Nowadays JavaScript has three different keywords to declare a variable — var, let and, const. Each has its own properties and particularities. Let’s start by making a simple comparison table ...

It assumes the reader has experience programming in JavaScript. Functions as a map between two sets. A functional programmer views the nature of a function in a similar way as does a mathematician: ..

i.e. intuitively it looks like the variable will only be available within the block but it is accessible outside the block .. in javascript's case, because var doesn't make a block level ...