JavaScript Basics

HTML Cheat SheetCSS Cheat SheetJavaScript BasicsJavaScript IntermediateJavaScript AdvancedJavaScript DOMAsynchronous JavaScript

0. Language overview

1.Grammar and Types

1.Basics

// Single statement : Instruction that perform actions.
console.log("Hello, world!");

// Expressions - Produce values 
2 + 2;

// Block of code: {} - Group multiple statements together.
{ 
  let a = 5;
  let b = 10;
  console.log(a + b); // Output: 15
}

// Case Sensitivity
let name = "Alice";
let Name = "Bob";
console.log(name); // Output: Alice
console.log(Name); // Output: Bob

// Unicode
let π = 3.14159;
let こんにちは = "Hello in Japanese"; 
console.log(π);         // Output: 3.14159
console.log(こんにちは); // Output: Hello in Japanese

// These are equivalent
let x = 5;
let y=10;
console.log(x + y); // Output: 15
// javascript ignores extra spaces, tabs, and newlines between statements.

// without semicolons
let a = 10
// with semicolons (recommended for clarity)

let b = 20;
console.log(a + b) // Output: 30

// (;) are optional in JavaScript because of Automatic Semicolon Insertion (ASI)
// Pitfall -- after return JavaScript inserts ; automatically.
// so we should put { on the same line as return

"use strict";
x = 10; // ReferenceError: x is not defined

var public = "Restricted word!"; 
// SyntaxError: Unexpected strict mode reserved word

var myVar = 10;
delete myVar; // SyntaxError: Delete of an unqualified identifier in strict mode

function sum(a, a) { // SyntaxError: Duplicate parameter name not allowed
  return a + a;
}

function showThis() {
  console.log(this); // undefined
}
showThis(); // Output: undefined

const obj = Object.freeze({ name: "Alice" });
// to use freeze we have to use strict mode else it won't work.
obj.name = "Bob"; // TypeError: Cannot assign to read only property 'name'

function updateValue(x) {
  arguments[0] = 20;
  console.log(x); // Output: 10 (unchanged) 
  // without strict mode Output: 20
}
updateValue(10);

2.Comments

3.Declarations

4.Data Structures and Types

Primitive Types

object / Reference Types

Type Conversion and Coercion

Explicit Coercion
Implicit Coercion
Type Coercion in Comparisons

DataType in Memory

5.Literals

2.Control flow and error handling

1.Block statement

2.Conditional statements

3.Exception handling statements

4.Break statement

3.Loops and iteration

1. for statement

2. do...while statement

3. while statement

4. labeled statement

5. break statement

6. continue statement

7. for...in statement (for Object's key)

8. for...of statement (for Array's, Map's & Set's values )

9. Iteration with Array methods

10. Iterators and Generators

4.Functions

1.Defining functions

2.Calling functions

3.Function scopes and closures

4.Using the arguments object

5.Function parameters

6.Arrow functions

7. The this keyword

8. Higher-order functions

9. call(), apply(), and bind() methods

10. Function overloading

11. Debouncing and Throttling Functions

12. Function caching/memoization

5.Expressions and operators

1.Expressions & operators

2.Assignment Operators

3.Comparison Operators

4.Arithmetic Operators

5.Bitwise Operators

6.Logical Operators

7.BigInt Operators

8.String Operators

9.Conditional (Ternary) Operator

10.Comma Operator

11.Unary Operators

12.Relational operators

13.Basic expressions

14.Destructuring Assignment

15.Spread & Rest Operators

6.Numbers and strings

1. Numbers

2. Number object

3. Math object

4. BigInts

5. Strings

6. String object and Methods

7. Template literals

console.log(`string text line 1
string text line 2`);
const name = "Alice";
const age = 25;
const message = `My name is ${name} and I am ${age} years old.`;

7.Representing dates & times

1. Creating, getting & setting Date Ojbect

2. Formatting, Comparing & Parsing Date Ojbect

8.Regular expressions (RegExp)

1. Creating Regular Expressions

2. Writing a regular expression pattern

3. Using regular expressions in JavaScript

9.Indexed collections

1. Creating Array like objects

2. Array methods

10.Keyed collections

1.Map object

2.Set object

11.Working with objects

1.Creating new objects

2.Objects and properties