nodejs/js: Text not appended in file in sequence

I am trying to write state info logs in a code block that uses express, unirest and async modules as shown below.

const fsTest = require("fs");

app.post(someURL, function (req, res) {
    fsTest.appendFile("c:/fs.txt","\r\nInOf")   
    async.series({
        one: function (callback) {
                fsTest.appendFile("c:/fs.txt","\r\nInOf2")
                someOperation();        
                fsTest.appendFile("c:/fs.txt","\r\nInOf3")
                callback(false,"");         
        },
        //form search query
        two: function (callback) {
            fsTest.appendFile("c:/fs.txt","\r\nInOf4")
            anotherOperation();
            urClient.post("https://" + server + url)
                .header('Content-Type', 'application/json')             
                .send(qrySearchJSON)
                .end(
                    function (response) {
                    });
        }
    },  function (err,oResp) {
            errHandler();
        });
});

But my logs always appear out of sequence as shown below:

InOf2
InOf4
InOf3
InOf

InOf
InOf2
InOf3
InOf4

InOf
InOf2
InOf4
InOf3

InOf2
InOf
InOf4
InOf3

InOf
InOf2
InOf3
InOf4

What could possibly be the reason and what approach should i take to fix it.

via StillNestling