shift <positions> – shifts every element of the array the number of positions to the left (with rotation).

sumPairs – sums the elements in the array by pairs (first + second, third + fourth, …).

o For example, [1, 2, 4, 5, 6, 7, 8] -> [3, 9, 13, 8].

;-)

function manipulateArr(nums, commands) {

    for (let el of commands) {

        let [command, num1, ...rest] = el.split(' ');
        num1 = Number(num1);
        rest = rest.map(Number);

        switch (command) {
            case 'add':
                nums.splice(num1, 0, rest[0]);
                break;
            case 'addMany':
                nums.splice(num1, 0, ...rest);
                break;
            case 'contains':
                console.log(nums.indexOf(num1));
                break;
            case 'remove':
                nums.splice(num1, 1);
                break;
            case 'shift':
                // nums = shift(nums);
                shift(num1);
                break;
            case 'sumPairs':
                let newArr = [];
                nums = sumPairs(nums, newArr);
                break;
            case 'print':
                console.log(`[ ${nums.join(', ')} ]`);
                break;
            default:
                break;
        }
    }


    function shift(shiftAmount) {
        for (let j = 0; j < shiftAmount; j++) {
            // for (let i = 0; i < nums.length - 1; i++) {
            //     let temp = nums[i];
            //     nums[i] = nums[i + 1];
            //     nums[i + 1] = temp;
            // }
            let temp = nums.shift();
            nums.push(temp);
        }
        // return arr;
    }

    function sumPairs(arr, newArr) {
        for (let i = 0; i < arr.length - 1; i = i + 2) {
            let currEl = arr[i];
            let nextEl = arr[i + 1];
            let sum = currEl + nextEl;
            newArr.push(sum);
        }
        
        if (arr.length % 2 !== 0){
            newArr.push(arr[arr.length - 1]);
        }
        return newArr;
    }
}