I recently got a call from a client whose custom WooCommerce product sync was acting… well, weird. They were pulling in about 15,000 SKUs from a third-party vendor. The sync would start fine, but then it would just stop halfway, or worse, return empty objects without actually crashing the server. Total nightmare for an inventory manager who just wants their stock to match.
The developer who built the original script was actually quite sharp, but he’d hit a wall with how he was handling the data stream. He was trying to reuse a data source that he’d already “looped through.” He didn’t realize that in modern development, especially when dealing with JavaScript Iterators, once you’ve walked through the door, the door disappears behind you.
The Difference Between Iterables and JavaScript Iterators
This is where things get linguistically confusing for most people. You’ve got iterables—like Arrays, Maps, and Strings—and then you’ve got iterators. An iterable is something you can loop over multiple times. An array is an iterable. You can loop over it at 9:00 AM and again at 10:00 AM, and the data is still there. An iterator, however, is the specific, stateful mechanism that does the work one step at a time. I saw a great breakdown of this over at Smashing Magazine recently, and it perfectly explains why my client’s sync was failing.
My first thought when debugging this was a memory leak in the PHP side of the sync, but the real issue was the “past tense” nature of iterators. You can’t “reset” an iterator once it’s spent. It’s not a static list sitting in memory; it’s a sequence of events. Once it’s “done,” it’s done. Period.
// The wrong way: Trying to reuse a spent iterator
const bbioon_data_source = Iterator.from([ 'Product A', 'Product B' ]);
// First pass works perfectly
bbioon_data_source.forEach(item => console.log('Syncing:', item));
// Second pass? Crickets. Nothing happens because the iterator is "iterated."
bbioon_data_source.forEach(item => console.log('Checking:', item));
In practice, following the iterator protocol means the object has a next() method. Every time you call it, you get an object with a value and a done boolean. That done property only flips to true when you try to access something beyond the final element. It’s a very specific handshake.
Sequential Access and the next() Method
When you call next(), you’re advancing the sequence. This is incredibly powerful for processing massive WooCommerce datasets because you aren’t loading 20,000 products into memory at once. You’re asking for them one by one. But you have to respect the internal state of the pointer.
const bbioon_product_iterator = Iterator.from([ 'SKU-001', 'SKU-002', 'SKU-003' ]);
console.log(bbioon_product_iterator.next()); // Result: { value: 'SKU-001', done: false }
console.log(bbioon_product_iterator.next()); // Result: { value: 'SKU-002', done: false }
// If we stop here, 'SKU-003' is still waiting.
// If we run a forEach now, it ONLY sees 'SKU-003'.
And that was the kicker for my client. They were “peeking” at the first few records to validate the connection using next(), and then trying to run the full sync loop on the same iterator object. They were essentially skipping the first few products in every batch and couldn’t figure out why their inventory was always off. Trust me on this: always know whether you’re dealing with the collection (the iterable) or the active pointer (the iterator).
Why This Technical Detail Matters
Understanding the “deep magic” of how JavaScript handles sequences prevents those hard-to-track bugs in complex syncs and custom dashboards. Iterators are a more controlled form of traversal. They aren’t the “wind it up and watch it go” for loops we used a decade ago. They are surgical tools for modern web applications.
Look, this stuff gets complicated fast. If you’re tired of debugging someone else’s mess and just want your site to work, drop my team a line. We’ve probably seen it before.
Are you still using standard for-loops for everything in your custom scripts, or have you started moving toward these more controlled iteration protocols?
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