An example of JS Closures with inner & outer functions

In JS, functions have a reference to all variables declared in the same scope as well as in any other outer scopes.

Consider the following:

var createCounter = function(n) {
    let currentCount = n - 1;
    return function() {
        currentCount = currentCount + 1;
        return currentCount       
    };
};

/** 
 * const counter = createCounter(10)
 * counter() // 10
 * counter() // 11
 * counter() // 12
 */

The inner anonymous function has access to the currentCount variable. When the inner function is returned by createCounter(10), the inner function has access to any declared variable in the same lexical environment scope.