Errors / JavaScript errors

Uncaught (in promise) …

Something the page asked for in the background failed, and no code was there to handle it.

Sometimes nothing a visitor can see; often a spinner that never stops or a click that does nothing.

Why it happens

• A fetch() or other promise rejected and nothing called .catch(), or the await was not in a try.
• A library rejects with a string or an object instead of an Error, so the message has no stack: Uncaught (in promise) undefined or a bare sentence.
• play() on a video or audio element refused by the browser's autoplay rules.

How to fix it

1. Read what follows (in promise): that is the rejection's reason, and often the real error.
2. Handle rejections where the promise is made; for code you do not own, a window.addEventListener("unhandledrejection", …) at least tells you where.
3. Reject with new Error(…), never a string, so the next person gets a stack.

Whose problem it is

Either the site's own code or a third party's, depending on where it comes from.

Often seen with

• TypeError: Failed to fetchThe page tried to get data from a server and the request never completed.
• play() failed / was interruptedA video or sound tried to play on its own and the browser stopped it.