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Let Var and Const Variable in Javascript: What’s the difference?

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Table of Contents

Let Var and Const in Javascript

Variables are Containers for Storing Data and the variable is the name of the memory location.

A variable is a kind of data storage that has labels. It’s a method for identifying and storing data and values for subsequent program executions. Imagine a container has a name, inside which you can store various items such as words, numbers, or other forms of data. You can declare variables using three keywords: let, var, or const. 

Var

  • One way to declare variables in JavaScript is with the var keyword. 
  • Before ES6 var was used to declare variables in JavaScript.
  • The var keyword indicates that the variable is to be available only within the function that declared it. 
  • Because of its peculiarities and possible problems, var is less frequently used in contemporary JavaScript.
var age 25;
console.log("Initial Age:", age); // Output: Initial Age: 25

// Update the variable
if (true) {
   var age=30; // This reassigns the variable, even though it's within a block 
   console.log("Updated Age Inside Block:", age); 
 }

//Output: Updated Age Inside Block: 30

// Display the updated value outside the block
 console.log("Updated Age Outside Block:", age);

// Output: Updated Age Outside Block: 30

Let

  • Let keyword is  used to declare variables in javascript
  • Introduced with ECMAScript 6 (ES6), it provides block scope. Exclusively within the block containing their definition, encased in curly braces, are variables declared using let accessible.
  • Because its behavior is more predictable than that of var, let is more commonly used.

Example 1:

let message="Hello"; // Declare a variable using let
message=message+" World"; // updated the variable
console.log(message) // output:Hello World

Example 2:

let age 25;
console.log("Initial Age:", age); // Output: Initial Age: 25

// Update the variable
if (true) {
   var let=30; // This creates new variable, because it is in new block 
   console.log("Updated Age Inside Block:", age); 
 }

//Output: Updated Age Inside Block: 30

// Display the updated value outside the block
 console.log("Updated Age Outside Block:", age);

// Output: Updated Age Outside Block: 25

“Let” has block scope so when we declared “age” using “let” inside “if block”.This will not change the value of “age” outside the block.

Const

  • In JavaScript, const is a keyword used to declare variables
  • Another new feature in ES6 is the ability to declare variables as constants with the const keyword. These variables should not have their values changed after initialization.
  • This is why the const keyword is so useful when you need a variable to have a constant value all through your code.
  • Using const not only communicates the immutability of a variable but also helps prevent accidental reassignment, making your code more robust and self-explanatory.
  • Like let, const has block scope.
const pi = 3.14;
console.log(pi); // Output: 3.14

// Attempting to reassign a const variable will result in an error
pi = 3.14159; // Error: Assignment to constant variable

Differences between var, let, and const:

Sr NoVarLetConst
1The scope of a var variable is functional.The scope of a let variable is block scope.The scope of a const variable is block scope.
2It can be updated and re-declared into the scope.It can be updated but cannot be redeclared into the scope.It cannot be updated or re-declared into the scope.
3It can be declared without initialization.It can be declared without initialization.It cannot be declared without initialization.
4It can be accessed without initialization as its default value is “undefined”.It cannot be accessed without initialization otherwise it will give ‘referenceError’.It cannot be accessed without initialization, as it cannot be declared without initialization.
5The variable can be hoisted, with initializing as a ‘default’ value.The variable can be hoisted, but not initialized (this is the reason for the error when we access the let variable before declaration/initialization)Variable can be hoisted, but not initialized (this is the reason for the error when we access the const variable before declaration/initialization)
// Using var 
var x = 5;
if (true) {
    var x = 10;
    console.log("Inside Block (var):", x); // Output: Inside Block (var): 10
}
console.log("Outside Block (var):", x); // Output: Outside Block (var): 10
// Using let
let y=5;
if (true) { 
let y= 10;
console.log("Inside Block (let):", y); // Output: Inside Block (let): 10
}
console.log("Outside Block (let):", y); // Output: Outside Block (let): 5
// Using const

const z = 5;
// const z 10; // Uncommenting this line will result in an error
console.log("Const Variable (const):", z); // Output: Const Variable (const): 5

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MYSQL JOIN

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NUMPY
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PANDAS
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HYPOTHESIS TESTING
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LINEAR REGRESSION
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POLYNOMIAL REGRESSION
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MULTIPLE REGRESSION
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RANDOM FOREST ALGORITHM
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SUPPORT VECTOR MACHINES
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  • Creating confusion matrix
NAIVE BAYES CLASSIFIER ALGORITHM
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    Classifier
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What is React.js?
History and evolution of React.js
React.js components and their role in
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JSX

Introduction to JSX
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What is Node.js?
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NODE.JS MODULES

Introduction to Node.js modules
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Manager)

NODE.JS HTTP

Creating HTTP server in Node.js
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EXPRESS.JS FRAMEWORK

Introduction to Express.js framework
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application
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Express.js
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NODE.JS DATABASE CONNECTIVITY

Introduction to database connectivity in
Node.js
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Node.js
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using MongoDB
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Node.js

NODE.JS SECURITY

Introduction to security in Node.js
Understanding and implementing
authentication and authorization in
Node.js
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