Callback Is Not a Function (But It Is)

Javascript callback function throws error "Callback is not a function" in firefox

It is because you are not always passing the callback into that method.

success: function (data) {
  var items = "";
  $.each(data, function (i, item) {
    items += "<option value=\"" + item.Value + "\">" + item.Text + "</option>";
  });
  $(childClass).html(items);
  $(childClass)[0].selectedIndex = 0;
  if(callback) callback();  //check before calling it. 
}

Javascript: TypeError: callback is not a function

In your version, you're calling func2() without specifying the callback function, which is a required argument. In your second example (with init commented out), you're correctly specifying the callback function in func2(function (res) { ... });.

Is the below snippet something you're looking for?

const Parent = (function () {
  const func1 = function () {
    func2(function (res) {
      console.log(res);
    });
  }

  const func2 = function (callback) {
    callback('abc'); // this is passing 'abc' to line 3 of this snippet
  }

  return {
    init: function () {
      func1();
      // func2(); // you don't want to call "func2" here, as "func1" calls it
      // Or you could run:
      func2(function (res) {
        console.log(res);
      });
      // But this makes "func1" redundant
    }
  };
});

Parent().init();

// Output
abc

TypeError: callback is not a function - Node JS

that's because when you're calling callback from getUser you're expecting that getRepositories(user) is called but actually getRepositories(username, callback) is called. print username on the console so you'll know.

THERE'S NO OVERLOADING IN JAVASCRIPT.

So what you need to do is either change the function's name, or do something like

function getUser(id, callback) {
    setTimeout(() => {
        console.log('Reading a user data from database...');
        // callback({ id: id, gitHubUsername: 'Gary' });
        callback("Gary", getCommits);
    }, 2000);
}

On another note, your getCommits(repos) continuously calls itself without any base condition. you're likely to receive RangeError: Maximum call stack size exceeded.

callback is not a function (but it is)

You forget to call and handle the callback.

user.checkIfUserExists(newUser, function(arg1, isUserExist) { 
    if (isUserExist) {
       //user exist
    }
});

I am getting this error: 'TypeError: callback is not a function', but the function is still executing

module.exports = getData();

You are accidentally calling your function over here. Since no arguments are passed to it at this point, callback is undefined, and so is not a function.

getting error callback is not a function in one condition

In your function, you first need to check if callback is actually a function:

function EditorDuplicate(TransactionId, callback) {
    // your code

    if (typeof callback === 'function') {
      callback();
    }
}

This is because callback becomes undefined when it is not passed in this call:

EditorDuplicate(TransactionId);

And trying to call undefined will throw an error.

Error callback is not a function in node.js

I'm unable to reproduce what you're seeing. Running your code under a couple of different conditions, I got the following results:

> var lobbies = [{ lobbyName: 'foo' }];
> doesLobbyExist('foo', console.log)
foo exists!
true
> doesLobbyExist('bar', console.log)
...
> var lobbies = [{ lobbyName: 'foo' }, { lobbyName: 'bar' }, { lobbyName: 'foo' }];
> doesLobbyExist('bar', console.log)
bar exists!
true
> doesLobbyExist('foo', console.log)
foo exists!
true
foo exists!
true
...
> var lobbies = [];
> doesLobbyExist('foo', console.log)
false

but there a couple of problems in your code:

• lobbyExists only gives a false response if there are no lobbies to check, it's possible for it to call the callback multiple times if there are multiple lobbies with the same name, and it doesn't return anything if a lobby isn't found.
• there's no easy way to break out of forEach. since you only want to call your callback function once, you'll want to either return or break once a lobby has been found. switching over to a for i loop allows us to do either one.
• comparison using == is what's known as loose or abstract equality, which may cause errors down the road - you can read up on the difference between loose (==) and strict (===) equality here.
• if the lobbyExists function is only iterating through an array, then there's no need to treat it as asynchronous and use a callback. synchronous callbacks are a thing, and are perfectly fine to use, but it's something to be aware of as you continue to develop.

with all that being said, these are the changes that I would suggest:

function doesLobbyExist(a, callback) {
    lobbyExists(a, function(random_data) {
        //possibly do other things to random_data...

        callback(random_data);
    });
}

function lobbyExists(a, callback) {
    for(var i = 0; i < lobbies.length; i++) {
        var l = lobbies[i];

        if(l.lobbyName === a) {
            console.log(a + " exists!");

            callback(true);

            return;
        }
    }

    callback(false);
}

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