Interview Algorithm Questions in Javascript() {...}
A mostly reasonable collection of technical software development interview questions solved in Javascript in ES5 and ES6
Table of Contents
 Array
 Strings
 Stacks and Queues
 Recursion
 Numbers
 Javascript Specific
 To Be Continued
Array
 1.1 Given an array of integers, find the largest product yielded from three of the integers
var unsortedArray = [10, 7, 29, 30, 5, 10, 70]; computeProduct(unsortedArray); // 21000 function sortIntegers(a, b) { return a  b; } // Greatest product is either (min1 * min2 * max1  max1 * max2 * max3) function computeProduct(unsorted) { var sortedArray = unsorted.sort(sortIntegers), product1 = 1, product2 = 1, array_n_element = sortedArray.length  1; // Get the product of three largest integers in sorted array for (var x = array_n_element; x > array_n_element  3; x) { product1 = product1 * sortedArray[x]; } product2 = sortedArray[0] * sortedArray[1] * sortedArray[array_n_element]; if (product1 > product2) return product1; return product2; }
 1.2 Being told that an unsorted array contains (n  1) of n consecutive numbers (where the bounds are defined), find the missing number in
O(n)
time// The output of the function should be 8 var arrayOfIntegers = [2, 5, 1, 4, 9, 6, 3, 7]; var upperBound = 9; var lowerBound = 1; findMissingNumber(arrayOfIntegers, upperBound, lowerBound); // 8 function findMissingNumber(arrayOfIntegers, upperBound, lowerBound) { // Iterate through array to find the sum of the numbers var sumOfIntegers = 0; for (var i = 0; i < arrayOfIntegers.length; i++) { sumOfIntegers += arrayOfIntegers[i]; } // Find theoretical sum of the consecutive numbers using a variation of Gauss Sum. // Formula: [(N * (N + 1)) / 2]  [(M * (M  1)) / 2]; // N is the upper bound and M is the lower bound upperLimitSum = (upperBound * (upperBound + 1)) / 2; lowerLimitSum = (lowerBound * (lowerBound  1)) / 2; theoreticalSum = upperLimitSum  lowerLimitSum; return theoreticalSum  sumOfIntegers; }
 1.3 Removing duplicates of an array and returning an array of only unique elements
// ES6 Implementation var array = [1, 2, 3, 5, 1, 5, 9, 1, 2, 8]; Array.from(new Set(array)); // [1, 2, 3, 5, 9, 8] // ES5 Implementation var array = [1, 2, 3, 5, 1, 5, 9, 1, 2, 8]; uniqueArray(array); // [1, 2, 3, 5, 9, 8] function uniqueArray(array) { var hashmap = {}; var unique = []; for(var i = 0; i < array.length; i++) { // If key returns undefined (unique), it is evaluated as false. if(!hashmap.hasOwnProperty(array[i])) { hashmap[array[i]] = 1; unique.push(array[i]); } } return unique; }
 1.4 Given an array of integers, find the largest difference between two elements such that the element of lesser value must come before the greater element
var array = [7, 8, 4, 9, 9, 15, 3, 1, 10]; // [7, 8, 4, 9, 9, 15, 3, 1, 10] would return `11` based on the difference between `4` and `15` // Notice: It is not `14` from the difference between `15` and `1` because 15 comes before 1. findLargestDifference(array); function findLargestDifference(array) { // If there is only one element, there is no difference if (array.length <= 1) return 1; // currentMin will keep track of the current lowest var currentMin = array[0]; var currentMaxDifference = 0; // We will iterate through the array and keep track of the current max difference // If we find a greater max difference, we will set the current max difference to that variable // Keep track of the current min as we iterate through the array, since we know the greatest // difference is yield from `largest value in future`  `smallest value before it` for (var i = 1; i < array.length; i++) { if (array[i] > currentMin && (array[i]  currentMin > currentMaxDifference)) { currentMaxDifference = array[i]  currentMin; } else if (array[i] <= currentMin) { currentMin = array[i]; } } // If negative or 0, there is no largest difference if (currentMaxDifference <= 0) return 1; return currentMaxDifference; }
 1.5 Given an array of integers, return an output array such that output[i] is equal to the product of all the elements in the array other than itself. (Solve this in O(n) without division)
var firstArray = [2, 2, 4, 1]; var secondArray = [0, 0, 0, 2]; var thirdArray = [2, 2, 3, 2]; productExceptSelf(firstArray); // [8, 8, 4, 16] productExceptSelf(secondArray); // [0, 0, 0, 0] productExceptSelf(thirdArray); // [12, 12, 8, 12] function productExceptSelf(numArray) { var product = 1; var size = numArray.length; var output = []; // From first array: [1, 2, 4, 16] // The last number in this case is already in the right spot (allows for us) // to just multiply by 1 in the next step. // This step essentially gets the product to the left of the index at index + 1 for (var x = 0; x < size; x++) { output.push(product); product = product * numArray[x]; } // From the back, we multiply the current output element (which represents the product // on the left of the index, and multiplies it by the product on the right of the element) var product = 1; for (var i = size  1; i > 1; i) { output[i] = output[i] * product; product = product * numArray[i]; } return output; }
 1.6 Find the intersection of two arrays. An intersection would be the common elements that exists within both arrays. In this case, these elements should be unique!
var firstArray = [2, 2, 4, 1]; var secondArray = [1, 2, 0, 2]; intersection(firstArray, secondArray); // [2, 1] function intersection(firstArray, secondArray) { // The logic here is to create a hashmap with the elements of the firstArray as the keys. // After that, you can use the hashmap's O(1) look up time to check if the element exists in the hash // If it does exist, add that element to the new array. var hashmap = {}; var intersectionArray = []; firstArray.forEach(function(element) { hashmap[element] = 1; }); // Since we only want to push unique elements in our case... we can implement a counter to keep track of what we already added secondArray.forEach(function(element) { if (hashmap[element] === 1) { intersectionArray.push(element); hashmap[element]++; } }); return intersectionArray; // Time complexity O(n), Space complexity O(n) }
Strings
 2.1 Given a string, reverse each word in the sentence
"Welcome to this Javascript Guide!"
should be become"emocleW ot siht tpircsavaJ !ediuG"
var string = "Welcome to this Javascript Guide!"; // Output becomes !ediuG tpircsavaJ siht ot emocleW var reverseEntireSentence = reverseBySeparator(string, ""); // Output becomes emocleW ot siht tpircsavaJ !ediuG var reverseEachWord = reverseBySeparator(reverseEntireSentence, " "); function reverseBySeparator(string, separator) { return string.split(separator).reverse().join(separator); }
 2.2 Given two strings, return true if they are anagrams of one another
"Mary" is an anagram of "Army"
var firstWord = "Mary"; var secondWord = "Army"; isAnagram(firstWord, secondWord); // true function isAnagram(first, second) { // For case insensitivity, change both words to lowercase. var a = first.toLowerCase(); var b = second.toLowerCase(); // Sort the strings, and join the resulting array to a string. Compare the results a = a.split("").sort().join(""); b = b.split("").sort().join(""); return a === b; }
 2.3 Check if a given string is a palindrome
"racecar" is a palindrome. "race car" should also be considered a palindrome. Case sensitivity should be taken into account
isPalindrome("racecar"); // true isPalindrome("race Car"); // true function isPalindrome(word) { // Replace all nonletter chars with "" and change to lowercase var lettersOnly = word.toLowerCase().replace(/\s/g, ""); // Compare the string with the reversed version of the string return lettersOnly === lettersOnly.split("").reverse().join(""); }

2.3 Check if a given string is a isomorphic
For two strings to be isomorphic, all occurrences of a character in string A can be replaced with another character to get string B. The order of the characters must be preserved. There must be onetoone mapping for ever char of string A to every char of string B. `paper` and `title` would return true. `egg` and `sad` would return false. `dgg` and `add` would return true.
isIsomorphic("egg", 'add'); // true isIsomorphic("paper", 'title'); // true isIsomorphic("kick", 'side'); // false function isIsomorphic(firstString, secondString) { // Check if the same lenght. If not, they cannot be isomorphic if (firstString.length !== secondString.length) return false var letterMap = {}; for (var i = 0; i < firstString.length; i++) { var letterA = firstString[i], letterB = secondString[i]; // If the letter does not exist, create a map and map it to the value // of the second letter if (letterMap[letterA] === undefined) { // If letterB has already been added to letterMap, then we can say: they are not isomorphic. if(secondString.indexOf(letterB) < i){ return false; } else { letterMap[letterA] = letterB; } } else if (letterMap[letterA] !== letterB) { // Eles if letterA already exists in the map, but it does not map to // letterB, that means that A is mapping to more than one letter. return false; } } // If after iterating through and conditions are satisfied, return true. // They are isomorphic return true; }
View on Codepen: http://codepen.io/kennymkchan/pen/mRZgaj?editors=0012
Stacks and Queues
 3.1 Implement enqueue and dequeue using only two stacks
var inputStack = []; // First stack var outputStack = []; // Second stack // For enqueue, just push the item into the first stack function enqueue(stackInput, item) { return stackInput.push(item); } function dequeue(stackInput, stackOutput) { // Reverse the stack such that the first element of the output stack is the // last element of the input stack. After that, pop the top of the output to // get the first element that was ever pushed into the input stack if (stackOutput.length <= 0) { while(stackInput.length > 0) { var elementToOutput = stackInput.pop(); stackOutput.push(elementToOutput); } } return stackOutput.pop(); }
 3.2 Create a function that will evaluate if a given expression has balanced parentheses  Using stacks In this example, we will only consider "{}" as valid parentheses
{}{}
would be considered balancing.{{{}}
is not balancedvar expression = "{{}}{}{}" var expressionFalse = "{}{{}"; isBalanced(expression); // true isBalanced(expressionFalse); // false isBalanced(""); // true function isBalanced(expression) { var checkString = expression; var stack = []; // If empty, parentheses are technically balanced if (checkString.length <= 0) return true; for (var i = 0; i < checkString.length; i++) { if(checkString[i] === '{') { stack.push(checkString[i]); } else if (checkString[i] === '}') { // Pop on an empty array is undefined if (stack.length > 0) { stack.pop(); } else { return false; } } } // If the array is not empty, it is not balanced if (stack.pop()) return false; return true; }
Recursion

4.1 Write a recursive function that returns the binary string of a given decimal number Given
4
as the decimal input, the function should return100
decimalToBinary(3); // 11 decimalToBinary(8); // 1000 decimalToBinary(1000); // 1111101000 function decimalToBinary(digit) { if(digit >= 1) { // If digit is not divisible by 2 then recursively return proceeding // binary of the digit minus 1, 1 is added for the leftover 1 digit if (digit % 2) { return decimalToBinary((digit  1) / 2) + 1; } else { // Recursively return proceeding binary digits return decimalToBinary(digit / 2) + 0; } } else { // Exit condition return ''; } }
View on Codepen: http://codepen.io/kennymkchan/pen/OWYYKb?editors=0012

4.2 Write a recursive function that performs a binary search
function recursiveBinarySearch(array, value, leftPosition, rightPosition) { // Value DNE if (leftPosition > rightPosition) return 1; var middlePivot = Math.floor((leftPosition + rightPosition) / 2); if (array[middlePivot] === value) { return middlePivot; } else if (array[middlePivot] > value) { return recursiveBinarySearch(array, value, leftPosition, middlePivot  1); } else { return recursiveBinarySearch(array, value, middlePivot + 1, rightPosition); } }
View on Codepen: http://codepen.io/kennymkchan/pen/ygWWmK?editors=0012
Numbers
 5.1 Given an integer, determine if it is a power of 2. If so, return that number, else return 1. (0 is not a power of two)
isPowerOfTwo(4); // true isPowerOfTwo(64); // true isPowerOfTwo(1); // true isPowerOfTwo(0); // false isPowerOfTwo(1); // false // For the nonzero case: function isPowerOfTwo(number) { // `&` uses the bitwise n. // In the case of number = 4; the expression would be identical to: // `return (4 & 3 === 0)` // In bitwise, 4 is 100, and 3 is 011. Using &, if two values at the same // spot is 1, then result is 1, else 0. In this case, it would return 000, // and thus, 4 satisfies are expression. // In turn, if the expression is `return (5 & 4 === 0)`, it would be false // since it returns 101 & 100 = 100 (NOT === 0) return number & (number  1) === 0; } // For zerocase: function isPowerOfTwoZeroCase(number) { return (number !== 0) && ((number & (number  1)) === 0); }
Javascript

6.1 Explain what is hoisting in Javascript
Hoisting is the concept in which Javascript, by default, moves all declarations to the top of the current scope. As such, a variable can be used before it has been declared. Note that Javascript only hoists declarations and not initializations

6.2 Describe the functionality of the
use strict;
directivethe `use strict` directive defines that the Javascript should be executed in `strict mode`. One major benefit that strict mode provides is that it prevents developers from using undeclared variables. Older versions of javascript would ignore this directive declaration
// Example of strict mode "use strict"; catchThemAll(); function catchThemAll() { x = 3.14; // Error will be thrown return x * x; }
 6.3 Explain
event bubbling
and how one may prevent itEvent bubbling is the concept in which an event triggers at the deepest possible element, and triggers on parent elements in nesting order. As a result, when clicking on a child element one may exhibit the handler of the parent activating. One way to prevent event bubbling is using `event.stopPropagation()` or `event.cancelBubble` on IE < 9

6.4 What is the difference between
==
and===
in JS?`===` is known as a strict operator. The key difference between `==` and `===` is that the strict operator matches for both value and type, as opposed to just the value.
// Example of comparators 0 == false; // true 0 === false; // false 2 == '2'; // true 2 === '2'; // false

6.5 What is the difference between
null
andundefined
In Javascript, null is an assignment value, and can be assigned to a variable representing that it has no value. Undefined, on the other hand, represents that a variable has been declared but there is no value associated with it

6.6 How does
prototypal inheritance
differ fromclassical inheritance
In classical inheritance, classes are immutable, may or may not support multiple inheritance, and may contain interfaces, final classes, and abstract classes. In contrast, prototypes are much more flexible in the sense that they may be mutable or immutable. The object may inherit from multiple prototypes, and only contains objects.