I am new to node.js. I am trying to understand Q.nfcall. I have the following Node.js code.

function mytest() {
  console.log('In mytest');
  return 'aaa';
}

Q.nfcall(mytest)
.then(
  function(value){
    console.log(value);
});

My expected output should be:

In mytest 
aaa

But the actual output is:

In mytest

After I changed Q.nfcall to Q.fcall in the code above, the output became what I expected:

In mytest 
aaa

Why's that? What's difference between Q.nfcall and Q.fcall? Thanks.

I am new to node.js. I am trying to understand Q.nfcall. I have the following Node.js code.

function mytest() {
  console.log('In mytest');
  return 'aaa';
}

Q.nfcall(mytest)
.then(
  function(value){
    console.log(value);
});

My expected output should be:

In mytest 
aaa

But the actual output is:

In mytest

After I changed Q.nfcall to Q.fcall in the code above, the output became what I expected:

In mytest 
aaa

Why's that? What's difference between Q.nfcall and Q.fcall? Thanks.

• javascript
• node.js
• q

3 Answers 3

From the Q documentation:

If you're working with functions that make use of the Node.js callback pattern, where callbacks are in the form of function(err, result), Q provides a few useful utility functions for converting between them. The most straightforward are probably Q.nfcall and Q.nfapply

What it means is that nfcall() expects Node-style function(cb) which calls cb(error, result). So when you write:

Q.nfcall(mytest)
.then(
  function(value){
    console.log(value);
});

Q expects that mytest calls passed callback with (error, value) and Q then calls your next callback with value. So your code should look something like this(here is Plunkr):

function mytest(cb) {
  console.log('In mytest');
  cb(null, 'aaa');
}

Q.nfcall(mytest)
.then(
  function(value){
    console.log('value', value);
});

You can look into nfcall() test cases to go deeper how it should be used.

The accepted answer is good but to address the difference:

• nfapply expects an array of arguments
• nfcall expects arguments provided individually

For Example:

Q.nfcall(FS.readFile, "foo.txt", "utf-8");
Q.nfapply(FS.readFile, ["foo.txt", "utf-8"]);

See https://github.com/kriskowal/q/wiki/API-Reference#qnfapplynodefunc-args

While the accepted answer is a good explanation of nfcall it's not the difference between fcall and nfcall.

Both methods return a promise.

fcall returns a promise that is resolved to the return value from the function it receives.

nfcall returns a promise that is resolved (or rejected) to the value of the result of the callback that the input function should receive when called.

e.g.

return Q.nfcall(FS.readFile, "foo.txt", "utf-8");

can be translated to the following code

return new Promise((resolve, reject)=>{
    FS.readFile("foo.txt","utf-8",function(err, result){
        if (err)
        {
            reject(err);
            return err;
        }
        resolve(result);
    });
});

Where

return Q.fcall(function () {
    return 10;
});

can be translated to

return new Promise((resolve,reject)=>{resolve(10);});