js string转char

let str = "Hello";
let charAtIndex1 = str[1]; // 'e'

let str = "Hello";
let charAt1 = str.charAt(1); // 'e'

let charCode = 69; // Unicode for 'E'
let char = String.fromCharCode(charCode); // 'E'

function printChars(str) {
    for (let i = 0; i < str.length; i++) {
        console.log(str.charAt(i));
    }
}

printChars("JavaScript"); // 输出: J a v a S c r i p t

function isLetter(char) {
    return /^[A-Za-z]$/.test(char);
}

console.log(isLetter('A')); // true
console.log(isLetter('1')); // false

let str = "Hello World";
let upperCaseStr = str.toUpperCase(); // "HELLO WORLD"
let lowerCaseStr = str.toLowerCase(); // "hello world"