me. The following code successfully instantiated and executed the generated module:

import fs from 'fs';
const bytes = fs.readFileSync('add.wasm');
const { instance } = await WebAssembly.instantiate(
    bytes,
    {
        Math: { pow: Math.pow }
    }
);
console.log(instance.exports.add(5, 3));

The Math: { pow: Math.pow } import appears to be required. Without it, the example did not work in my setup.

The Julia code from the README worked unchanged:

using WasmTarget

function add(a::Int32, b::Int32)::Int32
    return a + b
end

wasm_bytes = compile(add, (Int32, Int32))
write("add.wasm", wasm_bytes)

Environment

OS: Linux (x86_64-linux-gnu)
Julia: 1.12.6
WasmTarget.jl: v0.3.15
Node.js: v26.3.1

I don't have any prior experience with either this package or WebAssembly, so I may be missing something. I'm simply reporting what I observed while following the README example and sharing the changes that made it work on my system.

[Dale-Black]

Thanks for the careful write-up — you're exactly right, and this is a README bug on our end, not user error. 🙏

Every WasmTarget module imports Math.pow (it backs the float ^/pow path), and that import is emitted unconditionally — even a trivial add(::Int32, ::Int32) module contains exactly (import "Math" "pow" (func ...)). So WebAssembly.instantiate(bytes) with no import object fails, and your { Math: { pow: Math.pow } } is precisely what's needed. No server or bundler involved — that import object is the only setup the runtime needs.

I've updated the Quick Start example to instantiate with the import (and added the node:fs import so it runs as-is):

import fs from "node:fs";

const bytes = fs.readFileSync("add.wasm");
const { instance } = await WebAssembly.instantiate(bytes, {
  Math: { pow: Math.pow },
});
console.log(instance.exports.add(5, 3)); // → 8

There's also a note now for the next step up: modules that touch print/show or string interop additionally import the standardized wasm:js-string builtins and a small io module, instantiated with WebAssembly.instantiate(bytes, imports, { builtins: ['js-string'] }).

Fix is in #57. Thanks again — genuinely helpful pointer, and a great first issue. 🚀

[huangyxi]

Thank you for the quick response.

My point is not just to report that the README is outdated, but also to raise the underlying issue: should bindings for other Node.js libraries/functions be provided as well?

The missing Math.pow import seems like a symptom of a broader problem rather than just a documentation issue. Perhaps the LLM agent should be guided to address the root cause instead of only updating the README.

[Dale-Black]

Hey, thanks for pushing on this — you were right, it wasn't really a docs thing.

That Math.pow import was just dead leftover code. Float x^y used to call the host's Math.pow, but we replaced it with our own wasm version a while back and never deleted the line declaring the import. So every module was declaring it without ever calling it — and wasm won't let you instantiate with no imports object if the module declares any import, even an unused one. That's what broke your example.

Fixed now: pure compiled modules have zero imports, so your original snippet works as-is:

import fs from "node:fs";
const bytes = fs.readFileSync("add.wasm");
const { instance } = await WebAssembly.instantiate(bytes);
console.log(instance.exports.add(5, 3)); // 8

Thanks again, good catch.