Why does this generator return a function instead of a value

Question

Essentially I'm trying to make an http request with my koa application and was wondering what exactly I would doing wrong

var request = require('koa-request');
var beatsGen = beats();
var response1 = beatsGen.next().value;
function *beats (){
 var options = {
 url: 'https://api.github.com/repos/dionoid/koa-request',
 headers: { 'User-Agent': 'request' }
 }; 
 var response = yield request(options);
}

When I console log response1 this is what I get

respone from beats is function (callback) { 
 _request(uri, options, function (error, response, body) { 
 callback(error, response); 
 }) 
 }

I figured response1 would be someType of object containing the body parameter of the callback not the function itself. So why am I getting the function as the generators .next().value?

I'm new to generators and koa so I'm assume I'm making a stupid mistake here.

Answer 1

koa-request or any other library that returns thunks or promises/thenables for asynchronous calls is meant to be used with a co-routine library like co or the koa web application framework, which uses co to handle generator-based control flow.

ECMAScript 6 generators are not async-aware but ECMAScript 7 will have async and await to handle this natively.

The statement

var response1 = beatsGen.next().value;

returns the value returned by the first yield statement in beats, which is the thunk returned by request(options). A co-routine-aware library will inspect the return value from generator.next().value and wait for the callbacks to execute or thenables to resolve before resuming the generator body by calling generator.next() again.

Such a library is co:

var request = require('koa-request'),
 co = require('co');
function beats (){ //doesn't have to be a generator function
 var options = {
 url: 'https://api.github.com/repos/dionoid/koa-request',
 headers: { 'User-Agent': 'request' }
 }; 
 return request(options); //return the thunk
}
co(function *() {
 var response1 = yield beats;
 //use response
})

If this is meant to be used with koa:

app.use(function *() {
 this.body = yield beats();
});

BTW, koa-request would be better named co-request since all co-routine wrappers are prefixed with co-.

• Austin Davis: Thanks man. Didn't really realize of some of the underlying "magic" that was occurring. Just out of curiosity how would you recommend unit testing in koa. I wouldn't be able to fire the app.use middleware simply from a .exports function call, which is what I've done with express in the pass.
• c.P.u1: You could use co-mocha to test generators.