SOURCE

var maxProfit = function(prices) {
    var minP = prices[0], maxP = 0;
    prices.forEach(function (value,index,arry) {
          minP = Math.min(value,minP);
          maxP = Math.max(maxP,value - minP);
    });
    return maxP;
};
console.log(maxProfit([7,1,5,3,6,4]))