Difference between let, var and const

In vanilla JavaScript, variables are declared using ‘var‘ keyword. In version ES6, we can define the variables using let and const keywords too.

All three keywords have similar syntax for variable declaration and initialization, but they differ in their scope and usage.

1. Differences between var, let and const

We will see the main differences in short, and then we will explain further in the post.

• Variables declared by var and const keywords are function scoped and are scoped to the immediate function body.
  Also, variables declared with var keyword are hoisted (initialized with undefined before the code is run) which means they are accessible in their enclosing scope even before they are declared.
• Variables declared by let keyword are block scoped and are scoped to the immediate enclosing block.
  The let variables are not initialized until their definition is evaluated. Accessing them before the initialization results in a ReferenceError.

2. The 'var' Keyword

2.1. Variable Syntax

The var keyword has traditional variable definition syntax. Optionally, we can initialize the variable with a value.

If we do not initialize the variable in the var statement, it is automatically assigned the JavaScript value undefined.

var value; 	//value is 'undefined'

var data = 100;	//initialized with number 100

var blogName = 'howtodoinjava.com';	//initialized with string value

2.2. Function Scoped

• Variables declared with the var keyword are function scoped. It means that they can be accessed inside the same function only.

  function fun() 
  {
  	var dataX = 10;
  
  	if(true) 
  	{
  		var dataY = 20;	
  
  		console.log(dataX);	//Output 10
  		console.log(dataY);	//Output 20
  	}
  	
  	console.log(dataX);	//Output 10
  	console.log(dataY);	//Output 20
  }
  
  fun();
  
  console.log(dataX);	//Not available outside function; dataX is undefined
  console.log(dataY);	//Not available outside function; dataY is undefined
  

• Variables can be used before we either declare or initialize them. In this case, the value of the variable will be undefined, but there will not be any runtime error.

  function fun() 
  {
  	console.log(dataX);	//Output 'undefined'
  	
  	var dataX = 10;
  }
  fun();
  

2.3. Global Scoped If we do not use var Keyword

In JavaScript, if we do not use var keyword for variable declaration (implicit declaration) then variable will be created in global scope. e.g.

for(index = 0; index < array.length; index++){		//index is in global scope
   //code
}

Above for-loop will create a variable called index in the global scope. If some other code is also using a variable named 'index' variable, then we’ve just overwritten that other variable as well.

To avoid giving the variable global scope, we must use the var keyword in the variable declaration.

2.4. Variable Hoisting

Please note that variables declared with the var keyword are subject to hoisting.

Variable hoisting means that if we declare a variable (declared but not initialized) at the end of a function, the runtime will hoist it to the top and we will not have any error if we would have used that variable before being declared.

Read More: JavaScript Hoisting

3. The 'let' Keyword

The let keyword is very similar syntax to the var keyword – but it is more restrict in the scoping.

3.1. Block Scoped

1. Use the let statement to declare a variable, when the scope MUST BE restricted to the block in which it is declared.

   function fun() 
   {
   	let dataX = 10;	
   
   	if(true) 
   	{
   		let dataY = 20;	
   
   		console.log(dataX);	//Output 10
   		console.log(dataY);	//Output 20
   	}
   	
   	console.log(dataX);	//Output 10
   	console.log(dataY);	//dataY is 'undefined'
   }
   
   fun();
   

   See above highlighted line. If we would have used ‘var‘ keyword then dataY would have been available due to its global scope within function. Because we have used the let keyword, dataY is not visible beyond if block.

2. A variable declared using let cannot be used before its declaration or ReferenceError will result.

   function fun() 
   {
   	console.log(x);		//Output 'undefined'
   	console.log(y);		//Error - "Uncaught ReferenceError: y is not defined"
   	
   	var x = 10;
   	let y = 11;
   }
   fun();
   

3.2. No Variable Hoisting

Variables declared with let keyword are not hoisted. It means we can use the variable before we declare and initialize it.

4. The 'const' Keyword

The const keyword follows same rules as let keyword. Only difference is that const is used to drfine constants in the program.

4.1. Block Scoped

1. The const keyword declares a block-scoped variable with a constant value. It is basically variable declaration with ‘let‘ keyword where variable value is constant and cannot be changed.
2. The const follows the same scoping principles as the let keyword.
3. If we know that the variable we are declaring cannot and should not be allowed to reassign, then declare it with const, else use the let keyword.

4.2. const Syntax and Example

const PI = "3.14";

PI = 12;	//Uncaught TypeError: Assignment to constant variable.

Happy Learning !!