How do I read a text file using a function javascript?

Question

When I do console.log(readTextFile("./yaml/ss.txt")) it is just undefined.

function readTextFile(file)
    {
        var rawFile = new XMLHttpRequest();
        rawFile.open("GET", file, false);
        rawFile.onreadystatechange = function ()
        {
            if(rawFile.readyState === 4)
            {
                if(rawFile.status === 200 || rawFile.status == 0)
                {
                    var allText = rawFile.responseText;
                    return allText;
                }
            }
        }
    }
    rawFile.send(null);

console.log(readTextFile("./yaml/ss.txt"))

"./yaml/ss.txt" is a file and I have tried removing rawFile.send(null); and changing the return to just return "test";