fs.write json file to capture and save streaming json file

I want json stream stored in text file. When running the node server, the json file isn't appended to the json.txt file. What am I missing? Am new to to node, so be gentle..

Here is a code chunk I expect to capture the json content:

var fs = require('fs');
fs.writeFile("json.txt",{encoding:"utf8"}, function(err) {
if(err) {
console.log(err);
} else {
console.log("The file was saved!");
}
});

via Antex