uniqueInteger.js

// Initialize the counter property of the function object.
// Function declarations are hoisted so we really can
// do this assignment before the function declaration.
uniqueInteger.counter = 0;

// This function returns a different integer each time it is called.
// It uses a property of itself to remember the next value to be returned.
function uniqueInteger() {
    return uniqueInteger.counter++;  // Return and increment counter property
}
uniqueInteger()  // => 0
uniqueInteger()  // => 1