Finding two missing numbers that appears only once and twice respectively in JavaScript

Problem

We are required to write a JavaScript function that takes in an array in which all the numbers appear thrice except one which appears twice and one which appears only one. Our function should find and return these two numbers.

Example

Following is the code −

const arr = [1, 1, 1, 2, 2, 3];
const findMissing = (arr = []) => {
   let x = 0;
   let y = 0;
   for(let i = 0; i < arr.length; i++){
      if(arr.filter(a => a === arr[i]).length === 2){
         y = arr[i];
      };
      if(arr.filter(b => b === arr[i]).length === 1){
         x = arr[i];
      };
   };
   return [x, y];
};
console.log(findMissing(arr));

Output

Following is the console output −

[3, 2]