charCodeAt()
ES3+Returns the UTF-16 code unit at the specified index.
Syntax
string.charCodeAt(index)Parameters
index number The index of the character
Return Value
A number representing the UTF-16 code unit
Examples
const str = 'ABC';
console.log(str.charCodeAt(0));
console.log(str.charCodeAt(1));📌 When to Use
Use charCodeAt() when you need the numeric Unicode value of a character, such as for encryption algorithms, sorting, character classification, or converting characters to their numeric representations.
⚠️ Common Mistakes
Confusing charCodeAt() with codePointAt() - charCodeAt() returns UTF-16 code units, while codePointAt() handles full Unicode code points including emoji and special characters.
Forgetting that charCodeAt() returns NaN for invalid indices, not 0 or undefined.
Not handling surrogate pairs for characters outside the Basic Multilingual Plane (emoji, some Asian characters).
✅ Best Practices
Use codePointAt() instead of charCodeAt() when dealing with emoji or characters outside the BMP (code points > 0xFFFF).
Combine with String.fromCharCode() for character transformations like Caesar cipher or case shifting.
Use charCodeAt() for quick character classification: check if code is between 65-90 (A-Z) or 97-122 (a-z).
⚡ Performance Notes
charCodeAt() is extremely fast as it directly accesses the internal string representation. For processing large amounts of text character-by-character, charCodeAt() can be faster than charAt() followed by comparison, as it avoids creating intermediate string objects.
🌍 Real World Example
Character Encoding and Simple Cryptography
charCodeAt() is essential for implementing character-based algorithms, simple encryption, character classification, and ASCII-based operations.
// Simple Caesar cipher encryption
function caesarCipher(text, shift) {
let result = '';
for (let i = 0; i < text.length; i++) {
const code = text.charCodeAt(i);
// Uppercase letters (A-Z: 65-90)
if (code >= 65 && code <= 90) {
result += String.fromCharCode(((code - 65 + shift) % 26) + 65);
}
// Lowercase letters (a-z: 97-122)
else if (code >= 97 && code <= 122) {
result += String.fromCharCode(((code - 97 + shift) % 26) + 97);
}
else {
result += text.charAt(i);
}
}
return result;
}
console.log(caesarCipher('Hello World', 3)); // 'Khoor Zruog'
// Check if string contains only ASCII characters
function isAscii(str) {
for (let i = 0; i < str.length; i++) {
if (str.charCodeAt(i) > 127) {
return false;
}
}
return true;
}
console.log(isAscii('Hello')); // true
console.log(isAscii('Hello')); // true (Korean characters)
// Count letters, digits, and special characters
function analyzeString(str) {
let letters = 0, digits = 0, special = 0;
for (let i = 0; i < str.length; i++) {
const code = str.charCodeAt(i);
if ((code >= 65 && code <= 90) || (code >= 97 && code <= 122)) {
letters++;
} else if (code >= 48 && code <= 57) {
digits++;
} else {
special++;
}
}
return { letters, digits, special };
}
console.log(analyzeString('Hello123!')); // { letters: 5, digits: 3, special: 1 }