JavaScript Cheat Sheet

From the Author:

This beautiful notes was made by Wreaking#9752. Join my Discord Server: https://discord.gg/Z7JXdVbE2J

• I made these notes, because I usually forget some stuff about JavaScript. So I used to refer from these notes.
• I thought of sharing these notes with everyone, because everyone deservers to learn JavaScript.
• I tried my best to make it easy for you guys, so that you understand the basic concept.
• You need to practice it on a daily basis, if you are willing to learn JavaScript.
• I recommend you to use https://replit.com and select node.js or JavaScript.

Introduction:

First of all you need to be good at Algebra and basic Mathematics, and if you're not, make sure to brush up those concepts.

So, let's begin!

console.log("Welcome to JavaScript!")

Declaration & It's Uses:

---

Declaring variables in JS are,

• const,
• var,
• let

var was the most commonly used declaring variable in older days, but now-a-days, const and let are mostly used.

There are differences in using these 3 declaring variable, here's a simple table that explains it all,

Declaring Variable	Block Scoped	Creates Global Property	Re-assignable	Re-declarable
var	❌	✔	✔	✔
let	✔	❌	✔	❌
const	✔	❌	❌	❌

From the table, we can conclude that, if a variable is decalred using var or let, we can assign a new value to that same varible, even though an older value is assigned to it. For an example,

let a = 5
let b = 6
a = b + a

console.log(a)

Here, a will return the new value, although if it was delacred using const, it will not work anymore.

Note: Recently, JavaScript released two important keywords let and const. These two keywords provide Block Scope in JavaScript. Variables declared inside a {} block cannot be accessed from outside the block. For example,

{
  let x = 2;
}

Here, x cannot be used out of the block. But we can use var inside and outside the block,

{
  var x = 2;
}

In this case, x can be used outside the block, because var doesn't have Blocked Scope property.

Variables:

Variables are used to store temporary data. For an example,

var x = 5
var y = 6

In this example x and y both are variable, with the value of 5 and 6 stored in it respectively. we can use const and let instead of var.

In JavaScript, if you declare a variable without any of let, const or var, by default, it registers the variable as let. For example,

x = 5
y = 10
z = a + b

console.log(z) // Value of z will be 15

Data Types:

The set of types in the JavaScript language consists of Primitive Values and Objects.

• Primitive Values
  • Boolean
  • Null
  • Undefined
  • Number
  • BigInt
  • String
  • Symbol
• Objects (Collector of Properties)

Get a detailed infomration at here.

Difference between "=" and "=="

• "=": This is assigment operator
• "==": This is equal to operator
  

So, you can save a value to a variable by using, "=" and check whether the value of the variable is equal to another value by using "==".

Declaring variables in one line

• Use comma to seperate it,

let x = "Among us", y = "is sus", z = 100

Or you can do this different lines,

let x = 'Among us';
let y = 'is sus';
let z = 100;

You can also, declare undefined variables first, and then save data to them.

Note: Semi-colon (;) is used to finish a line. Although, the current versions of JS doesn't require ";" at end of the line.

We can add Strings, Numbers, Booleans like,

let x = "abcc" // <- is a string
let y = 100 // <- is a number
let z = true or false // <- is a boolean

If you don't assign any value to a variable, JS will register the value as undefined. Remember, a String is always defined inside "" or ''.

Try It Yourself:

Log the values of these given problems.

let x = 5 + 4 + 7
console.log(x)

let k = "10" + 4 + 4
console.log(k)

let y = "FBI" + " " + "OPEN" + " " + "UP"
console.log(y) 

Explanation:

const x = 5

In this code, const is a declaration. x is the a defined as a variable. = is assignment

Operators:

Operators are defined as the basic Mathematical addition, subraction, multiplication and more, +, -, *, /.

For example,

(2 + 2) * 10

Expression:

Expression is a combination of values, variables, and operators, which computes (to calculate an answer or amount by using a machine) to a value.

For example,

10 * 10 // evalutes 100

let x = 5
let z = x * 5

console.log(z)

The answer will be 25. We will learn about console logs later.

"Wrea" + "" + "king" // It evaluates (to determine or fix the value of) as "Wreaking"

"Wrea" + " " + "king" // It evaluates as "Wrea king", because we have added a space between the quotes (string).

Comments:

// -> Two forward slashes place one after another are known as a comment. It can be used to explain a particular statement or a bunch of statement. Texts written inside it are ignored by JS hence, they cannot be executed.

For example,

let y = 10
// let x = 8 -> This line will be ignored while execution of the code

/* */ -> We can use this to comment out a portion of a code. Code, inside this slashes, won't get executed.

For example,

/*
let x = "wow"
let z = "among"
let y = "sus" -> These three lines of codes will be ignored and won't be executed
*/

Identifiers:

JavaScript Identifiers are named as variables, functions, etc.

Caution:

• You cannot use JS reserved keywords as the name of a variable. Such as, break, new or boolean variable names are not a valid variable name.
• Variable names cannot contain any special or numeric characters. The only excepted speical characters are - & _.

We can use two variables to define two strings like,

let title, surname

title = "Wrea"
surname = "king"

console.log(title) // Result will be "Wrea"
console.log(surname) // Result will be "king"

Note: Before assigning the two variables to their values, if we console logged the two variables, we'd get undefined.

Camel Case:

What is it?
When multiple words are used to form a variable, camel case joins those words together, without any white space, and delineates (to describe) the start of each new word with a capital letter.

Camel Case are of 2 types,

• Upper Camel Case (Pascal Case): FirstName, LastName, MasterCard, InterCity.
• Lower Camel Case: firstName, lastName, masterCard, interCity.

Credits

Written by: Wreaking#9752 (Discord Server)
Edited & Updated by: DragoLuca#3636 (Discord Server)

References

• w3Schools
• MDN Web Docs

🔗 Links