make node.js fs.readFile as promised action using Q

Question

when I use deferred.resolve way to promise a action,I can't get the content of the file

function readFile(fileName) {
    var deferred = Q.defer();
    fs.readFile(fileName, 'utf-8', deferred.resolve);
    return deferred.promise;
};

readFile('test.txt').then(function (err, data) {
    console.log('data:' + data)
})

I get data:undefined output

but it works OK fine when I promised action httpGet

var httpGet = function (opts) {
    var deferred = Q.defer();
    http.get(opts, deferred.resolve);
    return deferred.promise;
};

httpGet('http://www.google.com').then(function (res) {
        console.log("Got response: " + res.statusCode);
        res.on('data', function (data) {
            console.log(data.toString());
        })
    }
);

Is there something wrong the code above and in this way how can i get the content of the file. or is there something different between fs.readFile and http.get?

Related http://stackoverflow.com/questions/22519784/how-do-i-convert-an-existing-callback-api-to-promises — Benjamin Gruenbaum, Apr 10 '14 at 07:35

Benjamin Gruenbaum · Accepted Answer · 2014-04-10T10:03:24.490

3

You can use Q.nfcall to call a NodeJS function promisified.

function httpGet(opts){
    return Q.nfcall(http.get,opts);
}

Or simply:

var httpGet = Q.nfbind(http.get,http)

This would also work for fs.readFile by the way.

If you want to do it manually. Q's deferred objects have a .makeNodeResolver function which lets you pass them around safely like you do:

var httpGet = function (opts) {
    var deferred = Q.defer();
    http.get(opts, deferred.makeNodeResolver());
    return deferred.promise;
};

One of the things .makeNodeResolver does is bind the .this value.

It is better to use .nfcall though.

edited Apr 10 '14 at 10:03

answered Apr 10 '14 at 05:29

Benjamin Gruenbaum

270,886
87
504
504

It is worth mentioning other libraries like Bluebird are orders of magnitude faster here as they use techniques like dynamic recompilation to create a promisified function that is (almost) as fast as the callback version. – Benjamin Gruenbaum Apr 10 '14 at 05:32

make node.js fs.readFile as promised action using Q

1 Answers1