What is the javascript var, let, const variables ?

What is the javascript var, let, const variables ?

Umut Cakmak
·Oct 15, 2021·

Subscribe to my newsletter and never miss my upcoming articles

var ;

  • You can declare the variable "var" globally or locally. The var variable has no block scope.

For example ;

function sayHello() {
  hey = "Hello";
  console.log(hey);
  var hey;

// The definition of the variables happens when the function starts.
}
  • var = i; It is a global variable, so after the for loop it is assigned a value of 5. It loops again with the variable 5 each time.
for (var i = 0; i < 5; i++) {
  setTimeout(function () {
      console.log(i);
  }, 1000);
}

// Answer 5
  • The var keyword does not return an error when re-declared.
var a = 0;
var a;
console.log(a); // 0

let ;

  • With ES6 we can declare a variable using the let keyword. It has similar properties to the var keyword.

  • Variables are declared using the let keyword are block-scoped, are not attached to the global object.

let counter = 0;
let counter;
console.log(counter);

// SyntaxError...

In ES6, the let keyword declares a new variable in each loop iteration.

for (let i = 0; i < 5; i++) {
  setTimeout(function () {
      console.log(i);
  }, 1000);
}

// Answer : 0,1,2,3,4

The let keyword cannot be used this way.

    console.log(counter); 
    let counter = 10;    

    // Error

const ;

  • Same as let but variables defined with const are defined as "constant". They cannot be changed, an error is received when trying to change them.
const myBirthDay = '28.11.1992';
myBirthDay = '01.01.1993'; // error , because the fixed value cannot be changed.
const QUANTITY = 0.5;
QUANTITY = 0.2; // TypeError
const BLACK; // SyntaxError
  • Values in the referenced object can be changed.
const umut  = { age: 20 };
umut.age = 28; // OK
console.log(person.age); // 28
umut = { age: 40 }; // TypeError

Thanks for reading.