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) {
var that = this;
_OLAPSchemaProvider.LoadCubesJSon(function(result) {
that.body = JSON.stringify(result.toString());
});
}
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:42:19
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:2110:50)
at Object.ajaxOptions.complete (G:\NAP\node_modules\xmla4js\src\Xmla.js:2021:34)
Solution
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;
}
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