access data from this nodeJS function - node.js

Question

I want to use "data" outside of this function. Please can some one show me how?
resemble('/Users/User/Documents/dev/engineerappcopy/VGimages/'+deviceName+'.png')
.compareTo('/Users/User/Documents/dev/engineerappcopy/VGimages/'+"nexUpdate"+'.png')
.ignoreColors().onComplete(function(data) {
browser.sleep(5000)
console.log(data);
data.getDiffImage().pack().
pipe(fs.createWriteStream('/Users/User/Documents/dev/engineerappcopy/VGimages/'+deviceName+'VG.png'));
});
I am aware that this is asynchronous, however I am struggling with this.

Answer 1

I would suggest you to use EventEmitters. You can use this if at all you want to indicate if any action is finished and optionally you can pass the data as well.
Create a new javscript file 'my-emitter.js'
my-emitter.js (ES6 version)
const EventEmitter = require('events');
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();
module.exports = myEmitter;
OR
my-emitter.js (Javascript version)
var EventEmitter = require('events').EventEmitter;
var util = require('util');
function MyEmitter(){
EventEmitter.call(this);
}
util.inherits(MyEmitter,EventEmitter);
myEmitter = new MyEmitter();
module.exports = myEmitter;
Your code snippet:
(Check the comments in the code). Emit an event saying that data is available after the async operation is complete
myEmitter.emit('img-op-complete',data);
var myEmitter = require('./my-emitter.js'); //Get your emitter from the module
resemble('/Users/User/Documents/dev/engineerappcopy/VGimages/'+deviceName+'.png')
.compareTo('/Users/User/Documents/dev/engineerappcopy/VGimages/'+"nexUpdate"+'.png')
.ignoreColors().onComplete(function(data) {
browser.sleep(5000)
//Emit the event that data is available and pass the data
myEmitter.emit('img-op-complete',data);
console.log(data);
data.getDiffImage().pack().
pipe(fs.createWriteStream('/Users/User/Documents/dev/engineerappcopy/VGimages/'+deviceName+'VG.png'));
});
othermodule.js
Where ever you want the data (if in other module), use the below piece of code
var myEmitter = require('./my-emitter.js'); //Get your emitter from the module
myEmitter.on('img-op-complete', function(data){
console.log(data); //You'll get your data here after op is done
})
Fore more info on events, https://nodejs.org/dist/latest-v6.x/docs/api/events.html
NOTE:
Promises is also nice solution, but if you use promises good design if data is needed within the same module. But events present a good design pattern in node.js

Answer 2

I would suggest go with promises, you would be able to use this promise across module also if you export it.
There are some really nice articles about how to use promises like Promise.
Coming to how to approach the above issue via promises,
function foo(){
return new Promise(function(resolve,reject){
resemble(<yourPath>)
.compareTo(<yourPath>)
.ignoreColors().onComplete(function(data) {
//for best practice do handle errors
if(err<anyError>){
reject(err);
} else{
resolve(data);
}
});
})
}
Now You can use the above promise where you want to use the data variable :
foo().then(function(<data>){
//do whatever you wish to do with the data
}).catch(function(<err>){
//handle the error
});