Strings & Template Literals

// Three ways
const single = 'Hello';
const double = "Hello";
const backtick = `Hello`;     // Template literal

// All three work — be consistent (most projects use double quotes or backticks)

const str = "JavaScript";

// Length
console.log(str.length);      // => 10

// Character access
console.log(str[0]);           // => "J"
console.log(str[str.length - 1]); // => "t"

// charAt (older way)
console.log(str.charAt(4));    // => "S"

const text = "Hello World";

console.log(text.toUpperCase());      // => "HELLO WORLD"
console.log(text.toLowerCase());      // => "hello world"
console.log(text);                     // => "Hello World" (original unchanged)

const padded = "  spaces around  ";
console.log(padded.trim());           // => "spaces around"
console.log(padded.trimStart());      // => "spaces around  "
console.log(padded.trimEnd());        // => "  spaces around"

const sentence = "The quick brown fox jumps over the lazy dog";

console.log(sentence.indexOf("fox"));       // => 16
console.log(sentence.indexOf("cat"));       // => -1 (not found)
console.log(sentence.includes("fox"));      // => true
console.log(sentence.startsWith("The"));    // => true
console.log(sentence.endsWith("dog"));      // => true

// lastIndexOf — finds last occurrence
console.log(sentence.lastIndexOf("the"));   // => 31

const str = "JavaScript";

console.log(str.slice(0, 4));     // => "Java"  (start at 0, end before 4)
console.log(str.slice(4));        // => "Script"  (from 4 to end)
console.log(str.slice(-6));       // => "Script"  (last 6 characters)

console.log(str.substring(0, 4)); // => "Java"  (similar to slice)
console.log(str.substr(4, 3));    // => "Scr"  (deprecated — avoid)

// Split string into array
const csv = "apple,banana,cherry";
console.log(csv.split(","));       // => ["apple", "banana", "cherry"]

const words = "one two three";
console.log(words.split(" "));     // => ["one", "two", "three"]

// Split into characters
console.log("hello".split(""));    // => ["h", "e", "l", "l", "o"]

// Join array into string (opposite of split)
const parts = ["2026", "06", "16"];
console.log(parts.join("-"));     // => "2026-06-16"

const text = "I like cats. Cats are great.";

// Replace FIRST match only
console.log(text.replace("cats", "dogs"));
// => "I like dogs. Cats are great."

// Replace ALL (case-sensitive)
console.log(text.replace(/cats/g, "dogs"));
// => "I like dogs. Cats are great."

// Replace ALL (case-insensitive)
console.log(text.replace(/cats/gi, "dogs"));
// => "I like dogs. dogs are great."

// Replace with function
const prices = "Total: $10.50";
const result = prices.replace(/\$(\d+\.\d+)/, (match, amount) => {
  return `$${(parseFloat(amount) * 1.1).toFixed(2)}`;  // Add 10% tax
});
console.log(result);  // => "Total: $11.55"

// Pad to a minimum length
console.log("5".padStart(3, "0"));     // => "005"
console.log("123".padStart(3, "0"));   // => "123" (already long enough)
console.log("hello".padEnd(10, "-"));  // => "hello-----"

console.log("ha".repeat(3));  // => "hahaha"

const name = "Alice";
const age = 30;

// String interpolation
console.log(`Hello, my name is ${name} and I am ${age} years old.`);
// => "Hello, my name is Alice and I am 30 years old."

// Expressions
console.log(`Next year I'll be ${age + 1}.`);
console.log(`Total: ${10 * 5} dollars.`);

// Multi-line strings
const html = `
  <div class="card">
    <h2>${name}</h2>
    <p>Age: ${age}</p>
  </div>
`;
console.log(html);

// Creating regex patterns
const pattern1 = /hello/;
const pattern2 = new RegExp("hello");

// test — returns boolean
console.log(/hello/.test("hello world"));  // => true
console.log(/hello/.test("goodbye"));      // => false

// match — returns array of matches (or null)
const message = "The colors are red, green, and blue";
console.log(message.match(/green/));
// => ["green", index: 15, input: "...", groups: undefined]

// matchAll — returns all matches (with flags)
const tags = "<h1>Title</h1><p>Text</p><h2>Subtitle</h2>";
const headings = tags.matchAll(/<h[1-6]>(.*?)<\/h[1-6]>/g);
for (const match of headings) {
  console.log(match[1]);  // Captured group
}
// Title
// Text
// Subtitle

// Common regex patterns
console.log(/^\d{5}$/.test("12345"));      // => true  (5-digit zip code)
console.log(/^\S+@\S+\.\S+$/.test("user@example.com")); // => true (basic email)
console.log(/^https?:\/\//.test("https://example.com")); // => true (URL starting with http)

/g  // Global — find all matches, not just first
/i  // Case-insensitive
/m  // Multiline — ^ and $ match line boundaries
/s  // Dotall — . matches newlines

\d  // Any digit (0-9)
\w  // Any word character (a-z, A-Z, 0-9, _)
\s  // Any whitespace (space, tab, newline)
\D  // NOT a digit
\W  // NOT a word character
.   // Any character except newline

+     // One or more
*     // Zero or more
?     // Zero or one
{3}   // Exactly 3
{2,4} // 2 to 4
{3,}  // 3 or more

// 1. Validate an email address using regex
function isValidEmail(email) {
  return /^\S+@\S+\.\S+$/.test(email);
}

// 2. Extract all numbers from a string
function extractNumbers(text) {
  return text.match(/\d+/g)?.map(Number) || [];
}
console.log(extractNumbers("Item 1 costs $25, item 2 costs $30"));  // => [1, 25, 2, 30]

// 3. Convert "hello_world_example" to "helloWorldExample" (snake to camelCase)
function snakeToCamel(str) {
  return str.replace(/_([a-z])/g, (_, letter) => letter.toUpperCase());
}
console.log(snakeToCamel("hello_world_example"));  // => "helloWorldExample"