All about Var, Let & Const

Afzal shaikh — Mon, 17 Aug 2020 10:57:58 +0000

Var,Let & Const

lets talk about Var first

we use it to declare a variable like this

    var x = 100;

but do you know you can decalre it again ??

    var x = 100;
    var x = 300;
    consol.log(x)//will successfully print 300 in your console

yes, the var variables are redefinable

which means in a long code you can define a variable twice by mistake and then spend a lot of time trying to figure out what went wrong

scoping -
var as we know is function scoped , which means its only availible in the function that it is created

    function name(){
        var name = "penguin";//only available in this function

        console.log(name);//will work
    }

    name() //will work

    console.log(name);//will scream at you in red color

so what happens when it is declared outside a function ?
it becomes globally scoped

    if(marks > 40){
        var grade = "pass"; //variable decalred not within a function 
        console.log("your grade is ${grade}")
    }

if you write this code , and then in your console type out console.log(grade) then it will still print the grade which means the variable grade is being stored even after the execution of that line which is not required, which means the var grade has leaked from it's block and can create problems later on

mainly these aree the points where Var differs with let and const
you can learn more about var about here

Now let's talk about 'Let' and 'Const'

first of all let and const cannot be redefined like var

no way to use the same variable name again

    let x = 100;
    let x = 300 ; //will not work and will scream at you in red

but as let and const are block scoped meaning they're available in the block they're defined in


let marks = 78
let isPass = false;//globally scoped 

if(marks > 50){//start of the block

    let isPass = true; //block scoped

}//end of the block

console.log(isPass)//will print 'false'

here we've used the same name isPass to define a variable two times but both of them refer to two different vairable values , which means they're not the same variable because of their scoping , while the first `isPass' is defined globally , what we have to do is to update it instead of redefining it for the desired outcome

`js

let isPass = true; // will not work
isPass = true; //will work

while a let defined variable can be changed like this a const defined cannot be changed however, you can change attributes in a const defined object , for e.g

`js

const student= {
    name = 'penguin'
    marks = 78
}

//we cannot reassign the whole variable like

student = { name = 'not penguin' } // this will not work

//but we can change the properties 

student.marks = 97; //this will work

so, i think this information is enough to understand differences between Var , Let & Const
Hopefully you've gained something from this
As this is my first post i hope you liked it, Thank you
i intend to post more about whatever i have learned you can follow me for more such posts on twitter @aFlying_Penguin

Forem: Afzal shaikh

All about Var, Let & Const

Var,Let & Const

lets talk about Var first

Now let's talk about 'Let' and 'Const'