Use JS best practices

[dalmia]

Reference: https://github.com/getify/You-Dont-Know-JS/blob/1st-ed/scope%20%26%20closures/ch3.md

• Law of Least Exposure: wrap functions being used by just one function within that function
• Improved Garbage Collection: if some data is not going to be used by later code within a block or function, put that data within its own block so that the Garbage Collector can safely get rid of it. Otherwise, it cannot as that data would be a part of the closure of a function appearing later in the code (read the reference for more details - the example just above 'let' loops)
• use const wherever possible, then let and then var.