JavaScript: Passing parameters to a callback function

Question

I'm trying to pass some parameter to a function used as callback , how can I do that?

This is my try:

 function tryMe(param1, param2) { alert(param1 + " and " + param2); } function callbackTester(callback, param1, param2) { callback(param1, param2); } callbackTester(tryMe, "hello", "goodbye");

Answer 1

If you want something slightly more general, you can use the arguments variable like so:

 function tryMe(param1, param2) { alert(param1 + " and " + param2); } function callbackTester(callback) { callback(arguments[1], arguments[2]); } callbackTester(tryMe, "hello", "goodbye");

But otherwise, your example works fine ( arguments[0] can be used in place of callback in the tester)

Answer 2

This would also work:

 // callback function function tryMe(param1, param2) { alert(param1 + " and " + param2); } // callback executer function callbackTester(callback) { callback(); } // test function callbackTester(function() { tryMe("hello", "goodbye"); });

Another Scenario :

 // callback function function tryMe(param1, param2, param3) { alert(param1 + " and " + param2 + " " + param3); } // callback executer function callbackTester(callback) { //this is the more obivous scenario as we use callback function //only when we have some missing value //get this data from ajax or compute var extraParam = "this data was missing"; //call the callback when we have the data callback(extraParam); } // test function callbackTester(function(k) { tryMe("hello", "goodbye", k); });

Answer 3

Your question is unclear. If you're asking how you can do this in a simpler way, you should take a look at the ECMAScript 5th edition method .bind() , which is a member of Function.prototype . Using it, you can do something like this:

function tryMe (param1, param2) {
    alert (param1 + " and " + param2);
}

function callbackTester (callback) {
    callback();
}

callbackTester(tryMe.bind(null, "hello", "goodbye"));

You can also use the following code, which adds the method if it isn't available in the current browser:

// From Prototype.js
if (!Function.prototype.bind) { // check if native implementation available
  Function.prototype.bind = function(){ 
    var fn = this, args = Array.prototype.slice.call(arguments),
        object = args.shift(); 
    return function(){ 
      return fn.apply(object, 
        args.concat(Array.prototype.slice.call(arguments))); 
    }; 
  };
}

Example

bind() - PrototypeJS Documentation

Answer 4

If you are not sure how many parameters are you going to be passed into callback functions, use apply function.

function tryMe (param1, param2) {
  alert (param1 + " and " + param2);
}

function callbackTester(callback,params){
    callback.apply(this,params);
}

callbackTester(tryMe,['hello','goodbye']);

Answer 5

When you have a callback that will be called by something other than your code with a specific number of params and you want to pass in additional params you can pass a wrapper function as the callback and inside the wrapper pass the additional param(s).

function login(accessedViaPopup) {
    //pass FB.login a call back function wrapper that will accept the
    //response param and then call my "real" callback with the additional param
    FB.login(function(response){
        fb_login_callback(response,accessedViaPopup);
    });
}

//handles respone from fb login call
function fb_login_callback(response, accessedViaPopup) {
    //do stuff
}

Answer 6

Wrap the 'child' function(s) being passed as/with arguments within function wrappers to prevent them being evaluated when the 'parent' function is called.

function outcome(){
    return false;
}

function process(callbackSuccess, callbackFailure){
    if ( outcome() )
        callbackSuccess();
    else
        callbackFailure();
}

process(function(){alert("OKAY");},function(){alert("OOPS");})

Answer 7

Code from a question with any number of parameters and a callback context:

function SomeFunction(name) {
    this.name = name;
}
function tryMe(param1, param2) {
    console.log(this.name + ":  " + param1 + " and " + param2);
}
function tryMeMore(param1, param2, param3) {
    console.log(this.name + ": " + param1 + " and " + param2 + " and even " + param3);
}
function callbackTester(callback, callbackContext) {
    callback.apply(callbackContext, Array.prototype.splice.call(arguments, 2));
}
callbackTester(tryMe, new SomeFunction("context1"), "hello", "goodbye");
callbackTester(tryMeMore, new SomeFunction("context2"), "hello", "goodbye", "hasta la vista");

// context1: hello and goodbye
// context2: hello and goodbye and even hasta la vista

Answer 8

Use curried function as in this simple example.

 const BTN = document.querySelector('button') const RES = document.querySelector('p') const changeText = newText => () => { RES.textContent = newText } BTN.addEventListener('click', changeText('Clicked!'))

 <button>ClickMe</button> <p>Not clicked<p>

Answer 9

A new version for the scenario where the callback will be called by some other function, not your own code, and you want to add additional parameters.

For example, let's pretend that you have a lot of nested calls with success and error callbacks. I will use angular promises for this example but any javascript code with callbacks would be the same for the purpose.

someObject.doSomething(param1, function(result1) {
  console.log("Got result from doSomething: " + result1);
  result.doSomethingElse(param2, function(result2) {
    console.log("Got result from doSomethingElse: " + result2);
  }, function(error2) {
    console.log("Got error from doSomethingElse: " + error2);
  });
}, function(error1) {
  console.log("Got error from doSomething: " + error1);
});

Now you may want to unclutter your code by defining a function to log errors, keeping the origin of the error for debugging purposes. This is how you would proceed to refactor your code:

someObject.doSomething(param1, function (result1) {
  console.log("Got result from doSomething: " + result1);
  result.doSomethingElse(param2, function (result2) {
    console.log("Got result from doSomethingElse: " + result2);
  }, handleError.bind(null, "doSomethingElse"));
}, handleError.bind(null, "doSomething"));

/*
 * Log errors, capturing the error of a callback and prepending an id
 */
var handleError = function (id, error) {
  var id = id || "";
  console.log("Got error from " + id + ": " + error);
};

The calling function will still add the error parameter after your callback function parameters.

Answer 10

Let me give you a very plain Node.js style example of using a callback:

/**
 * Function expects these arguments: 
 * 2 numbers and a callback function(err, result)
 */
var myTest = function(arg1, arg2, callback) {
  if (typeof arg1 !== "number") {
    return callback('Arg 1 is not a number!', null); // Args: 1)Error, 2)No result
  }
  if (typeof arg2 !== "number") {
    return callback('Arg 2 is not a number!', null); // Args: 1)Error, 2)No result
  }
  if (arg1 === arg2) {
    // Do somethign complex here..
    callback(null, 'Actions ended, arg1 was equal to arg2'); // Args: 1)No error, 2)Result
  } else if (arg1 > arg2) {
    // Do somethign complex here..
    callback(null, 'Actions ended, arg1 was > from arg2'); // Args: 1)No error, 2)Result
  } else {
    // Do somethign else complex here..
    callback(null, 'Actions ended, arg1 was < from arg2'); // Args: 1)No error, 2)Result
  }
};


/**
 * Call it this way: 
 * Third argument is an anonymous function with 2 args for error and result
 */
myTest(3, 6, function(err, result) {
  var resultElement = document.getElementById("my_result");
  if (err) {
    resultElement.innerHTML = 'Error! ' + err;
    resultElement.style.color = "red";
    //throw err; // if you want
  } else {
    resultElement.innerHTML = 'Result: ' + result;
    resultElement.style.color = "green";
  }
});

and the HTML that will render the result:

<div id="my_result">
  Result will come here!
</div>

You can play with it here: https://jsfiddle.net/q8gnvcts/ - for example try to pass string instead of number: myTest('some string', 6, function(err, result) .. and see the result.

I hope this example helps because it represents the very basic idea of callback functions.

Answer 11

function tryMe(param1, param2) {
  console.log(param1 + " and " + param2);
}

function tryMe2(param1) {
  console.log(param1);
}

function callbackTester(callback, ...params) {
  callback(...params);
}



callbackTester(tryMe, "hello", "goodbye");

callbackTester(tryMe2, "hello");

read more about the spread syntax

Answer 12

Faced this recently, to get it(especially if the parent function has multiple arguments doing different stuffs not related to the callback, is to have the callback placed with its argument in an arrow function passed as an argument.

function tryMe(param1, param2) {
  alert(param1 + " and " + param2);
}

function callbackTester(callback, someArg, AnotherArg) {
  callback();
  
}

callbackTester(()=> tryMe("hello", "goodbye"), "someArg", "AnotherArg");

...or simply if you dont have multiple arguments doing other stuff.

function tryMe(param1, param2) {
  alert(param1 + " and " + param2);
}

function callbackTester(callback) {
  callback();
}

callbackTester(()=> tryMe("hello", "goodbye"));

Answer 13

I was looking for the same thing and end up with the solution and here it's a simple example if anybody wants to go through this.

var FA = function(data){
   console.log("IN A:"+data)
   FC(data,"LastName");
};
var FC = function(data,d2){
   console.log("IN C:"+data,d2)
};
var FB = function(data){
   console.log("IN B:"+data);
    FA(data)
};
FB('FirstName')

Also posted on the other question here

Answer 14

//Suppose function not taking any parameter means just add the GetAlterConfirmation(function(result) {});
GetAlterConfirmation('test','messageText',function(result) {
                        alert(result);
    }); //Function into document load or any other click event.


function GetAlterConfirmation(titleText, messageText, _callback){
         bootbox.confirm({
                    title: titleText,
                    message: messageText,
                    buttons: {
                        cancel: {
                            label: '<i class="fa fa-times"></i> Cancel'
                        },
                        confirm: {
                            label: '<i class="fa fa-check"></i> Confirm'
                        }
                    },
                    callback: function (result) {
                        return _callback(result); 
                    }
                });

Answer 15

I'm trying to pass some parameter to a function used as callback , how can I do that?

I think he is implying that he wants to call the function this callbackTester(tryMe, "hello", "goodbye") . To do this we can use the Rest Operator (...) . This operator takes the arguments that a function receives and dumps them into a real array that we will use to access in our callback function.

Now, some other developers might also argue that we could use the arguments "array". That will be fine, but we should be careful with it. arguments is not a real array but an array-like object with a length property.

Here is a working snippet using the Rest Operator:

 function tryMe(params) { console.log(params.join(', ')); } function callbackTester(callback, ...params) { callback(params); } callbackTester(tryMe, 'hello', 'goodbye', 'hi again'); callbackTester(tryMe, 'hello', 'goodbye'); callbackTester(tryMe, 'hello');

Answer 16

Just use the bind() function which is primarily used to set the this value. However, we can also use it to pass parameters without calling the function due to bind() returning a new function with the sequence of arguments provided.

Example:

 function foo(param1, param2, param3) { console.log(param1, param2, param3); } setTimeout(foo.bind(null, 'foo', 'bar', 'baz'), 1000);

In the snippet above, the setTimeout function takes 2 arguments, the callback function and a minimum time in ms for the function to be called, so when passing the callback function we're going to use bind and specify the parameters

Note: The first parameter of bind is the value that we want to set for this , and because we're not interested on that, null was passed, the subsequent parameters in bind are going to be the parameters for the callback.