In JavaScript, strings consist of UTF-16 code units - which basically means that most characters are represented by a 16-bit number (2 bytes). This allows for more than just the 128 ASCII characters, but also characters from non-English languages and other symbols.
Some characters (like emojis 😀) require more than 16 bits to represent, so JavaScript uses a pair of 16-bit numbers (2 code units) to represent them.
Long story short, you're generally safe to use unicode characters (like emojis) in your strings. Just be aware that some characters will take up more than one "character" in the string. This is totally chill:
const kermit = "🐸";
name variable with the bear emoji ('🐻') and run the code again.