var, let, and const in JavaScript.

Question

Amrith Chandran · Answer

Let's understand the differences between var, let, and
const in JavaScript with the help of simple examples.
var
Declared variables have function scope (not block scope).They can be redeclared and updated.Supports hoisting (moved to the top of the scope, initialized with
undefined).
Example-
var name = "Manish";
console.log(name); // output- Manish
var name = "Sharma"; // Redeclare the name variable with new value
console.log(name); // output- Sharma
if (true) {
 var age = 30; // var ignores block level scope
}
console.log(age); // 30 (accessible outside block)
and 
let
Declared variables are block-scoped (only within the {}).They can be updated, but not redeclared in the same scope.They are not hoisted in the same way (they exist in a "temporal dead zone" until defined).
Example-
let city = "Delhi";
console.log(city); //output- Delhi
city = "Mumbai"; // can update or reasign a new value
console.log(city); //output- Mumbai
let city = "Noida"; // Return error: can't redeclared
if (true) {
 let country = "India";
 console.log(country); //output- India
}
// console.log(country); //Return error (not accessible outside block)
and 
const
Declared variables are block-scoped (only within the {}).It is mandatory to initialize it when declaring it.It cannot be updated or redeclared.For objects/arrays, you can modify their contents (but not 
reassign them).
Example-
const pi = 3.1416;
// pi = 3.14; // Error - cannot reassign
const user = { name: "Manish" };
user.name = "Sharma"; // allowed (object content can change)
console.log(user.name); // outout- Sharma
// user = { name: "New" }; // Error - cannot reassign object
Let's see a quick example to compare all.
function demo() {
 var a = 10;
 let b = 20;
 const c = 30;
 if (true) {
   var a = 100; // same variable (function-scoped)
   let b = 200; // new variable (block-scoped)
   const c = 300; // new variable (block-scoped)
   console.log(a, b, c); // output- 100 200 300
 }
 console.log(a, b, c); // output- 100 20 30
}
demo();
and 
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var, let, and const in JavaScript.

Amrith Chandran

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1 Answers

var

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const

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