NodeJS prevent multiple access on file

I have a rest api that receives files.Each file that my node-application stores in the local storage is added to a xml & json document. How can I prevent, that the app doesn't write two files at almost the same time in the document, what would cause an invalid document.

At the moment I put all files into an array and store only the last one from the array. That prevents that the xml/json documents are only accessed once.

this.files = new Array();
this.running = false;    

this.app.push('/file', function(req, res){
    this.files.add(req.body.file);
    if(!this.running)
        this.storeFile();
} 

this.storeFile = function(){
    this.running = true;
    fs.write(files[files.length-1], this.succ, this.err);

this.succ(){
    files.splice(fileIndex);
    if files.length greater than 0
        app.storeFile();
    else
        this.running = false;
}

This doesn't prevent, that several files are written at the same time. If running = false and two push events come in at almost the same time. The loop is executed twice and each file is stored twice. And the xml/json file is accessed/opened/written twice which causes an error/exception.