secuancial_search.js

const arr = [3,2,5,6,23,86,34,45];
const toFind = 23;

function indexOf (arr, num) {
    for (let i = 0; i < arr.length; i++) {
        if (arr[i] === toFind) {
            return i;
        }
    }
    return -1;
}

console.log(indexOf(arr, toFind));