Difference between let and var keyword

var keyword

The var statement is used to declare a variable in JavaScript. A variable declared with the var keyword is defined throughout the program.

Example

Output:

let keyword

The let statement is used to declare a local variable in TypeScript. It is similar to the var keyword, but it has some restriction in scoping in comparison of the var keyword. The let keyword can enhance our code readability and decreases the chance of programming error. A variable declared with the let keyword is limited to the block-scoped only.

Note: The key difference between var and let is not in the syntax, but it differs in the semantics.

Example

Output:

The above code snippet throws an error because the variable "hello" is not defined globally.

Var vs. Let Keyword

function varGreeter(){
  var a = 10;        
  var a = 20; //a is replaced
  console.log(a);
}
varGreeter();

function varGreeter(){
  let a = 10;        
 let a = 20; //SyntaxError: 
 //Identifier 'a' has already been declared
  console.log(a);
}
varGreeter();

{
  console.log(c); // undefined. 
  //Due to hoisting
  var c = 2;
}

{
  console.log(b); // ReferenceError: 
  //b is not defined
  let b = 3;
}