Tuesday, 9 January, 2018 UTC


This interview question is also available as a video on my Youtube channel.
You might not know about me that I have conducted tech interviews with over 500 software developers from more than twenty countries with the objective of identifying and hiring the best talent. I have also been in the candidate position, interviewing for positions ranging from junior developer to CTO.
In this series, I am exposing the secrets of JavaScript interviewing. You will get a chance to solve a tech interview exercise every single week. You will not only refresh your JavaScript skills, but above all, you will learn the mindset required for solving interview exercises. We will start with simple tasks, and then transition to complex exercises that you could even use to build your portfolio.
Exercise: Suppose an 8*6 Connect-Four table is given. Each cell of the table is either empty (null), or contains the player’s number from the possible values 1 and 2. Determine if any player has won the game by connecting four of their symbols horizontally or vertically. For simplicity, ignore diagonal matches.
I encourage you to solve this exercise on your own before checking the reference solution. Even if you solve this exercise, you may learn a lot from my reference solution, as I will reveal my thought process to you both from an interviewer’s and from a candidate’s perspective.
Solution: As there is no example data, we have to model the table ourselves.
const createEmptyTable = () => 
    new Array( 8 ).fill( null ).map( 
        () => new Array( 6 ).fill( null ) 
This simple arrow function returns an empty 6*8 array:
let table = createEmptyTable()
(8) [Array(6), Array(6), Array(6), Array(6), Array(6), Array(6), Array(6), Array(6)]
0 : (6) [null, null, null, null, null, null]
1 : (6) [null, null, null, null, null, null]
2 : (6) [null, null, null, null, null, null]
3 : (6) [null, null, null, null, null, null]
4 : (6) [null, null, null, null, null, null]
5 : (6) [null, null, null, null, null, null]
6 : (6) [null, null, null, null, null, null]
7 : (6) [null, null, null, null, null, null]
It is evident that we will need a function that checks all elements of the array for four consecutive matches. I encourage you to implement this function yourself. Reading my solution will be more beneficial to you in case you put in the effort to understand what is going on.
const checkElements = ( [head, ...tail], matchCount = 0, lastElement = null ) => {
    if ( matchCount === 3 && head === lastElement ) return true;
    if ( tail.length === 0 ) return false;
    if ( head === null ) return checkElements( tail );
    if ( head === lastElement ) return checkElements( tail, matchCount + 1, head );
    return checkElements( tail, 1, head );
The solution is based on simple recursion. If we find four matches, the function returns true.
If there are no more elements left, and there is no match possible anymore, the function returns false. Note that the second if is only reachable if the first condition is evaluated to false. In general, due to the return statements, we know that in each line, all if conditions of the lines above are false.
In the last two conditions, we check if the head is null, or matches the sequence we are looking for. In both cases, our task is to recursively call our function with the correct argument list. Eventually, in the last line, we know that head contains a non-null element that is different than the last element. In this case, we have to restart the matching process.
Note that if you know how regular expressions work, you could simply write a regex to perform the same work:
const checkElements = arr => /([12]),\1,\1,\1/.test( arr.toString() );
[12] is an arbitrary character that is either a 1 or a 2. We capture it using parentheses, then repeat the captured character using the \1 capture group reference. We insert the commas in-between. If you want to brush up your regex skills, check out my articles on regular expressions.
Columns are easy to match using the reduce function. Check out exercise 2 of this article for more details on how the reduce function works.
const checkColumns = table => 
        (hasMatch, column) => hasMatch || checkElements( column ), 
If you still have trouble interpreting what is going on, insert a console log inside the arrow function, and study the logged output:
const checkColumns = table => 
        (hasMatch, column) => {
            console.log( hasMatch, column );
            return hasMatch || checkElements( column ); 
We now need to check the rows. We could google how transposing an array works in JavaScript. However, there is no need to make the solution more complicated than it is. A simple for loop will do:
const checkRows = table => {
    for ( let i = 0; i < table[0].length; ++i ) {
        let rowArray = table.map( column => column[i] );
        if ( checkElements( rowArray ) ) return true;
    return false;
The function works as follows: the for loop goes through each element of the first column. Note that in a table, each column has the same number of elements. For this reason, we can form rowArray by taking the ith element from each column using the map function. The map function takes each column of the table, and substitutes it with the ith element in the column. For more exercises on the map function, check out exercises 2 and 3 from this article.
Now that we have an array of consecutive elements, we can use our checkElements function to derive the matches. As soon as we find a match, we can return true. If execution reaches the end of the for loop, we know that none of the rows matched. Therefore, we can safely return false.
Let’s create a function that checks the whole table for matches:
const checkTable = table => 
    checkRows( table ) ||
    checkColumns( table );
Now that we have solved this exercise together, don’t forget that there are four pillars for a successful tech interview:
  • Tech skills
  • Problem solving skills
  • Soft skills
  • Personal Branding
This blog addresses the first two skills. My other blog, devcareermastery.com addresses soft-skills, and my book The Developer’s Edge – How to Double Your Career Speed with Soft-Skills helps you with soft-skills as well as with your personal brand.
If you are interested in fine-tuning your JavaScript skills as well as your soft skills, I highly recommend that you consider purchasing The JavaScript Developer’s Career Bundle. This bundle contains my book ES6 in Practice, 4.5 hours of JavaScript videos, and The Developer’s Edge. You will not only be fully up-to-date with the latest version of JavaScript, but you will also learn how to build a fulfilling career for yourself.
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