Unicode
JavaScript was designed at a time when Unicode was expected to have at most 65,536 characters. It has since grown to have a capacity of more than 1 million characters.
JavaScript's characters are 16 bits. That is enough to cover the original 65,536 (which is now known as the Basic Multilingual Plane). Each of the remaining million characters can be represented as a pair of characters. Unicode considers the pair to be a single character. JavaScript thinks the pair is two distinct characters.
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