TypeScript Variables – var, let and const

In vanilla JavaScript, variables are declared using ‘var‘ keyword. In ES6 (TypeScript), we can define variables using let and const keywords also.

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

Let’s learn the differences between var, let and const.

1. var keyword

‘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

1.1 Scope

• Variables declared with the var keyword have function scope (i.e. global scope inside function in which they are declared); this means that they can be accessed by any function sharing the same scope.

  function fun() 
  {
  	var dataX = 10;	//Globally available inside function fun()
  
  	if(true) 
  	{
  		var dataY = 20;	//Globally available inside function fun() 
  
  		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 declare or initialize them. Their value will be undefined in this case, but there will not be any runtime error.

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

1.2. Hoisting of var declarations

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

1.3. 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. let keyword

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

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 an error 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. const keyword

1. The const keyword declares a block-scoped variable with a constant value. It is basically variable declaration with ‘var‘ keyword where variable value is constant and cannot be changed.
2. 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.

3.1. const Syntax and Usage

const PI = "3.14";

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

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Happy Learning !!