Fix Iterator Protocol Error in Node.js

Iterator Protocol Errors in Node.js

You've probably seen TypeError: X is not iterable or X is not a function when using for...of, spread syntax, or destructuring. This means the object you're iterating doesn't implement the iterator protocol.

When This Happens

• Passing a plain object to for...of (objects aren't iterable by default)
• Spreading a non-iterable value: [...myObj]
• Using Array.from() on something without Symbol.iterator
• An async generator being consumed with a sync for...of instead of for await...of

Quick Fix for Objects

const config = { host: 'localhost', port: 3000, env: 'dev' };

// ERROR: config is not iterable
// for (const item of config) { ... }

// Fix: iterate over entries
for (const [key, value] of Object.entries(config)) {
  console.log(`${key}: ${value}`);
}

Custom Iterable Class

If you want your own class to be iterable:

class NumberRange {
  constructor(start, end) {
    this.start = start;
    this.end = end;
  }

  [Symbol.iterator]() {
    let current = this.start;
    const end = this.end;
    return {
      next() {
        return current <= end
          ? { value: current++, done: false }
          : { done: true };
      }
    };
  }
}

const range = new NumberRange(1, 5);
console.log([...range]); // [1, 2, 3, 4, 5]

Async Iterators

Don't forget to use for await...of with async generators:

for await (const chunk of readableStream) {
  process.stdout.write(chunk);
}

Bugsly captures these TypeErrors with full stack traces, including the variable name and call site, so you can pinpoint exactly which value wasn't iterable.