Before I write anything I just want to let you know that this is solely my personal understanding of the language I might be absolutely wrong about something but hey this is why you and I are here, if you find something wrong just file a PR or DM me on twitter.

What is javascript

Short answer, its a programming language.

Javascript variables

Javascript variables are just a bucket to hold values.

Data Types

In javascript variables don't have data type, but values do, unlike other programming language where you declare a variable using some keyword like (int, string etc) in javascript we dont.

Let's have a look into some of the primitve data type in JS.

• undefinded
• String
• Number
• Boolean
• Object
• Symbol

• BigInt

  Apart from these we have other data type as well. These are

• Object
• Array (Sub type of Object)
• Function (Sub type of Object)
• null

Ok so before you fire PR, let me explain the phrase "variables don't have data type but values do"

var a
typeof a // undefined
a = 10
typeof a // Number

Now the question arise that hey isn't undefined a type. Well good question but javascript assigns undefined to everything not defined. So if you check

typeof c //undefined

its undefined even though we didn't even declare it.

JS type Coercion

Type coercion is process of converting value from one type to another. These are of two types

1. Explicit
2. Implicit

Explicit conversion is when you express the intent to convert between types, a good example will be

"I got" + String(80) + "in english"

Javascript being weakly-typed language values can also get converted between different types automatically this is known as Implicit coercion. For example

"a" + 2 // a2

Boolean

Unline other language JS has two Booleans, I like to call them truthy and falsy instead of true or false. Now what does truthy or falsy means , let's have a look.

Equality Comparisons

JavaScript has two different value-comparison operations

1. Loose equality comparison ==
2. Strict equality comparison ===

Loose equality comparison allow coercion but Strict equality comparison disallow it, it checks for the type too.

And I lied to you it has 3 equality comparison operator instead of 2, the new one object.is has been added recently but it uses the above two comparison operator inside so I didn't added it in the list 😉.

Scope

In js scope is current context of execution. Its the place where JS is looking for thing ( values and expressions ) if they are not present inside it will look for outer scope.

Function expression

Its basically just functioned asiigned as value to some variable.

var handleRequest = function () {}

IIFE these are imediately invoked function expression that runs as soon as it is defined.

(function () {
  //your code
})()

IIFE prevents polluting global scope variable.

Closure

Its a term defined for the case where a function remembers the variables outside of it even if you pass that function elsewhere.

function ask(question){
    return function holdYourQuestion()){
        console.log(question)
    }
}
 var myQuestion = ask('What is closure')

It preserves variable of a function to use them at different time of need.

this

A function's this referrs the execution context fot that call, determined entirely by how the function was called. Therefore, this has dynamic context and can be different each time function is called.

Arrow functions don't provide their own this binding.

Fun fact with bind() we can set the value of this regardless of how its called.

Prototype

Its an Object where instances are linked to. Or in mdn's way a mechanism by which JavaScript object inherit features from one another.

function Person(name) {
  this.name = name
}

Person.prototype.greet = function (phrase) {
  console.log(this.name, phrase)
}

var hrishikesh = new Person("Hrishikesh ")

hrishikesh.greet("Hello 👋") // hrishikesh hello

new creates object of the constructor function linked with prototype methods. So when a method is created with protoype its going to be linked to every instance of the construnctor

That's it for javascript101