6.md

Difference between var and let keyword in javascript.

The var and let keywords are both used to declare variables in JavaScript, but there are some important differences between them.

• Scope: Variables declared with var are function-scoped, while variables declared with let are block-scoped. This means that a var variable is accessible from anywhere within the function in which it is declared, while a let variable is only accessible from within the block in which it is declared.
• Hoisting: var declarations are hoisted to the top of their scope, while let declarations are not. This means that a var variable can be accessed even before it is declared, while a let variable cannot be accessed until it is declared.
• Redeclaration: var variables can be redeclared, while let variables cannot. This means that you can declare a var variable multiple times within the same scope, but you can only declare a let variable once within the same block.
• Temporal dead zone: let variables have a temporal dead zone, while var variables do not. This means that you cannot access a let variable until the end of the statement in which it is declared.

Here is an example of the difference between var and let scoping:

function foo() {
  var x = 1;
  {
    console.log(x); // 1
    var x = 2;
    console.log(x); // 2
  }
  console.log(x); // 1
}

foo();

In this example, the first console.log() statement will print 1, the second console.log() statement will print 2, and the third console.log() statement will print 1. This is because the var variable x is function-scoped, so the second var declaration of x does not shadow the first var declaration of x.

Here is an example of the difference between var and let hoisting:

function foo() {
  console.log(x); // ReferenceError: x is not defined
  let x = 1;
}

foo();

In this example, the console.log() statement will throw a ReferenceError because the let variable x is not declared yet. This is because let declarations are not hoisted, so the let declaration of x is not visible until the end of the foo() function.

Here is an example of the difference between var and let redeclaration:

function foo() {
  var x = 1;
  x = 2;
  var x = 3; // Error: x is already defined
}

foo();

In this example, the second var x = 2; statement will not compile because the var variable x has already been declared. This is because var variables can be redeclared.

Here is an example of the difference between var and let temporal dead zone:

function foo() {
  let x;
  if (true) {
    x = 1;
  }
  console.log(x); // ReferenceError: x is not defined
}

foo();

In this example, the console.log() statement will throw a ReferenceError because the let variable x is not defined yet. This is because the let variable x is not visible until the end of the if statement.