let and const keywords in ES6

let and const are new keywords introduced in ES6 which enable us to change the way we work with variables and constants. Let and const are both block scoped. Prior to ES6, we had only function scope and global scope. Block scope restricts variable access to blocks, i.e. constructs with curly braces, such as if...else, for loops, etc. As an example:

In the past, we would declare variables using var as follows:

if(true){
  var age = 23
}
console.log(age);

But with ES6, if we now use let to declare a variable inside a block of code, it fails with a legitimate error:

if(true){
  let age = 23
}
console.log(age);

Code block above fails with an error such as:

"error"
"ReferenceError: age is not defined
    at yozibopigo.js:9:38
    at https://static.jsbin.com/js/prod/runner-4.1.8.min.js:1:13924
    at https://static.jsbin.com/js/prod/runner-4.1.8.min.js:1:10866"

You can read more about scopes here.

Question: Why wasn’t just var modified to be block-scoped?

It wasn’t done so due to the backward compatibility reasons. If var was changed to be block scoped a lot of existing code would go useless.

The const keyword is a new block-scoped keyword that is used to define constants. Thus, once the value of a constant has been defined, it cannot be changed later on. The following code, for example, would result in an error:

const AGE = 23
AGE = 27

"TypeError: Assignment to constant variable.
    at yozibopigo.js:2:5
    at https://static.jsbin.com/js/prod/runner-4.1.8.min.js:1:13924
    at https://static.jsbin.com/js/prod/runner-4.1.8.min.js:1:10866"

It is important to keep in mind that when a constant is defined, it does not hold the value but merely refers to the value being defined. Which is why, when a refernce type such as an array or an object is defined as a constant, they can be altered without an issue.

Since, const holds a reference type as explained above, following code block works just fine:

const AGES = [23, 24, 25]
AGES.push(26)