Caleb Cox•3d ago

Iterator is not defined when creating a snapshot with init_ops_and_esm

I've been following the roll your own runtime blog post series to make a simple CLI powered by deno_core. It's worked great, but I'm having trouble on part 3: setting up snapshots. My runtime.js file is just this:

class StdinIterator extends Iterator {
  next() {
    const line = Deno.core.ops.op_stdin_line();
    if (line === null) {
      return { done: true };
    }
    return { value: line, done: false };
  }
}

globalThis.stdin = new StdinIterator();

class StdinIterator extends Iterator {
  next() {
    const line = Deno.core.ops.op_stdin_line();
    if (line === null) {
      return { done: true };
    }
    return { value: line, done: false };
  }
}

globalThis.stdin = new StdinIterator();

In my build.rs, I'm creating the snapshot like this:

create_snapshot(
    CreateSnapshotOptions {
        cargo_manifest_dir: env!("CARGO_MANIFEST_DIR"),
        startup_snapshot: None,
        skip_op_registration: false,
        extensions: vec![kaw::init_ops_and_esm()],
        with_runtime_cb: None,
        extension_transpiler: None,
    },
    None,
)

create_snapshot(
    CreateSnapshotOptions {
        cargo_manifest_dir: env!("CARGO_MANIFEST_DIR"),
        startup_snapshot: None,
        skip_op_registration: false,
        extensions: vec![kaw::init_ops_and_esm()],
        with_runtime_cb: None,
        extension_transpiler: None,
    },
    None,
)

But I get the error Failed to initialize JsRuntime for snapshotting: Uncaught ReferenceError: Iterator is not defined. Inheriting from Iterator works fine without using snapshots. Also, I found that switching to extensions: vec![kaw::init_ops()] in build.rs when creating the snapshot seems to work fine, but the docs seem to indicate that I should use init_ops_and_esm when creating the snapshot and init_ops only when using the snapshot in main.rs. So my question is is it OK to create the snapshot with init_ops instead of init_ops_and_esm? If not, is there a way to access Iterator inside snapshotted code? Thanks in advance for any help! It's been really fun getting Rust and JS working together with deno_core!

2 Replies

Caleb Cox•3d ago

I was able to work around this by rewriting my runtime.js script to use generators:

const lines = function* () {
  while (true) {
    const line = Deno.core.ops.op_stdin_line();
    if (line === null) {
      return;
    }
    yield line;
  }
};


globalThis.stdin = lines();

const lines = function* () {
  while (true) {
    const line = Deno.core.ops.op_stdin_line();
    if (line === null) {
      return;
    }
    yield line;
  }
};


globalThis.stdin = lines();

bartlomieju•3d ago

Some globals are not available in the snapshot 😭 but V8 does provide a list of what's not working