Nodejs write strings to file

I try using Nodejs to write string and json data to output.json file but it is not working. To be more specific, i want to first write a string to a file C. Next, read some data from file B and use the data to write to file C again. I test the code separately, the separate part of writing string and writing json works perfectly but it is not working if i use them together.

Expected output:

this is some string
json data

However, my output is:

json data

Here is my code:

// write strings to file
var fs = require('fs');
fs.writeFile('/data/outputjson', 'this is some string', function(err) {
    if(err) {
        return console.log(err);
    }
    console.log("The file was written successfully!");
});

// read file
var jsonfile = require('jsonfile');
var file = 'data/routes.json';
var routes;
var links=[];
jsonfile.readFile(file, function(err, obj) {
  routes=obj;
  for(var i=0;i<routes.length;i++){
    links.push({
      source: routes[i]["geocoded_waypoints"][0]["place_id"],
      target: routes[i]["geocoded_waypoints"][1]["place_id"]
    });
  }
  console.log(links);

  // write json to file
  var jsonfile = require('jsonfile')
  var file = 'data/output.json'
  jsonfile.writeFileSync(file, links)
})

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