Steps:

  • Create a set object having values within the range of initial and final values of the provided array
  • Then compare it with the provided array to retrieve the missing value
function findMissingNumberInArray(arr) {
    const mySet = new Set();
    const firstEl = arr[0];
    const lastEl = arr[arr.length - 1];
    for(let i = firstEl; i <= lastEl; i++) {
        mySet.add(i);
    }
    for(let x = 0; x < arr.length; x++) {
        if(mySet.has(arr[x])) {
            mySet.delete(arr[x]);
        }
    }
    return Array.from(mySet);
}

findMissingNumberInArray([1,4,6,7,8,9]);

References: