Show the difference between a++ and ++a

plusplus-a-or-a-plusplus.js

/**
 * update quantity based on button type
 * make sure we don't go below zero
 */
if( action_type === 'minus' ) {

    // this returned my value, then decremented it, so it never saved
    // hitting the minus button always resulted in the same number
    quantity = quantity ? quantity-- : 0;

    // this decremented my value, then returned it
    // which works as expected
    quantity = quantity ? --quantity : 0;

} else if ( action_type === 'plus' ) {
    // either of these are fine, because I'm not returning the value, 
    // just using it later down the live
    item_quantity++;
    ++item_quantity;
}