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JavaScript Basics Cheat Sheet by

JavaScript - Basics Concepts

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"]
array definition
 
array.length //3
number of elements in array
 
array[0] // a
access first element
 
array[n] //0=<n­<ar­ray.length
access nth element
 
array[4] // error
4 grater then length of array
 
array[array.length - 1] // c
access last element

Comments

// this is a one line comment
 
/* this is a multiple 
    lines comment 
    also called block comment */
 

Object

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

Object usage

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

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

Function

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

Function with parameter

function greet(name="Alice") {
   console.log("Hello")
}
parameter name with default value "­Ali­ce"
 

Metadata

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  • Published: 3rd April, 2025

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