Array Manipulator

function arrayManipulator(array, manipulations) {
    for (let i = 0; i < manipulations.length; i++) {
        let manipulation = manipulations[i].split(" ");
        let command = manipulation.shift();
        let firstNum = Number(manipulation.shift());
        let secNum = Number(manipulation[0]);

        switch (command) {
            case "add":
                array.splice(firstNum, 0, secNum);
                break;

            case "addMany":
                let index = firstNum;
                for (let j = 0; j < manipulation.length; j++) {
                    array.splice(index + j, 0, Number(manipulation[j]));
                }
                break;

            case "contains":
                if (array.includes(firstNum)) {
                    console.log(array.indexOf(firstNum));
                } else {
                    console.log(-1);
                }
                break;

            case "remove":
                array.splice(firstNum, 1);
                break;

            case "shift":
                let positions = firstNum;
                let shifted = array.splice(0, positions);
                for (let j = 0; j < shifted.length; j++) {
                    array.push(shifted[j]);
                }
                break;
            case "sumPairs":
                let sumArr = [];
                for (let j = 0; j < array.length; j += 2) {
                    if (j + 1 > array.length - 1) {
                        sumArr.push(array[j] + 0);
                    } else {
                        sumArr.push(array[j] + array[j + 1]);
                    }
                }
                let sliced = sumArr.slice(0);
                array = sliced;
                break;
            case "print":
                console.log(`[ ${array.join(", ")} ]`);
                break;
        }
    }

}