What is the significance of using: string.split("").reverse().join(")?

I want to say first. Very beginner. Be kind. However. I am attempting to reverse a string. Put a string into a function as an argument and reverse its order. For example 'hello' should return 'olleh', etc.

I have complete this by doing:

function revString(stringArg) {
    return stringArg.split("").reverse().join(");
}

However. By doing it like this:

function revString (stringArg) {
    stringArg.split("");
    stringArg.reverse();
    stringArg.join("");
    return stringArg;
}

The resulting output for the second function is that reverse() is not a function. Can someone tell me what the difference here is?