In the following code snippet, if you can explain why the values differ for the variables declared with var and let/const, then you may skip this article. Thank you for giving it a look.

console.log(a); //Output: undefined
var a = 1;
console.log(a); //Output: 1

console.log(b); //Output: Reference Error: b is not defined
const b = 2;
console.log(b); //Output: 2

If not, then let's dive in.

Temporal Scoping

The reason is temporal-scoping. It was introduced in ES6 with the introduction of let/const.

A variable declared with let/const is unavailable before the declaration. Whereas, if declared with var, a variable is available in the entire scoping context. However, a value is assigned to it only when the variable is initialized. That is why, in the above example, a is available to use before its declaration, but it's still undefined.

Conclusion

ES6 (ECMAScript 6 now) has been around for many years now. The concepts introduced with it are well-supported in all major browsers. However, this aspect of scoping is not talked about much. I hope that you learned something interesting from this post. Until the next one. Cheers!