Target no-modules failed to find its .wasm file automatically

[yume-chan]

Describe the Bug

When building with --target no-modules, the output js file should be able to find its .wasm file.

Steps to Reproduce

1. Clone the without-a-bundler-no-modules example and build
2. Modify index.html as:

   - await wasm_bindgen('./pkg/without_a_bundler_no_modules_bg.wasm');
   + await wasm_bindgen();

3. Open it in a browser (I'm using Microsoft Edge 89.0.774.54)

Expected Behavior

It loads the .wasm file and runs.

Actual Behavior

Uncaught (in promise) CompileError: WebAssembly.instantiate(): expected magic word 00 61 73 6d, found 3c 68 74 6d @+0

Additional Context

I treat it as a bug because the generated code did try to detect the correct url for its .wasm file, but it failed.

Related code:

    async function init(input) {
        if (typeof input === 'undefined') {
            let src;
            if (typeof document === 'undefined') {
                src = location.href;
            } else {
                src = document.currentScript.src;
            }
            input = src.replace(/\.js$/, '_bg.wasm');
        }
//...

The problem is that, document.currentScript refers to the current processing script, so as most of time init was been called from another script (and in this case, a script tag without src), this code fails.

The correct method is caching document.currentScript.src when it's initializing, for example:

+     const cachedSrc = document.currentScript.src;
      async function init(input) {
          if (typeof input === 'undefined') {
              let src;
              if (typeof document === 'undefined') {
                  src = location.href;
              } else {
-                 src = document.currentScript.src;
+                 src = cachedSrc;
            }
            input = src.replace(/\.js$/, '_bg.wasm');
        }

I verified it works after the modification:

[ghost]

We can't literally cache it like you had shown, because the code needs to check for the existence of the document object.

What we can do instead is cache the entire expression.

If document doesn't exist, and is created later, then I doubt that we'd want that document object, and the location won't change without reloading the scripts, so the entire thing can be a constant.

const cached_source = ("document" in globalThis
    ? document.currentScript.src
    : location.href
).replace(/\.js$/, "_bg.wasm");

async function init(input = cached_source) {
    ...
}

[ghost]

I would like to create a small PR updating both, ESM and non-ESM to cache the default sources so they will have equal performance at runtime, but I'd like to know, in what cases is it expected that the client would have a location, but not a document?