Answer 1

In a nutshell without weird words or exceptions.

"let" has a local scope, you can use it when you want to make a variable usable for the local scope only.

Example:

for (let i in array) { }

... variable "i" will be available inside the for loop only.

let is not supported in all browser yet.

var is global and generally you can access to a variable declared with this keyword everywhere (i want to emphasize "generally")

const is a constant and his value cannot be changed in runtime, just this. Use it when you want to make variables without change their value, for example:

const screen_width = innerWidth;

Answer 2

I recommend to deepen a bit following these articles: http://ccoenraets.github.io/es6-tutorial/

Answer 3

'var' has a function scope , no matter where it is declared within a function it can be used anywhere inside it
In 'var' Using first and declaring later will take it's value as 'undefined' (no error)
In 'let' Using first and declaring later will cause error - "unknown variable"

So using this is technically wrong -

function doSomething(){

for(var i = 0; i<10 ; i++){

}

for(var i = 0; i<10 ; i++){

}
}
Instead 'let' should be used which has kind of a bracket scope as @Maz said

The variables we use in high-level-languages like Java,C++, etc have bracket scope like 'let' (I always thought var has it too before finding out about let )

https://www.youtube.com/watch?v=q8SHaDQdul0
🎥 16.1: let vs var - Topics of JavaScript/ES6 - YouTube

Hope this clarifies var ⛹