In the release of ES6 (ECMAScript 2015) a new way of declaring variables in JavaScript has been introduced. Actually 2 new ways. let and const. With these two, we now have 3 different ways to declare variables. var, let, const

What are the differences? The main difference is the scoping 

• let
  Block scope
  
  
• const
  Block scope
  
  
• var
  Function scope

In nutshell, block scope is the code that is enclosed in curly braces

The javascript engineers have been troubled often with the callbacks and the callback hell. The rescue are the promises: an easy and convenient way to handle the asynchronous calls.

A promise is an alternative way which allows us to handle asynchronous operations in a more elegant way than callbacks

Let's assume a simple story which will help us understand this. (well OK, I know that its not the best example you've ever read)

A father asks his son to go and buy a newspaper. The father is awaiting for the newspaper and will start reading it when his son returns back. On the other hand, the father knows that his son, who loves playing out in the yard, has some possibilities to fall down and start crying. In this case he won't read his newspaper and he will have to care of his son.

There are two cases in the story; the father will read the newspaper. The father won't read the newspaper.

• What happens on a single part of JS code?
  
• Is the code interpreted or compiled?
• What is the variable shadowing?
• What is the variable hoisting?
• What are the closures

In the following slide presentation I will try to answer the aforementioned questions by showing what the JS engine does on a simple part of code
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Javascript compilation - execution

* In the company that I work for, we have a ceremony, called DOJO, in which any employer can present his/her ideas about any subject he/she wants to share (though we are trying to be focused on technological stuff). As such, this slide presentation was one of our DOJOs