callback function in custom function - javascript

is there a way to call a custom function after another completes. Lots of jquery includes a way to do a call back, like when an animation completes you can just give it another function as a last parameter. But is there anyway to string together function1(params, function1(){alert('2');}) or something? I honestly have no idea how it would look.
Thank you

You can create a function pointer with something like:
var yourCallback = function() {
alert('2');
}
You can think of it as an object containing the function itself (not its return value). Since it is an object, you can pass it around to other functions.
Then pass that to the function you are calling as an argument (do not invoke it with (), as that would pass its return value):
function1(params, yourCallback);
Finally, in function1, invoke your callback when you need it:
function function1(args, callback) {
//Some code
if (callback) {
callback();
}
}

function doSomething(arg1, callback){
// bla bla bla
callback();
}

You'd structure your function something like this:
function one(x, y, z, callback) {
// do operations here
callback();
}
Then, since functions are objects you could pass in a function through the callback parameter, IE:
one(1, 2, 3, function(){alert('hello');});
Or
var func = function() { alert('hello'); };
one(1, 2, 3, func);
Or
function func() {
alert('hello');
}
one(1, 2, 3, func);

While callbacks are convenient, overuse can lead to ugly and unmaintainable code. Besides standard callbacks shown in other answers, you might want to look at other patterns like deferred/promises and AOP.
In jQuery, promises are implemented with when() and then().
requirejs is also a good startpoint.

Technically the pattern you're asking for is:
function myFunctionNeedsACallback(param1, callback) {
//do stuff with param1
if(typeof callback === "function"){
callback.call(yourThisParam, anyOtherParams);
}
}
While this is a useful pattern... overuse will lead to ugly, untestable, and unmaintainable code. Use with discretion.
Your example would work just fine:
myFunctionNeedsACallback(param, function() { alert('2'); });
//will alert 2 after running the rest of the contents of the function
EDIT
To address thomas's situation:
function ajaxCall1(){
return $.ajax(/*your parameters NO SUCCESS CALLBACK NEEDED*/);
}
function ajaxCall2(){
return $.ajax(/*your parameters NO SUCCESS CALLBACK NEEDED*/);
}
function ajaxCall3(){
return $.ajax(/*your parameters NO SUCCESS CALLBACK NEEDED*/);
}
$.when(ajaxCall1(), ajaxCall2(), ajaxCall3()).then(function(results1, results2, results3) {
//do stuff with all 3 results without "nesting" things
});

Related

TypeError: "listener" argument must be a function. Using npm pixelmatch in node JS [duplicate]

How do I pass a function as a parameter without the function executing in the "parent" function or using eval()? (Since I've read that it's insecure.)
I have this:
addContact(entityId, refreshContactList());
It works, but the problem is that refreshContactList fires when the function is called, rather than when it's used in the function.
I could get around it using eval(), but it's not the best practice, according to what I've read. How can I pass a function as a parameter in JavaScript?
You just need to remove the parenthesis:
addContact(entityId, refreshContactList);
This then passes the function without executing it first.
Here is an example:
function addContact(id, refreshCallback) {
refreshCallback();
// You can also pass arguments if you need to
// refreshCallback(id);
}
function refreshContactList() {
alert('Hello World');
}
addContact(1, refreshContactList);
If you want to pass a function, just reference it by name without the parentheses:
function foo(x) {
alert(x);
}
function bar(func) {
func("Hello World!");
}
//alerts "Hello World!"
bar(foo);
But sometimes you might want to pass a function with arguments included, but not have it called until the callback is invoked. To do this, when calling it, just wrap it in an anonymous function, like this:
function foo(x) {
alert(x);
}
function bar(func) {
func();
}
//alerts "Hello World!" (from within bar AFTER being passed)
bar(function(){ foo("Hello World!") });
If you prefer, you could also use the apply function and have a third parameter that is an array of the arguments, like such:
function eat(food1, food2) {
alert("I like to eat " + food1 + " and " + food2 );
}
function myFunc(callback, args) {
//do stuff
//...
//execute callback when finished
callback.apply(this, args);
}
//alerts "I like to eat pickles and peanut butter"
myFunc(eat, ["pickles", "peanut butter"]);
Example 1:
funct("z", function (x) { return x; });
function funct(a, foo){
foo(a) // this will return a
}
Example 2:
function foodemo(value){
return 'hello '+value;
}
function funct(a, foo){
alert(foo(a));
}
//call funct
funct('world!',foodemo); //=> 'hello world!'
look at this
To pass the function as parameter, simply remove the brackets!
function ToBeCalled(){
alert("I was called");
}
function iNeedParameter( paramFunc) {
//it is a good idea to check if the parameter is actually not null
//and that it is a function
if (paramFunc && (typeof paramFunc == "function")) {
paramFunc();
}
}
//this calls iNeedParameter and sends the other function to it
iNeedParameter(ToBeCalled);
The idea behind this is that a function is quite similar to a variable. Instead of writing
function ToBeCalled() { /* something */ }
you might as well write
var ToBeCalledVariable = function () { /* something */ }
There are minor differences between the two, but anyway - both of them are valid ways to define a function.
Now, if you define a function and explicitly assign it to a variable, it seems quite logical, that you can pass it as parameter to another function, and you don't need brackets:
anotherFunction(ToBeCalledVariable);
There is a phrase amongst JavaScript programmers: "Eval is Evil" so try to avoid it at all costs!
In addition to Steve Fenton's answer, you can also pass functions directly.
function addContact(entity, refreshFn) {
refreshFn();
}
function callAddContact() {
addContact("entity", function() { DoThis(); });
}
I chopped all my hair off with that issue. I couldn't make the examples above working, so I ended like :
function foo(blabla){
var func = new Function(blabla);
func();
}
// to call it, I just pass the js function I wanted as a string in the new one...
foo("alert('test')");
And that's working like a charm ... for what I needed at least. Hope it might help some.
I suggest to put the parameters in an array, and then split them up using the .apply() function. So now we can easily pass a function with lots of parameters and execute it in a simple way.
function addContact(parameters, refreshCallback) {
refreshCallback.apply(this, parameters);
}
function refreshContactList(int, int, string) {
alert(int + int);
console.log(string);
}
addContact([1,2,"str"], refreshContactList); //parameters should be putted in an array
You can also use eval() to do the same thing.
//A function to call
function needToBeCalled(p1, p2)
{
alert(p1+"="+p2);
}
//A function where needToBeCalled passed as an argument with necessary params
//Here params is comma separated string
function callAnotherFunction(aFunction, params)
{
eval(aFunction + "("+params+")");
}
//A function Call
callAnotherFunction("needToBeCalled", "10,20");
That's it. I was also looking for this solution and tried solutions provided in other answers but finally got it work from above example.
Here it's another approach :
function a(first,second)
{
return (second)(first);
}
a('Hello',function(e){alert(e+ ' world!');}); //=> Hello world
In fact, seems like a bit complicated, is not.
get method as a parameter:
function JS_method(_callBack) {
_callBack("called");
}
You can give as a parameter method:
JS_method(function (d) {
//Finally this will work.
alert(d)
});
The other answers do an excellent job describing what's going on, but one important "gotcha" is to make sure that whatever you pass through is indeed a reference to a function.
For instance, if you pass through a string instead of a function you'll get an error:
function function1(my_function_parameter){
my_function_parameter();
}
function function2(){
alert('Hello world');
}
function1(function2); //This will work
function1("function2"); //This breaks!
See JsFiddle
Some time when you need to deal with event handler so need to pass event too as an argument , most of the modern library like react, angular might need this.
I need to override OnSubmit function(function from third party library) with some custom validation on reactjs and I passed the function and event both like below
ORIGINALLY
<button className="img-submit" type="button" onClick=
{onSubmit}>Upload Image</button>
MADE A NEW FUNCTION upload and called passed onSubmit and event as arguments
<button className="img-submit" type="button" onClick={this.upload.bind(this,event,onSubmit)}>Upload Image</button>
upload(event,fn){
//custom codes are done here
fn(event);
}
By using ES6:
const invoke = (callback) => {
callback()
}
invoke(()=>{
console.log("Hello World");
})
If you can pass your whole function as string, this code may help you.
convertToFunc( "runThis('Micheal')" )
function convertToFunc( str) {
new Function( str )()
}
function runThis( name ){
console.log("Hello", name) // prints Hello Micheal
}
You can use a JSON as well to store and send JS functions.
Check the following:
var myJSON =
{
"myFunc1" : function (){
alert("a");
},
"myFunc2" : function (functionParameter){
functionParameter();
}
}
function main(){
myJSON.myFunc2(myJSON.myFunc1);
}
This will print 'a'.
The following has the same effect with the above:
var myFunc1 = function (){
alert('a');
}
var myFunc2 = function (functionParameter){
functionParameter();
}
function main(){
myFunc2(myFunc1);
}
Which is also has the same effect with the following:
function myFunc1(){
alert('a');
}
function myFunc2 (functionParameter){
functionParameter();
}
function main(){
myFunc2(myFunc1);
}
And a object paradigm using Class as object prototype:
function Class(){
this.myFunc1 = function(msg){
alert(msg);
}
this.myFunc2 = function(callBackParameter){
callBackParameter('message');
}
}
function main(){
var myClass = new Class();
myClass.myFunc2(myClass.myFunc1);
}

JavaScript: How to structure more than two callbacks

I've tried searching for this and cannot find the answer anywhere.
I've been using callbacks like this so far:
function one(param, callback){
//do stuff to param1
callback()
}
function two(param){
//do more stuff to param1
}
one(myParam, two)
This seems to work fine. However, I want to use three functions. This doesn't work:
function one(param, callback){
//do stuff to param1
callback();
}
function two(param, callback){
//do more stuff to param1
callback();
}
function three(param){
//do even more stuff to param1
}
one(myParam, two(three))
I'm guessing this is because of the brackets when passing two as a parameter in one, making 'two' excecute immediately.
How do I structure my code so I can excecute the function in the correct order?
This is because you setting three as a parameter of two:
function one(param1, callback){
alert(param1);
callback("Second Called", three);
}
function two(param1, callback){
//do more stuff to param1
alert(param1);
callback("Third Called");
}
function three(param1){
alert(param1);
//do even more stuff to param1
}
one("First Called", two);
You need to make little correction.
function one(param1, callback){
//do stuff to param1
// assuming param1 contains the processed value
callback(param1,three);
}
function two(param1, callback){
//do more stuff to param1
callback(param1);
}
function three(param1){
//do even more stuff to param1
}
// call the function like this
one(myParam,two);
Have a look on the library Async.js. Its good for handling asynchronous activity. You can better handle such cases and your code will be much more readable.
You could simply control the sequence of execution using an array and a forEach loop.
var callbacks = [one, two, three];
callbacks.forEach(function(callback){
callback(param);
});
Ofcourse the assumption here is that the scope of param is global to all of these callback functions.
function one(param, callback){
//param = "apple"
callback(param);
}
function two(paramFromOne){
//paramFromOne = "apple"
three(paramFromOne)
}
function three(paramFromTwoViaOne){
//paramFromTwoViaOne = "apple"
}
//you are invoking two before it can do anything inside the callback context of one
//one("apple", two(three))
//do this instead
one("apple", two)
If you want to specify callbacks that two uses you have to use a closure
function one(param, callback){
//param = "apple"
callback(param) //the callback here is the returned function in two
}
function two(callback){ //three
return function(paramViaOne) {
//paramViaOne = "apple"
callback(paramViaOne);
};
}
function three(paramFromTwoViaOne){
//paramFromTwoViaOne = "apple"
}
one("apple", two(three)) //two(three) returns an anonymous function that is used by one

Cleaner callbacks in JavaScript

If I want to break a callback implementation out of a method's parameter footprint for cleaner code I can do (for example)
foo.bar(a, callback(), b);
function callback() {
stuff;
}
instead of
foo.bar(a, function() {
stuff;
}, b);
But what do I do if the method passes something into the callback like three.js's loader functions? (http://threejs.org/docs/#Reference/Loaders/OBJMTLLoader)
foo.bar(callback(object));
function callback(object) {
object.stuff();
}
doesn't seem to work.
Got it. The format should be:
foo.bar(callback);
function callback(object) {
object.stuff();
}
The 2 snippets you've posted are actually different - when you pass an anonymous function as an argument it isn't run immediately, but in the "foo.bar" function itself. However, when you pass it as "callback()", that function runs immediately (it is useful in some cases, for example: if the function returns another function). So, pass it without "()".
For example:
function a(){
alert(1);
}
function b(callback){
callback(); //run the callback function.
}
b(a);
And, if you want to see an example of the second option:
function a(){
return function() {
alert(1);
};
}
a()(); //Alert
b(a()); //Alert
b(a) //nothing happens

Pass a JavaScript function as parameter

How do I pass a function as a parameter without the function executing in the "parent" function or using eval()? (Since I've read that it's insecure.)
I have this:
addContact(entityId, refreshContactList());
It works, but the problem is that refreshContactList fires when the function is called, rather than when it's used in the function.
I could get around it using eval(), but it's not the best practice, according to what I've read. How can I pass a function as a parameter in JavaScript?
You just need to remove the parenthesis:
addContact(entityId, refreshContactList);
This then passes the function without executing it first.
Here is an example:
function addContact(id, refreshCallback) {
refreshCallback();
// You can also pass arguments if you need to
// refreshCallback(id);
}
function refreshContactList() {
alert('Hello World');
}
addContact(1, refreshContactList);
If you want to pass a function, just reference it by name without the parentheses:
function foo(x) {
alert(x);
}
function bar(func) {
func("Hello World!");
}
//alerts "Hello World!"
bar(foo);
But sometimes you might want to pass a function with arguments included, but not have it called until the callback is invoked. To do this, when calling it, just wrap it in an anonymous function, like this:
function foo(x) {
alert(x);
}
function bar(func) {
func();
}
//alerts "Hello World!" (from within bar AFTER being passed)
bar(function(){ foo("Hello World!") });
If you prefer, you could also use the apply function and have a third parameter that is an array of the arguments, like such:
function eat(food1, food2) {
alert("I like to eat " + food1 + " and " + food2 );
}
function myFunc(callback, args) {
//do stuff
//...
//execute callback when finished
callback.apply(this, args);
}
//alerts "I like to eat pickles and peanut butter"
myFunc(eat, ["pickles", "peanut butter"]);
Example 1:
funct("z", function (x) { return x; });
function funct(a, foo){
foo(a) // this will return a
}
Example 2:
function foodemo(value){
return 'hello '+value;
}
function funct(a, foo){
alert(foo(a));
}
//call funct
funct('world!',foodemo); //=> 'hello world!'
look at this
To pass the function as parameter, simply remove the brackets!
function ToBeCalled(){
alert("I was called");
}
function iNeedParameter( paramFunc) {
//it is a good idea to check if the parameter is actually not null
//and that it is a function
if (paramFunc && (typeof paramFunc == "function")) {
paramFunc();
}
}
//this calls iNeedParameter and sends the other function to it
iNeedParameter(ToBeCalled);
The idea behind this is that a function is quite similar to a variable. Instead of writing
function ToBeCalled() { /* something */ }
you might as well write
var ToBeCalledVariable = function () { /* something */ }
There are minor differences between the two, but anyway - both of them are valid ways to define a function.
Now, if you define a function and explicitly assign it to a variable, it seems quite logical, that you can pass it as parameter to another function, and you don't need brackets:
anotherFunction(ToBeCalledVariable);
There is a phrase amongst JavaScript programmers: "Eval is Evil" so try to avoid it at all costs!
In addition to Steve Fenton's answer, you can also pass functions directly.
function addContact(entity, refreshFn) {
refreshFn();
}
function callAddContact() {
addContact("entity", function() { DoThis(); });
}
I chopped all my hair off with that issue. I couldn't make the examples above working, so I ended like :
function foo(blabla){
var func = new Function(blabla);
func();
}
// to call it, I just pass the js function I wanted as a string in the new one...
foo("alert('test')");
And that's working like a charm ... for what I needed at least. Hope it might help some.
I suggest to put the parameters in an array, and then split them up using the .apply() function. So now we can easily pass a function with lots of parameters and execute it in a simple way.
function addContact(parameters, refreshCallback) {
refreshCallback.apply(this, parameters);
}
function refreshContactList(int, int, string) {
alert(int + int);
console.log(string);
}
addContact([1,2,"str"], refreshContactList); //parameters should be putted in an array
You can also use eval() to do the same thing.
//A function to call
function needToBeCalled(p1, p2)
{
alert(p1+"="+p2);
}
//A function where needToBeCalled passed as an argument with necessary params
//Here params is comma separated string
function callAnotherFunction(aFunction, params)
{
eval(aFunction + "("+params+")");
}
//A function Call
callAnotherFunction("needToBeCalled", "10,20");
That's it. I was also looking for this solution and tried solutions provided in other answers but finally got it work from above example.
Here it's another approach :
function a(first,second)
{
return (second)(first);
}
a('Hello',function(e){alert(e+ ' world!');}); //=> Hello world
In fact, seems like a bit complicated, is not.
get method as a parameter:
function JS_method(_callBack) {
_callBack("called");
}
You can give as a parameter method:
JS_method(function (d) {
//Finally this will work.
alert(d)
});
The other answers do an excellent job describing what's going on, but one important "gotcha" is to make sure that whatever you pass through is indeed a reference to a function.
For instance, if you pass through a string instead of a function you'll get an error:
function function1(my_function_parameter){
my_function_parameter();
}
function function2(){
alert('Hello world');
}
function1(function2); //This will work
function1("function2"); //This breaks!
See JsFiddle
Some time when you need to deal with event handler so need to pass event too as an argument , most of the modern library like react, angular might need this.
I need to override OnSubmit function(function from third party library) with some custom validation on reactjs and I passed the function and event both like below
ORIGINALLY
<button className="img-submit" type="button" onClick=
{onSubmit}>Upload Image</button>
MADE A NEW FUNCTION upload and called passed onSubmit and event as arguments
<button className="img-submit" type="button" onClick={this.upload.bind(this,event,onSubmit)}>Upload Image</button>
upload(event,fn){
//custom codes are done here
fn(event);
}
By using ES6:
const invoke = (callback) => {
callback()
}
invoke(()=>{
console.log("Hello World");
})
If you can pass your whole function as string, this code may help you.
convertToFunc( "runThis('Micheal')" )
function convertToFunc( str) {
new Function( str )()
}
function runThis( name ){
console.log("Hello", name) // prints Hello Micheal
}
You can use a JSON as well to store and send JS functions.
Check the following:
var myJSON =
{
"myFunc1" : function (){
alert("a");
},
"myFunc2" : function (functionParameter){
functionParameter();
}
}
function main(){
myJSON.myFunc2(myJSON.myFunc1);
}
This will print 'a'.
The following has the same effect with the above:
var myFunc1 = function (){
alert('a');
}
var myFunc2 = function (functionParameter){
functionParameter();
}
function main(){
myFunc2(myFunc1);
}
Which is also has the same effect with the following:
function myFunc1(){
alert('a');
}
function myFunc2 (functionParameter){
functionParameter();
}
function main(){
myFunc2(myFunc1);
}
And a object paradigm using Class as object prototype:
function Class(){
this.myFunc1 = function(msg){
alert(msg);
}
this.myFunc2 = function(callBackParameter){
callBackParameter('message');
}
}
function main(){
var myClass = new Class();
myClass.myFunc2(myClass.myFunc1);
}

Javascript callback function with parameters [duplicate]

This question already has answers here:
Pass an extra argument to a callback function
(5 answers)
Closed 6 years ago.
This question looks like a duplicate, as the title is nearly replicated. But, my issue seems simpler and I can't find the answer to it.
I have a Javascript function that executes another callback function, it works like this:
<script type='text/javascript'>
firstfunction(callbackfunction);
</script>
where callback function is defined as:
callbackfunction(response) {
if (response=='loggedin'){
// ... do stuff
}}
but I want it to be something like this:
callbackfunction(response, param) {
if (response=='loggedin'){
// ... do stuff with param
}}
My question is, does it work to pass the parameter like this:
<script type='text/javascript'>
firstfunction(callbackfunction(param));
</script>
or am I doing it wrong?
In direct answer to your question, this does not work:
firstfunction(callbackfunction(param));
That will execute callbackfunction immediately and pass the return value from executing it as the argument to firstfunction which is unlikely what you want.
It is unclear from your question whether you should just change firstfunction() to pass two parameters to callbackfunction() when it calls the callback or whether you should make an anonymous function that calls the callback function with arguments.
These two options would look like this:
function firstfunction(callback) {
// code here
callback(arg1, arg2);
}
firstfunction(callbackfunction);
or
function firstfunction(callback) {
// code here
callback();
}
firstfunction(function() {
callbackfunction(xxx, yyy);
});
Use an anonymous function:
function foo( callback ) {
callback();
}
function baz( param ) {
console.log( param );
}
foo( function(){ baz('param') });
Adding parameters when calling a function.
https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Global_Objects/Function/apply
xdaz already answered the simple version.
Here is an example with variable amount of parameters.
function someObject(){
this.callbacks=new Array();
this.addCallback=function(cb){
this.callbacks[this.callbacks.length]=cb
}
this.callCallbacks=function(){
//var arr=arguments; this does not seem to work
//arr[arr.length]="param2";
var arr = new Array();
for(i in arguments){
arr[i]=arguments[i];
}
arr[arr.length]="another param";
i=0;
for(i in this.callbacks){
this.callbacks[i].apply(null,arr);
//this.callbacks[i].apply(this,arr);
//this is a ref to currrent object
}
}
this.callCallbacksSimple=function(arg){
for(i in this.callbacks){
this.callbacks[i](arg,"simple parameter");
}
}
}
function callbackFunction(){
for(i in arguments){
console.log("Received argument: " + arguments[i]);
}
}
var ObjectInstance=new someObject();
ObjectInstance.addCallback(callbackFunction);
ObjectInstance.callCallbacks("call callbacks");
ObjectInstance.callCallbacksSimple("call callbacks");
function is key word, you can't use it as function name.
Let say your function name is foo, then you could do like below:
var param = 'what ever';
foo(function(response) {
callbackfunction(response, param);
});
I think this is what you're looking for.
Lets say you're using jQuery ajax to do something, and you're passing it named callbacks. Here we have an onError callback that you might use to log or handle errors in your application. It conforms to the jQuery Ajax error callback signature, except for an extra parameter that you might have wanted to add at the back
function onError(jqXHR, textStatus, errorThrown, yourOwnVariableThing) {
console.error('Something went wrong with ' + yourOwnVariableThing);
}
this is where your function would be called - but you want an extra parameter
$.ajax("/api/getSomeData/")
.done(onDone)
.fail(onError)
.always(onComplete);
So this is what you can do to add the extra parameter
$.ajax("/api/getSomeData/")
.done(onDone)
.fail(onError.bind(this, arguments[0], arguments[1], arguments[2], 'Moo Moo');
.always(onComplete);
arguments is an array in JavaScript that contains all arguments passed to a function, and so you're just passing those arguments along to the callback.
Arguments
Bind

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