;-)

Code =>

function mergeArrays(arr1, arr2) {
    let newArray = [];

    for (let i = 0; i < arr1.length; i++) {
        // don't modify elements in both arrays, will trigger fail tests
        // arr1[i] = Number(arr1[i]);
        // arr2[i] = Number(arr2[i]);
        let num1 = Number(arr1[i]);
        let num2 = Number(arr2[i]);

        if (i % 2 != 0) {
            newArray.push(`${arr1[i]}${arr2[i]}`);
        } else if (i % 2 == 0) {
            newArray.push(num1 + num2);
        }
    }

    console.log(newArray.join(' - '));
}

Alternatives =>

function solve(array1, array2) {
    let newArray = [];
    for (let i = 0; i < array1.length; i++) {
        if (i % 2 === 0) {
            newArray[i] = Number(array1[i]) + Number(array2[i]);
        } else {
            newArray[i] = array1[i] + array2[i];
        }
    }

    console.log(newArray.join(' - '));
}

function solve2(arr1, arr2) {
    let arr3 = arr1.map((el, i) => {
        return i % 2 === 0
            ? Number(el) + Number(arr2[i])
            : el.concat(arr2[i]);
    })
    console.log(arr3.join(' - '));
}