const str = 'Hello, World! Welcome to JavaScript.';



// Includes (ES6+ — use this!)

str.includes('World');        // true

str.includes('world');         // false (case-sensitive)

str.includes('Python');        // false



// Starts with / ends with

str.startsWith('Hello');       // true

str.startsWith('World', 7);    // true (from index 7)

str.endsWith('Script.');       // true

str.endsWith('.');             // true



// Index of first/last occurrence

str.indexOf('o');              // 4 (first 'o')

str.indexOf('o', 5);          // 8 (first 'o' after index 5)

str.lastIndexOf('o');          // 22 (last 'o')

str.indexOf('xyz');            // -1 (not found)



// Match with regex

str.match(/o/g);               // ['o', 'o', 'o'] — all matches

str.match(/javascript/i);      // ['JavaScript'] — case insensitive

const str = 'JavaScript is awesome';



// charAt — single character

str.charAt(0);                 // "J"

str[str.length - 1];           // "e" (last character)



// slice — extract portion (negative = from end)

str.slice(0, 10);              // "JavaScript"

str.slice(11);                 // "is awesome"

str.slice(-7);                 // "awesome"

str.slice(0, -8);              // "JavaScript is"



// substring — similar to slice but no negative indices

str.substring(0, 10);          // "JavaScript"



// split — string to array

'a,b,c,d'.split(',');          // ['a', 'b', 'c', 'd']

'hello world'.split(' ');       // ['hello', 'world']

'2026-05-16'.split('-');        // ['2026', '05', '16']

''.split(',');                  // [''] (empty array with one element)



// match + groups for extraction

const url = 'https://example.com/path?q=1';

url.match(/^https?:\/\/([^\/]+)/)?.[1];  // "example.com"

// Case conversion

'hello'.toUpperCase();          // "HELLO"

'HELLO'.toLowerCase();          // "hello"



// Trim whitespace

'  hello  '.trim();            // "hello"

'  hello  '.trimStart();       // "hello  "

'  hello  '.trimEnd();         // "  hello"



// Replace

'hello world'.replace('world', 'universe');

// "hello universe"



'aaa bbb aaa'.replace('a', 'x');

// "xaa bbb aaa" (only first occurrence!)



'aaa bbb aaa'.replaceAll('a', 'x');

// "xxx bbb xxx" (all occurrences!)



// Replace with regex

'hello123world456'.replace(/\d+/g, '-');

// "hello-world-"



// Replace with function

'price: $100, tax: $15'.replace(/\$(\d+)/g, (_, num) => {

  return `$${Number(num) * 1.1}`; // Add 10% tax

});

// "price: $110, tax: $16.5"



// Repeat

'ha'.repeat(3);                // "hahaha"

'-'.repeat(20);                // "--------------------"



// Pad

'5'.padStart(3, '0');          // "005"

'42'.padEnd(5, '.');           // "42..."

'abc'.padStart(10, '-');       // "-------abc"

// String → Number

Number('42');                   // 42

parseInt('42px', 10);          // 42 (stops at non-digit)

parseFloat('3.14rem');         // 3.14

+'100';                         // 100 (unary plus)

'100' * 1;                     // 100



// Number → String

String(42);                    // "42"

(42).toString();                // "42"

'' + 42;                       // "42"

42 + '';                        // "42"



// Boolean → String

String(true);                   // "true"

String(false);                  // "false"



// Anything → JSON string

JSON.stringify({ a: 1 });       // '{"a": 1}'

JSON.stringify({ a: 1 }, null, 2);

// {

//   "a": 1

// }

const name = 'Alex';

const age = 30;



// Basic interpolation

`My name is ${name} and I am ${age} years old.`;

// "My name is Alex and I am 30 years old."



// Expressions in templates

`Next year I'll be ${age + 1}.`;

// "Next year I'll be 31."



// Multi-line strings

const html = `

  <div class="card">

    <h2>${name}</h2>

    <p>Age: ${age}</p>

  </div>

`;



// Tagged templates (advanced)

function highlight(strings, ...values) {

  return strings.reduce((result, str, i) => {

    const value = values[i] ? `<mark>${values[i]}</mark>` : '';

    return result + str + value;

  }, '');

}



highlight`User ${name} has ${age} items`;

// "User <mark>Alex</mark> has <mark>30</mark> items"

// Emoji length (surprise! 😱)

'👨‍👩‍👧‍👦'.length;              // 11 (not 1!) — it's multiple code points

[...'👨‍👩‍👧‍👦'].length;        // 1 (spread into array of graphemes)



// Code points

'A'.codePointAt(0);            // 65

String.fromCodePoint(9733);     // "★"



// Locale-aware operations

'café'.normalize('NFD');        // Decomposed accents (for accent-insensitive search)

'I'.localeCompare('i');         // 0 (locale-aware comparison)



// Intl APIs

new Intl.NumberFormat('en-US').format(1234567.89);

// "1,234,567.89"



new Intl.NumberFormat('de-DE').format(1234567.89);

// "1.234.567,89"



new Intl.DateTimeFormat('en-US', { dateStyle: 'full' }).format(new Date());

// "Friday, May 16, 2026"

function slugify(text) {

  return text

    .toLowerCase()

    .trim()

    .replace(/[^\w\s-]/g, '')   // Remove special chars

    .replace(/[\s_]+/g, '-')     // Spaces to hyphens

    .replace(/^-+|-+$/g, '');   // Remove leading/trailing hyphens

}



slugify('Hello World! How Are You?');

// "hello-world-how-are-you"

function truncate(str, maxLength, suffix = '...') {

  if (str.length <= maxLength) return str;

  return str.slice(0, maxLength - suffix.length) + suffix;

}



truncate('This is a very long sentence that should be truncated.', 20);

// "This is a very lo..."

function titleCase(str) {

  return str.toLowerCase().replace(

    /\b\w/g,

    char => char.toUpperCase()

  );

}



titleCase('hello WORLD from javascript');

// "Hello World From Javascript"

function snakeToCamel(str) {

  return str.replace(/_([a-z])/g, (_, letter) => letter.toUpperCase());

}

snakeToCamel('my_variable_name'); // "myVariableName"



function camelToSnake(str) {

  return str.replace(/[A-Z]/g, letter => `_${letter.toLowerCase()}`);

}

camelToSnake('myVariableName'); // "my_variable_name"

function extractNumbers(str) {

  return str.match(/\d+(\.\d+)?/g) || [];

}



extractNumbers('Order #12345: $99.99 shipping');

// ['12345', '99.99']