JS Cheatsheet

js-cheatsheet.md

JavaScript Cheat Sheet

1. Basics

• Variables:

  let x = 10;      // Block-scoped, mutable
  const y = 20;    // Block-scoped, immutable
  var z = 30;      // Function-scoped, avoid using

• Data Types:

  // Primitive types
  let str = "Hello"; // String
  let num = 42;      // Number
  let bool = true;   // Boolean
  let nothing = null; // Null
  let undef;          // Undefined
  let big = 9007199254740991n; // BigInt
  let sym = Symbol("id");      // Symbol
  
  // Objects
  let obj = { name: "Alice", age: 25 }; // Object
  let arr = [1, 2, 3];                  // Array
  let func = () => console.log("Hi!");  // Function

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2. Operators

• Arithmetic: +, -, *, /, %, **
• Comparison: ===, !==, <, >, <=, >=
• Logical: &&, ||, !
• Assignment: =, +=, -=, *=, /=

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3. Control Structures

• Conditional Statements:

  if (x > 10) {
      console.log("Greater than 10");
  } else if (x === 10) {
      console.log("Exactly 10");
  } else {
      console.log("Less than 10");
  }

• Loops:

  for (let i = 0; i < 5; i++) {
      console.log(i);
  }
  
  while (x > 0) {
      x--;
  }
  
  do {
      x++;
  } while (x < 5);

---

4. Functions

• Function Declaration:

  function add(a, b) {
      return a + b;
  }

• Arrow Function:

  const multiply = (a, b) => a * b;

• Default Parameters:

  const greet = (name = "Guest") => console.log(`Hello, ${name}`);

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5. Objects and Arrays

• Object Access:

  let person = { name: "Alice", age: 25 };
  console.log(person.name); // Dot notation
  console.log(person["age"]); // Bracket notation

• Array Methods:

  let arr = [1, 2, 3];
  arr.push(4);  // Add to end
  arr.pop();    // Remove from end
  arr.shift();  // Remove from start
  arr.unshift(0); // Add to start
  arr.map(x => x * 2); // Transform array
  arr.filter(x => x > 1); // Filter array
  arr.reduce((sum, x) => sum + x, 0); // Reduce array

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6. ES6+ Features

• Template Literals:

  let name = "Alice";
  console.log(`Hello, ${name}!`);

• Destructuring:

  let { name, age } = person;
  let [first, second] = arr;

• Spread and Rest:

  let arr2 = [...arr, 4, 5];
  let sum = (...nums) => nums.reduce((a, b) => a + b, 0);

• Promises:

  const fetchData = () => new Promise((resolve, reject) => {
      resolve("Data fetched");
  });
  
  fetchData().then(data => console.log(data)).catch(err => console.log(err));

---

7. Modules

• ES6 Module:

  // module.js
  export const add = (a, b) => a + b;
  
  // main.js
  import { add } from './module.js';
  console.log(add(2, 3));

• CommonJS:

  // module.js
  module.exports = { add };
  
  // main.js
  const { add } = require('./module');

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JavaScript Learning Resources

1. Interactive Tutorials

   • JavaScript.info: Comprehensive tutorials with interactive examples.
   • MDN Web Docs: Official and detailed reference for JavaScript.

2. Courses

   • FreeCodeCamp JavaScript: Hands-on exercises and projects.
   • Codecademy JavaScript: Beginner-friendly course with an interactive environment.

3. Books

   • Eloquent JavaScript (Free Online): A great in-depth book for all levels.
   • You Don’t Know JS (GitHub): Deep dive into JavaScript internals.

4. Practice Platforms

   • HackerRank JavaScript Challenges
   • Codewars: Practice solving coding problems with JavaScript.