JavaScript Strings: Creating Text and Using String Methods

const single = 'Hello';
// single quotesconst double = "Hello";                  // double quotes — identical behaviorconst backtick = `Hello`;                // template literals — with superpowers ✨

const quote1 = "It's working!";
// ✅ double outside, apostrophe insideconst quote2 = 'She said "hi"';            // ✅ single outside, double insideconst quote3 = 'It\'s "fine"';             // ⚙️ escaping works but gets ugly fast

const name = "Priya";
const score = 95;
// ① Interpolation — embed values with ${ }:console.log(`${name} scored ${score}%! 🎉`);        // "Priya scored 95%! 🎉"
// ② Multi-line strings — no \n needed:const emailTemplate = `Dear ${name},
Your score: $ {
  score
}% ✅Keep it up!`;console.log(emailTemplate);   // prints with real line breaks intact!

let text = "hello";
text.toUpperCase();
// 🤔 did it change?console.log(text);               // "hello" — NO! 😱
text[0] = "H";
// 😱 also silently does nothing!console.log(text);               // still "hello"

let text = "hello";
const shouted = text.toUpperCase();
// 📄 a NEW string is createdconsole.log(shouted);                 // "HELLO" ✅console.log(text);                    // "hello" — original intact
// To "change" a string, reassign the variable:text = text.toUpperCase();console.log(text);                    // "HELLO" ✅ — new string, same label

const word = "JavaScript";
console.log(word.length);
// 10 — how many charactersconsole.log(word[0]);         // "J" — first character (zero-indexed!)console.log(word.at(-1));     // "t" — last character (modern .at() ✨)console.log(word.charAt(4));  // "S" — the classic way

const lang = "JavaScript";
//            0123456789
console.log(lang.slice(0, 4));
// "Java" — start to (not incl.) endconsole.log(lang.slice(4));       // "Script" — to the endconsole.log(lang.slice(-6));      // "Script" — negative = from the end! ✨

console.log(lang.substring(4, 0));
// "Java" — SWAPS the arguments 😮console.log(lang.slice(4, 0));      // "" — no swap, just emptyconsole.log(lang.substring(-3));    // "JavaScript" — negatives become 0 😬

const sentence = "JavaScript makes the web fun, JavaScript everywhere";
// ❓ Existence checks (from Lesson 11's boolean friends):sentence.includes("web");          // true — contains it anywheresentence.startsWith("Java");       // true — begins with itsentence.endsWith("everywhere");   // true — ends with it
// 📍 Position checks:sentence.indexOf("web");           // 19 — first position (or -1 if absent)sentence.indexOf("JavaScript");    // 0 — first occurrencesentence.lastIndexOf("JavaScript");// 31 — the LAST occurrence

const email = "priya@example.com";
if (email.includes("@")) {
  …
} // validation gate ✅if (email.indexOf("@") === -1) { … }              // the -1 "not found" signalconsole.log(email.slice(email.indexOf("@") + 1)); // "example.com" — extract the domain! 🎯

const name = "pRiYa ShArMa";
name.toUpperCase();
// "PRIYA SHARMA"name.toLowerCase();    // "priya sharma"
// 🎁 The "capitalize" recipe (no built-in method — make your own!):const proper = name.toLowerCase();const capitalized = proper[0].toUpperCase() + proper.slice(1);console.log(capitalized);    // "Priya sharma" ✅

const input = "   priya@example.com   ";
console.log(input.trim());
// "priya@example.com" — both ends ✅console.log(input.trimStart());  // leading onlyconsole.log(input.trimEnd());    // trailing only

const msg = "I love cats. Cats are great. cats cats cats!";
msg.replace("cats", "dogs");
// "I love dogs. Cats are great…" — FIRST match only ⚠️msg.replaceAll("cats", "dogs");    // replaces ALL lowercase "cats" ✅msg.replace(/cats/gi, "dogs");     // regex from Lesson 28: ALL + case-insensitive! 🔍

const id = "42";
id.padStart(6, "0");
// "000042" — invoice-style IDs! 🧾
"9".padStart(2, "0");
// "09" — clock digits (Lesson 34's clock!)
"Total:".padEnd(15, ".") + "₹99";
// "Total:.........₹99" — receipt dots

"⭐".repeat(5));
// "⭐⭐⭐⭐⭐" — instant ratings!
"=".repeat(20);
// "====================" — divider lines

const csv = "red,green,blue";
csv.split(",");
// ["red", "green", "blue"] — split on commas ✅
"hello".split("");
// ["h","e","l","l","o"] — every character!
"a-b-c".split("-", 2);
// ["a", "b"] — optional limit

const words =["JavaScript", "is", "fun"];
words.join(" ");
// "JavaScript is fun" ✅
words.join("");
// "JavaScriptisfun"
words.join(" 🔥 ");
// "JavaScript 🔥 is 🔥 fun"

const sentence = "I love JavaScript";
const reversed = sentence .split(" ") // ["I", "love", "JavaScript"]
.reverse() // ["JavaScript", "love", "I"]  (array method!)
.join(" ");
// "JavaScript love I"
console.log(reversed);
// "JavaScript love I" ✅

// 👤 Username Generator — string methods working as a team
function generateUsername(fullName, birthYear) {
  const cleaned = fullName.trim();
  // ✂️ clean edges
  const parts = cleaned.split(" ");
  // 🌉 bridge to array
  const first = parts[0].toLowerCase();
  // 🔧 transform
  const lastInitial = parts.length > 1 ? parts[parts.length - 1][0].toLowerCase() // 🔍 first letter of last name
  : "";
  const yearDigits = String(birthYear).slice(-2);
  // 📏 last two digits
  const username = `${first}${lastInitial}${yearDigits}`;
  // ✨ template assembly
  return username;
}function isUsernameTaken(username, takenList) {
  return takenList.includes(username.toLowerCase());
  // case-safe check
}console.log(generateUsername("  Priya Sharma  ", 2001));
// "priyas01" ✅
console.log(generateUsername("Amit", 1999));
// "amit99" ✅
const taken =["priyas01", "amit99"];
console.log(isUsernameTaken("PRIYAS01", taken));
// true — case-insensitive ✅
console.log(isUsernameTaken("newuser", taken));
// false

priyas01amit99truefalse