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Do 3 arrays contain a common element, if they contain output it.
For example: [1,2,3], [2,3], [2,4] -> answer = 2

let arr1 = [1, 3, 11, 32, 44, 99];
let arr2 = [4, 12, 15, 99];
let arr3 = [4, 11, 13, 15, 23, 43];

function searchThreeSameNum(arr1, arr2, arr3) 
 let i = 0;
 let j = 0;

 while (1) 

I will be glad if you give me any tips to improve my code. Thanks in advance.
Sorry, I'm not an English speaker.

Your code assumes that each of the 3 arrays is sorted. Otherwise the < operator would not work. It's ok to assume this. You should have mentioned this in your question.

You use the == operator for comparing the numbers and the lengths. You should better use the === since the == operator considers 0 and "0" equal, which is not good in most cases.

It does not matter which of the 3 arrays comes first. The result will always be the same. Therefore it would be nice if the code looked the same for each of the 3 arrays. Your current code looks different for arr3.

I would write the code differently:

function smallestCommonElement(a, b, c) 
 let i = 0, j = 0, k = 0;

 while (i < a.length && j < b.length && k < c.length) 
 const max = Math.max(Math.max(a[i], b[j]), c[k]);

 let n = 0;
 while (i < a.length && a[i] < max) i++, n++;
 while (j < b.length && b[j] < max) j++, n++;
 while (k < c.length && c[k] < max) k++, n++;

 if (n === 0)
 return a[i];
 
 return null;

The idea is to start at the beginning of the arrays. In each step, look at the current values and find the maximum number. Advance each array to this maximum number. If none of the 3 arrays has been advanced, this means that the current values from all the arrays must be the same. In that case, return this value. Otherwise the values must be different, so try again. Do all this until one of the arrays is at the end, in which case there is no common element.

Looking again at your code, there is a bug. Given the arrays [2, 3], [2, 3], [3], your code will return 'No equal number' even though the 3 appears in each array. Using a debugger (or pen and paper), you should step through your code to see where the bug is.

It's an edge case, and it happens in the part of the code that differs from the other parts. That's why I suggested that the code for all 3 arrays should look the same. It's one less chance of introducing bugs.

Although your code seems to work, it is difficult to read. Loops inside loops and many if, else if blocks and continue or return. Let's start with some big issues:

You use an endless while loop when it is clear you don't have to. The code below would perform the same function:

function searchThreeSameNum(arr1, arr2, arr3) 
 let i = 0, j = 0;
 while (i < arr1.length && j < arr2.length) 
 if (arr1[i] < arr2[j]) 
 i++;
 continue;
 else if (arr1[i] > arr2[j]) 
 j++;
 continue;
 else if (arr1[i] == arr2[j]) 
 for (let k = 0; k < arr3.length; k++) 
 if (arr1[i] == arr3[k]) return arr1[i];
 
 
 
 return 'No equal numbers';

This has one less return 'No equal numbers'; (code repetition) and only one return from within the while loop.

Now let's look at what the loop is actually doing. It runs through array 1 and 2 and tries to find equal pairs of values in them. If a pair is found it searches in the third array for the same value and returns it when it is found.

There is a handy array method called includes(). It could replace the whole looping of the third array, like this:

function searchThreeSameNum(arr1, arr2, arr3) 
 let i = 0, j = 0;
 while (i < arr1.length && j < arr2.length) 
 if (arr1[i] < arr2[j]) 
 i++;
 continue;
 else if (arr1[i] > arr2[j]) 
 j++;
 continue;
 else if (arr1[i] == arr2[j]) 
 if (arr3.includes(arr1[i])) return arr1[i];
 
 
 return 'No equal numbers';

By now you can probably guess we don't need the complex while loop at all. We could just loop through array 1 and see if its values are contained in array 2 and 3:

function searchThreeSameNum2(arr1, arr2, arr3) 
 for (number of arr1) 
 if (arr2.includes(number) && arr3.includes(number)) return number;
 
 return 'No equal numbers';

The assumption here is that array 1 is properly sorted. If it isn't you can sort it.

function searchThreeSameNum2(arr1, arr2, arr3) 
 let sorted = arr1.sort((a, b) => a - b);
 for (number of sorted) 
 if (arr2.includes(number) && arr3.includes(number)) return number;
 
 return 'No equal numbers';

Arrays 2 and 3 don't need to be sorted.

In summary

1. If you use a while loop, always break the loop with a proper condition in the right place. Do not use endless loops.


2. Try not to use complex execution flow constructions, with lot of else, continue and return. They are difficult to read and to debug.


3. Use the build in array methods, they are very handy, but don't overdo it.