NodeJS TypeError Callback is not a function

Question

I have the following piece of code:

function fun1(callback){
  var result =  "result of function 1";
  callback(result)
};
function fun2(callback){
  var result =  "result of function 2";
  callback(result)
};

fun1(log); // displays "result of function 1"
fun2(log); // displays "result of function 2"
fun2(fun1(log)); // Type Error ...

Where log is simple function (i.e. console.log(data)...) I'm wondering why fun2(fun1(log)) does not display "result of function 1" as one would expect. What is missing? Many thanks - Christian

sagie · Accepted Answer · 2016-11-14 12:40:48Z

2

fun2 expects to get a function. what you gave is the result (which is 'undefined') of func1. meaning

var something = fun1(log);
fun2(something);

So not sure what you expect to do in fun2 but you need to provide a callback and not undefined.

you could do:

function fun1(callback){
  var result =  "result of function 1";
  callback(result)
  return callback
};
function fun2(callback){
  var result =  "result of function 2";
  callback(result)
};

fun1(log); // displays "result of function 1"
fun2(log); // displays "result of function 2"
fun2(fun1(log));

answered Nov 14, 2016 at 12:40

sagie

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2 Comments

Christian68 Over a year ago

Thanks Sagie for your quick response. Is this the standard way of doing when you want to chain functions together?

sagie Over a year ago

there are many ways to chain functions, for example to return a function, to use promise and so on... there is no 1 specific standard way.

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