Can not set Header in KOA When using callback

Question

Recently I worked on a new project that used javascript callbacks. And I was using koa framework. But when I called this route :

function * getCubes(next) {         


        

Answer 1

The problem is that your async call LoadCubesJSon() takes a while to return but Koa isn't aware of that and continues with the control flow. That's basically what yield is for.

"Yieldable" objects include promises, generators and thunks (among others).

I personally prefer to manually create a promise with the 'Q' library. But you can use any other promise library or node-thunkify to create a thunk.

Here is short but working example with Q:

var koa = require('koa');
var q = require('q');
var app = koa();

app.use(function *() {
    // We manually create a promise first.
    var deferred = q.defer();

    // setTimeout simulates an async call.
    // Inside the traditional callback we would then resolve the promise with the callback return value.
    setTimeout(function () {
        deferred.resolve('Hello World');
    }, 1000);

    // Meanwhile, we return the promise to yield for.
    this.body = yield deferred.promise;
});

app.listen(3000);

So your code would look as follows:

function * getCubes(next) {
    var deferred = q.defer();

    _OLAPSchemaProvider.LoadCubesJSon(function (result) {
        var output = JSON.stringify(result.toString());
        deferred.resolve(output);
    });

    this.body = yield deferred.promise;
}