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Javascript Cheatsheet

Javascript is a high-level, dynamically typed programming language commonly used for client-side web development and server-side programming with Node.js.

Variables

Javascript has dynamic typing, meaning that a variable's type can change at runtime. Variables can be declared using the keywords var, let, or const.

var x;
let y;
const z = 10;

var is function scoped, while let and const are block scoped. const is used to declare a constant value that cannot be re-assigned.

Data Types

Javascript has the following primitive data types:

  • String: a sequence of characters, denoted with quotes '' or "".
  • Number: numeric values, including integers and floating point numbers.
  • Boolean: either true or false.
  • Undefined: a variable that has been declared, but has no value.
  • Null: explicitly represents the absence of any object value.
Data Type Description Size (in bytes)
Number Represents numeric values, including integers and floating-point numbers 8
String Represents textual data The size of a string is proportional to the length of the string, typically 2 bytes per character
Boolean Represents a logical value 1
Undefined Represents an undefined value 0
Null Represents a null value 0
Object Represents a collection of properties and methods Varies
Symbol Represents a unique identifier Varies

Arrays

Arrays are ordered collections of values that can hold any data type. They are declared with square brackets [].

let numbers = [1, 2, 3];
let mixed = [1, "two", false];

Objects

Objects are collections of key-value pairs. They are declared with curly braces {}.

let person = {
  name: "John Doe",
  age: 30,
  occupation: "developer",
};

Operators

Javascript has various operators for performing arithmetic, assignment, comparison, and logical operations.

Arithmetic Operators

  • +: addition
  • -: subtraction
  • *: multiplication
  • /: division
  • %: modulus (remainder after division)

Assignment Operators

  • =: assignment
  • +=: addition and assignment
  • -=: subtraction and assignment
  • *=: multiplication and assignment
  • /=: division and assignment

Comparison Operators

  • ==: equality
  • ===: strict equality (value and type)
  • !=: inequality
  • !==: strict inequality (value and type)
  • >: greater than
  • <: less than
  • >=: greater than or equal to
  • <=: less than or equal to
  • ?: ternary operator

Logical Operators

  • &&: logical AND
  • ||: logical OR
  • !: logical NOT

Type Operators

  • typeof: returns the type of a variable
  • instanceof: returns true if object is an instance of an object type

Conditional Statements

Conditional statements are used to make decisions in code based on whether a condition is true or false.

if (condition) {
  // code to be executed if condition is true
} else if (condition) {
  // code to be executed if condition is true
} else {
  // code to be executed if all conditions are false
}

Loops

Loops are used to repeatedly execute a block of code.

For Loop

A for loop is used to execute a block of code a specified number of times.

for (let i = 0; i < 10; i++) {
  console.log(i);
}

For-In Loop

A for-in loop is used to iterate over the properties of an object.

let person = {
  name: "John Doe",
  age: 30,
};

for (let prop in person) {
  console.log(prop + ": " + person[prop]);
}

For-Of Loop

A for-of loop is used to iterate over the values of an iterable object, such as an array.

let colors = ["red", "green", "blue"];

for (let color of colors) {
  console.log(color);
}

While Loop

A while loop is used to execute a block of code as long as a specified condition is true.

let i = 0;

while (i < 10) {
  console.log(i);
  i++;
}

Do-While Loop

A do-while loop is similar to a while loop, but the block of code will always be executed at least once.

let i = 0;

do {
  console.log(i);
  i++;
} while (i < 10);

Additional Resources

  • Mozilla Developer Network (MDN): MDN is an excellent resource for learning more about JavaScript. It includes detailed documentation on the language itself, as well as tutorials and guides on specific topics.

  • JavaScript.info: JavaScript.info is a comprehensive guide to JavaScript that covers the language from the basics to advanced topics. It includes interactive exercises and examples to help you learn and practice.

  • W3Schools: W3Schools is a popular website for learning web development, including JavaScript. It includes tutorials and examples on a wide range of topics, from basic syntax to advanced techniques.

  • Stack Overflow: Stack Overflow is a community-driven Q&A site for programmers. It includes a wealth of information on JavaScript, from common questions to advanced topics.

  • GitHub: GitHub is a popular code hosting platform where you can find a wide range of open source JavaScript projects. It's a great place to learn from other developers and contribute to the community.