JS: String.fromCharCode

str.length

Number of 16 bits units in string str. This is the same as number of characters in string if all the characters are ASCII. 〔see JS: String Code Unit〕

console.log("a".length === 1);

// GRINNING CAT FACE WITH SMILING EYES, 128568, U+1F638
console.log("😸".length === 2);

Here is a function that returns number of chars in string.

// function that returns number of chars in string
const stringRealLength = ((str) => {
  let i = 0;
  for (let c of str) {
    i += 1;
  }
  return i;
});

// test
console.log(stringRealLength("😸") === 1);

// GRINNING CAT FACE WITH SMILING EYES, 128568, U+1F638
console.log("😸".length === 2);