What is the difference between let and var in Javascript?

In JavaScript, let and var are both used to declare variables. However, they have some key differences that you should be aware of.

Scope

The main difference between let and var is that let has block-level scope, while var has function-level scope. This means that a let variable is only available within the block of code in which it is declared, while a var variable is available throughout the entire function in which it is declared.

For example:

if (true) {
  let x = 5;
}
console.log(x); // ReferenceError: x is not defined

if (true) {
  var y = 5;
}
console.log(y); // 5

In the example above, the let variable x is only available within the block of code inside the if statement, and an error is thrown when we try to access it outside the block. On the other hand, the var variable y is available throughout the entire function, and we are able to access it outside the block.

Temporal Dead Zone

Another difference between let and var is that let variables are subject to the “temporal dead zone” (TDZ). This means that a let variable is not accessible until it has been fully declared.

For Example:

console.log(x); // ReferenceError: x is not defined
let x = 5;

In the example above, an error is thrown when we try to access the let variable x before it has been fully declared. On the other hand, a var variable is accessible before it has been fully declared, as shown in the example below:

console.log(y); // undefined
var y = 5;

In the example above, the var variable y is accessible before it has been fully declared, and its value is undefined.

Re-declaration:

A let variable cannot be re-declared within the same block of code, but a var variable can be re-declared within the same function.

For example:

let x = 5;
let x = 10; // SyntaxError: Identifier 'x' has already been declared

var y = 5;
var y = 10; // no error

In the example above, an error is thrown when we try to re-declare the let variable x, but no error is thrown when we re-declare the var variable y.

In general, it is recommended to use let over var whenever possible, as let has more intuitive behavior and can help prevent common errors. However, var may still be used in certain cases, such as when you need to declare a variable with function-level scope.