I'm solving a problem (leetcode 557) Given a string s, reverse the order of characters in each word within a sentence while still preserving whitespace and initial word order.

Example 1:

Input: s = "Let's take LeetCode contest"
Output: "s'teL ekat edoCteeL tsetnoc"

Example 2:

Input: s = "God Ding"
Output: "doG gniD"

When I write the solution below this way I get a wrong answer which is not reversed.

var reverseWords = function(s) {
    let separate = s.split("");
    let res = separate.map((str) => {
        return str.split("").reverse().join("");
    });

    return res.join("")
};

But when I write as shown below my answer is accepted

var reverseWords = function(s) {
    let separate = s.split(" ");
    let res = separate.map((str) => {
        return str.split("").reverse().join("");
    });

    return res.join(" ")
};

I am really confused about how the spacing in quotation marks of the second and last line of code is affecting the solution. Thanks in advance