Greedy Algorithm in JavaScript - Max Profit on Stock Prices (with comments)

greedyAlgorithm.js

/**
* Greedy Algorithm (maximum daily profit from stock sale) - "let me see what my options are, first..."
*
* Overview:
* ---------
* By using Greedy Algorithms we can pass over the data once (O(n) time), storing values we find to be optimal, per our criteria, by
* comparing them to current values. We have to go over the entire set to do this, but we only have to do it once - yay!
* 
* From Wikipedia: "A greedy algorithm is an algorithm that follows the problem solving heuristic of making the locally optimal
* choice at each stage with the hope of finding a global optimum."
*
* NOTE: This is my take on an interview answer to question taken from: https://www.interviewcake.com/question/javascript/stock-price
*
* @param pricesArr {arr}
* @returns int
*/
function getMaxProfit(pricesArr) {
  // make sure we have at least 2 prices
  if (stockPricesYesterday.length < 2) {
      throw new Error('Getting a profit requires at least 2 prices');
  }
  
  // init our first possible "buy price" (minPrice)
  var minPrice = pricesArr[0]
  // init our first possible maxProfit
  // the first 2 values in our array that can derive a profit figure
  // subtract our first "sell" price from our first "buy" price
  var maxProfit = pricesArr[1] - pricesArr[0]
  // init minIndex for comparisons on where we last set our minPrice
  var minIndex = 0
  
  // begin at 1, since we already have our first
  for (var i = 1, length = pricesArr.length; i < length; ++i) {  
    if (pricesArr[i] - minPrice > maxProfit) {
      maxProfit = pricesArr[i] - minPrice
    }
   
    // set new minPrice: if our current price is lower than our 
    // set minimum price of the day, then set the new minimum price
    // to equal that of the current price
    // NOTE: we check to eliminate the last price of day from being set, as the day is over
    if (pricesArr[i] < minPrice && i !== length - 1) {
      minPrice = pricesArr[i]
      minIndex = i
    }
  }
  
  return maxProfit
}

// boo, bearish market!
var stockPricesYesterday = [18, 17, 13, 11, 5, 1];
getMaxProfit(stockPricesYesterday)

For line 19, it should be pricesArr.length

Line no 30 and 44 not needed.