Using Callbacks With nodejs in KOA

Question

Recently I work on a new project and this project use JavaScript callbacks in nodejs. Now we use KOA but the problem happens when we try to use ES6 Generators and callbacks.

//Calback function
function load(callback){
  result = null;
  //Do something with xmla4js and ajax
  callback(result);
  return result;
}

Now in KOA I need to call load and response json to client so i use this code below :

router= require('koa-router');
app = koa();
app.use(router(app));

app.get('load',loadjson);

function *loadJson(){
  var that = this;
  load(function(result){
    that.body = result;
  });
}

but i get this error :

_http_outgoing.js:331
throw new Error('Can\'t set headers after they are sent.');
      ^
Error: Can't set headers after they are sent.
at ServerResponse.OutgoingMessage.setHeader (_http_outgoing.js:331:11)
at Object.module.exports.set (G:\NAP\node_modules\koa\lib\response.js:396:16)
at Object.length (G:\NAP\node_modules\koa\lib\response.js:178:10)
at Object.body (G:\NAP\node_modules\koa\lib\response.js:149:19)
at Object.body (G:\NAP\node_modules\koa\node_modules\delegates\index.js:91:31)
at G:\NAP\Server\OlapServer\index.js:40:19
at G:\NAP\Server\OlapServer\OLAPSchemaProvider.js:1599:9
at _LoadCubes.xmlaRequest.success   (G:\NAP\Server\OlapServer\OLAPSchemaProvider.js:1107:13)
at Object.Xmla._requestSuccess (G:\NAP\node_modules\xmla4js\src\Xmla.js:2113:50)
at Object.ajaxOptions.complete (G:\NAP\node_modules\xmla4js\src\Xmla.js:2024:34)

Answer 1

Just to clarify things, let's write your callback as

//Calback function
function load(callback){
    setTimeout(function() {
        var result = JSON.stringify({ 'my': 'json'});
        callback(/* error: */ null, result);
    }, 500);
}

in Koa world, this is called a thunk, meaning that it is an asynchronous function that takes only one argument: a callback with the prototype (err, res). you can check https://github.com/visionmedia/node-thunkify for a better explanation.

now you have to write your middleware with

function *loadJson(){
  this.type = 'application/json';
  this.body = yield load;
}

Answer 2

this is mainly because KOA is generator based, if your on the top of the middleware it does not support callbacks. so its not waiting for the function to finish. best solution would be to convert your function into a promise. promise works great with KOA.