Running Sum of 1d Array

const nums = [1,2,3,4], nums2 = [1,1,1,1,1];

var runningSum1 = function(nums) {
    if (!Array.isArray(nums)) return undefined;
    let result = [];
    let lastSum = 0;
    for (let i=0; i < nums.length; i++) {
        lastSum += nums[i];
        result.push(lastSum);
    }
    return result;
};

// best solution
var runningSum3 = function(nums) {
    if (!Array.isArray(nums)) return undefined;
    for (let i=1; i < nums.length; i++) {
        nums[i] += nums[i-1];
    }
    return nums;
};

const varyingSum2 = (nums) => {
    if (!Array.isArray(nums)) return undefined;
    return nums.reduce((acc, val) => acc + val);
}

const runningSum2 = (nums) => {
    for (i = 1; i < nums.length; i++) {
        nums[i] += nums[i-1];
    }
    return nums;
}

//console.log(runningSum2(nums));
console.log(runningSum3(nums));