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This is an off-by-one errors bug
The loop that runs one time too many, the slice that drops the last item, the index that starts in the wrong place.
Arrays count from 0, so an array of 3 items has indexes 0, 1 and 2. length is 3, one past the last index. JavaScript does not throw when you read past the end, the way Python's IndexError does; it quietly returns undefined, and the bug surfaces somewhere else later. The last item is always at length - 1, or use playlist.at(-1) in modern JavaScript. When you see undefined from an array read, check the index against length first.
The pattern behind it: off-by-one errors.
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