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JavaScript Cheat Sheet (DRAFT) by

JavaScript - Basics Concepts

This is a draft cheat sheet. It is a work in progress and is not finished yet.

Variables

let x = 10;
block scope
most used
 
const MAX = 100;
constant values
 
var y = 10;
global or function scope
old usage

Operators

let x = 10;
assignment
x == "­Hel­lo"
comparison
x === "­Hel­lo"
strict comparison
&&
logical AND
||
logical OR
!
logical negation

Data Types

let num = 10;
Number
let str = "­Hel­lo";
String
let isTrue = true
Boolean
let arr = [1, 2, 3]
Array
let obj = { a:1, b:2, c:"t­hre­e" }
Object

Array

let array = ["a", "­b", "­c", "­d", "­e"]
array definition
array.l­ength //5
number of elements in array
array[0] // a
access the first element
array[n] // 0 <= n < lenght of array
access nth element
array[5] // error
5 grater then length of array
array[­arr­ay.l­ength - 1] // e
access the last element

Object usage

object.name // Alice
dot notation
object­["na­me"] // Alice
property access notation
object.age // 30
object.gender // undefined
non-ex­istent property
 

Functions

function greet() {
   console.log("Hello")
}
function definition

try...c­at­ch...f­inally

try{
   //code might throw an error
} catch(err) {
   console.error("Error occurred", err.message);
} finally {
   console.log("Operation completed");
}
try: This block contains code that might throw an exception (an error)
catch: This block handles the exception if an error occurs in the try block
finally: This block always executes, regardless of whether an error occurs or not

Object

let object = {
   name: "Alice",
   age: 30
}
Unlike JSON, a JavaScript object does not have double quotes (") in the property name