JS: String.prototype.charCodeAt (Char to ID)

str.charCodeAt(index)

• Return a integer that's the Code Point at position index of str.
• Return NaN when out of bound.

// character: a (codepoint 97, #x61)
console.assert(
 "abc".charCodeAt(0) === 97,
);

// character: b (codepoint 98, #x62)
console.assert(
 "abc".charCodeAt(1) === 98,
);

// character: α (codepoint 945, #x3b1)
console.assert(
 "α".charCodeAt(0) === 945,
);

// s------------------------------

// return NaN when out of bound
console.assert(
 Number.isNaN("".charCodeAt(0)),
);

🛑 warning: if string contains a character outside of Basic Multilingual Plane (such as most emoji 🦋 or rarely used Chinese character), result of string methods may be unexpected. See JS: String Index and Code Unit.

console.assert(
 "🦋".charCodeAt(0) === 55358,
);

console.assert(
 "🦋".charCodeAt(0).toString(16) === "d83e",
);

// d83e is the first part of surrogate pair in utf 16 encoding for the butterfly char

/*
🦋
Name: BUTTERFLY
Codepoint: 129419
HEXD: 1F98B
UTF8: F0 9F A6 8B
UTF16: D83E DD8B
*/

// get the second part of surrogate pair
console.assert(
 "🦋".charCodeAt(1).toString(16) === "dd8b",
);