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// 1: STRING LENGTH

const message = "Hello World";

console.log(message.length); // 11

const password = "abc123";

if (password.length < 8) {

console.log("Password too short!");

}

const empty = "";

console.log(empty.length); // 0

const text = "Hello\nWorld";

console.log(text.length); // 11 (not 12)

// 2: ACCESSING CHARACTERS

const word = "JavaScript";

console.log(word[0]); // "J"

console.log(word[4]); // "S"

console.log(word[9]); // "t"

console.log(word[100]); // undefined

console.log(word.charCodeAt(0)); // 74 (Unicode for "J")

// 3: CASE MANIPULATION

// const message = "Hello World";

// console.log(message.toUpperCase()); // "HELLO WORLD"

// console.log(message.toLowerCase()); // "hello world"

// case-insensitive

const userInput = "JAVASCRIPT";

if (userInput.toLowerCase() === "javascript") {

console.log("Match!");

}

const original = "JavaScript";

const upper = original.toUpperCase();

console.log(original); // "JavaScript" (unchanged)

console.log(upper); // "JAVASCRIPT"

// 4: TRIMMING WHITESPACE

const messy = " Hello World ";

console.log(messy.trim()); // "Hello World"

console.log(messy.trimStart()); // "Hello World "

console.log(messy.trimEnd()); // " Hello World"

let username = " john_doe ";

username = username.trim();

console.log(username); // "john_doe"

// 5: REPLACING TEXT

// const sentence = "I love JavaScript";

// const newSentence = sentence.replace("JavaScript", "coding");

// console.log(newSentence); // "I love coding"

// const text = "cat cat cat";

// console.log(text.replace("cat", "dog")); // "dog cat cat"

// console.log(text.replaceAll("cat", "dog")); // "dog dog dog"

// console.log(text.replace(/cat/g, "dog")); // "dog dog dog"

// const numbers = "1 2 3";

// const doubled = numbers.replace(/\d/g, function(match) {

// return match * 2;

// });

// console.log(doubled); // "2 4 6"

// 6: PADDING STRINGS

const number = "5";

console.log(number.padStart(3, "0")); // "005"

console.log(number.padEnd(3, "0")); // "500"

const day = "7";

const month = "3";

console.log(`${day.padStart(2, "0")}/${month.padStart(2, "0")}/2025`); // "07/03/2025"

console.log("Hi".padStart(5)); // " Hi"

// 7: REPEATING STRINGS

console.log("Ha".repeat(3)); // "HaHaHa"

console.log("-".repeat(20)); // "--------------------"

const stars = "*".repeat(5);

console.log(stars); // "*****"

const title = "Chapter 1";

console.log(title);

console.log("=".repeat(title.length));

// // Chapter 1

// // =========

// 8: PRACTICAL EXAMPLES

// Example 1: Username Validation

// function validateUsername(username) {

// username = username.trim();

// if (username.length < 3) {

// return "Username too short";

// }

// if (username.length > 20) {

// return "Username too long";

// }

// return "Valid username";

// }

// console.log(validateUsername(" john ")); // "Valid username"

// console.log(validateUsername("ab")); // "Username too short"

// Example 2: Masking Credit Card

function maskCard(cardNumber) {

const visibleDigits = 4;

const maskedLength = cardNumber.length - visibleDigits;

const masked = "*".repeat(maskedLength);

const lastFour = cardNumber[cardNumber.length - 4] +

cardNumber[cardNumber.length - 3] +

cardNumber[cardNumber.length - 2] +

cardNumber[cardNumber.length - 1];

return masked + lastFour;

}

console.log(maskCard("1234567890123456")); // "************3456"

// Example 3: Format Phone Number

function formatPhone(phone) {

const cleaned = phone.replaceAll("-", "").replaceAll(" ", "");

if (cleaned.length !== 10) {

return "Invalid phone number";

}

const area = cleaned[0] + cleaned[1] + cleaned[2];

const prefix = cleaned[3] + cleaned[4] + cleaned[5];

const line = cleaned[6] + cleaned[7] + cleaned[8] + cleaned[9];

return `(${area}) ${prefix}-${line}`;

}

console.log(formatPhone("1234567890")); // "(123) 456-7890"