Source : Understanding closures in JavaScript

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What is a closure?
A closure is a feature in JavaScript where an inner function has access to the outer (enclosing) function’s variables — a scope chain.
The closure has three scope chains:
it has access to its own scope — variables defined between its curly brackets
it has access to the outer function’s variables
it has access to the global variables

When to use Closure?
Closure is useful in hiding implementation detail in JavaScript. In other words, it can be useful to create private variables or functions.