variable not defined node.js

Question

I'm trying to use Node.js to get a response from an API, I want to clean the API response and use the result.

So to access the first API I have the following code.

To store and use the result I believe I need to store the JSON output globally.

However, I can't work out how to do this.

Example -

var request = require('request');

request({url: 'https://www.car2go.com/api/v2.1/vehicles?loc=wien&oauth_consumer_key=car2gowebsite&format=json', json: true}, function(err, res, json) {
if (err) {
    throw err;
}
car2go = json.placemarks;
for (i = 0; i < car2go.length; i++) {
    delete car2go[i].address;
    delete car2go[i].charging;
    delete car2go[i].exterior;
    delete car2go[i].interior;
    delete car2go[i].smartPhoneRequired;
    delete car2go[i].vin
    car2go[i].vendor = 'car2go';
    car2go[i].city = 'wien';
    car2go[i].carmake = 'Smart';
    car2go[i].carmodel = 'Fortwo';
}
console.log(car2go);
});

This prints the desired result however I know that this is because my variable is defined within the function.

I want to access the variable outside of the function.

To test if I could do this I changed the code to -

var request = require('request');

request({url: 'https://www.car2go.com/api/v2.1/vehicles?loc=wien&oauth_consumer_key=car2gowebsite&format=json', json: true}, function(err, res, json) {
if (err) {
    throw err;
}
car2go = json.placemarks;
for (i = 0; i < car2go.length; i++) {
    delete car2go[i].address;
    delete car2go[i].charging;
    delete car2go[i].exterior;
    delete car2go[i].interior;
    delete car2go[i].smartPhoneRequired;
    delete car2go[i].vin
    car2go[i].vendor = 'car2go';
    car2go[i].city = 'wien';
    car2go[i].carmake = 'Smart';
    car2go[i].carmodel = 'Fortwo';
}
});

console.log(car2go);

But if I do this I get

ReferenceError: car2go is not defined

I am running Node v0.12.2 on Mac OS Yosemite (10.10.3).

Admittedly I am very new to node and I am more familiar with R, Python and PL SQL.

Answer 1

There is no way to get reference to it outside of the callback function because the console.log line runs before the callback function is invoked. The reason you have to pass a callback function into the request API is because the request library needs to invoke that function when it's done making the request. Meanwhile, your app moves on and does other things (such as running that console.log line) while it waits for the callback function to fire.

That said, there are a number of ways to deal with asynchronous code. My favorite way is with promises. I use a library called bluebird for handling promises.

var request = require('request');
var Promise = require('bluebird');
var requestP = Promise.promisify(request);

The call to Promise.promisify(request) returns a new function that doesn't take a callback function, but instead returns a promise.

requestP({ url: 'https://www.car2go.com/api/v2.1/vehicles?loc=wien&oauth_consumer_key=car2gowebsite&format=json', json: true })
  .spread(function(res, json) {
    var car2go = json.placemarks;
    for (i = 0; i < car2go.length; i++) {
      delete car2go[i].address;
      delete car2go[i].charging;
      delete car2go[i].exterior;
      delete car2go[i].interior;
      delete car2go[i].smartPhoneRequired;
      delete car2go[i].vin
      car2go[i].vendor = 'car2go';
      car2go[i].city = 'wien';
      car2go[i].carmake = 'Smart';
      car2go[i].carmodel = 'Fortwo';
    }
  })
  .then(function (car2go) {
    console.log(car2go);
  })
  .catch(function (err) {
    console.error(err);
  });

Note: .spread is the same as .then except if the resolved value is an array (which it will be because the callback passed to the request library accepts 2 arguments, which bluebird will translate into an array that the promise resolves to) .spread will split up the array back into multiple arguments passed into the function you give to .spread.

Promise.resolve(['hi', 'there']).then(function (result) {
  console.log(result); // "['hi', 'there']"
});

Promise.resolve(['hi', 'there']).spread(function (str1, str2) {
  console.log(str1); // 'hi'
  console.log(str2); // 'there'
});

You're not going to be able to return that value all the way back out to the same context from which you began the asynchronous call, but you can at least write code that looks somewhat synchronous when using promises.

Without promises you'll be forced to call functions from within functions from within functions from within functions ;)