Example of updateJson() with promise

update-json.js

var promise = require('bluebird');
var fs = primise.promisifyAll(require('fs'));

function updateJson(ticker, value) {
  
  fs.readFileAsync('stocktest.json', 'utf8')
  
    .then(function(contents) {

      var stocksJson = JSON.parse(contents);
      
      if (!stocksJson[ticker]) {
        throw new Error(ticker + ' doesn\'t exist on the json');
      }
      
      stocksJson[ticker].price = value;
      
      return fs.writeFileAsync('stocktest.json', JSON.stringify(stocksJson, null, 4));
    })
    
    .then(function() {
      console.log('File successfully written');
    })
    
    .catch(function(error) {
      console.error(error);
    })
    
    .error(function(error) {
      console.error('unable to read file, because: ', error.message);
    })
  ;
}