How many bytes is an emoji?

Depends on how you count. In UTF-8, most emojis take 4 bytes; compound ones (family emoji, ZWJ sequences, flags) can take 25+ bytes. In JavaScript's internal UTF-16, most emojis are 2 code units (4 bytes); compound ones can be 10+ code units. Never assume one emoji = one character — always use Unicode-aware string operations.