Geek asks and answers

Unable to set header in KOA. When using callback

I recently worked on a new project that used javascript callbacks. And I used 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)
+3
source share
1 answer

The problem is that your asynchronous call LoadCubesJSon()takes time to return, but Koa is unaware of this and continues the flow of control. This is basically it yield.

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

'Q' . node-thunkify thunk.

, 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);

, :

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;
}
+6

More articles:


All Articles