Develop Reference

Javascript invoke function without parentheses [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);
}

Currying - How is returned function called with a second argument in JavaScript?

How does the second argument get called in liftf(add)(1)?
function add (first, second) {
return first + second;
}
function liftf (binary) {
return function (first) {
return function (second) {
return binary(first, second);
};
};
}
var inc = liftf(add)(1);
I understand how lift(add) is called and stored.
I am confused on how a function is returned but then called with (1).
I first explored if it operated on the same principle of an IIFE but it doesn't seem to. IFFE's would be (function() {}()) vs funciton() {}().
The 'chained' function arguments confuse me and I want to understand what's going on.
Thanks!

If an expression evaluates to a function, then that function can be invoked with parentheses and the list of arguments.
Since the expression liftf(add) returns a function, you call the returned function with the parameter 1 in parentheses: liftf(add)(1)
Another way to look at it is if you set liftf(add) to a variable, then you could call the function stored in that variable:
var additionFunc = liftf(add) // Stores the new function in additionFunc
var result = additionFunc(1) // Evaluates the new function
Let's also look at IIFEs. Suppose we have one like this:
(function(x) {return x + 1})(5)
The (function() { /* ... */ }) expression evaluates to a function, which is then evaluated by the parentheses and argument (5).

Let's look at a simpler example of currying:
function add(x) {
return function (y) {
return x + y;
};
}
If you called it with only one argument, like so:
add(2);
It would expand to:
function (y) {
return 2 + y;
}
Which would expect a y argument (if you called it). You could run that like this:
function (y) {
return 2 + y;
}(5)
Or more succintcly:
add(2)(5);
Each function just expands to a new anonymous function when currying, so even though it looks weird, once you expand the code out, it will start to make sense.

How to pass a callback function with multiple possible parameter lists

Here are two callback function:
function callback_a(){
alert('a');
}
function callback_b(p){
alert('b says'+ p)'
}
If I want use callback_a
function test(callback){
if(condition){
callback();
}
}
test(callback_a);
But the function test isn't applicable to callback_b, So how to implement a common function that you can passing some callbacks function with multiple possible parameter lists.

There are three options:
The easiest way is to use spread operator:
function test(callback, ...callback_args) {
callback(...callback_args);
}
in this case the invocation of test for function callback_b would be like this:
test(callback_b,"b")
The second way is using arguments which are scoped to any function in JavaScript:
function test(callback) {
callback.apply(null, arguments.slice(1));
}
the invocation of test for function callback_b would be the same:
test(callback_b,"b")
Another options is to use partially applied functions. In this case you should define b_callback like this (ES6 syntax):
let callback_b = (p) => () => void{
alert('b says'+ p)'
}
or without ES6:
function callback_b(p) {
return function(){
alert('b says'+ p)'
}
}
and invoke it like this:
test(callback_b("b"))

There is a special object called arguments that gets created when a function is invoked. It's an array-like object that represents the arguments passed in to a function:
It can be used like this:
test();
// no arguments passed, but it still gets created:
// arguments.length = 0
// arguments >> []
test(a);
// ONE argument passed:
// arguments.length = 1
// arguments >> [a]
test(a,b,c,d);
// FOUR arguments passed:
// arguments.length = 4
// arguments >> [a,b,c,d]
Knowing this, one can call a callback with the rest of the arguments passed in from the parent function using apply like this:
function test(callback) {
callback.apply(null, Array.prototype.slice.call(arguments, 1));
}
// arguments passed into test are available in the function scope when
// .slice is used here to only pass the portion of the arguments
// array relevant to the callback (i.e. any arguments minus the
// first argument which is the callback itself.)
//
// N.B. The arguments object isn't an array but an array like object so
// .slice isn't available on it directly, hence .call was used here)
Might be worth reading up on:
The arguments object
Function.prototype.apply, Function.prototype.call and Function.prototype.bind as they are way to bind a context and arguments to a function (i.e. they'll work with the arguments object to call a function where you may not know how many arguments will be passed)

So how to implement a common function that you can passing some callbacks function with multiple possible parameter lists.
Basically, you don't. The function receiving the callback is in charge of what the callback receives as arguments. When you call Array#forEach, it's Array#forEach that decides what arguments your callback gets. Similarly, String#replace defines what it will call its callback with.
Your job is to say what test will do, what it will call its callback with. Then it's the job of the person using test to write their callback appropriately. For instance: You might document test as calling the callback with no arguments. If the caller wants to use callback_b, then it's up to them to handle the fact that callback_b expects a parameter. There are several ways they can do that:
The could wrap it in another function:
test(function() {
callback_b("appropriate value here");
});
...or use Function#bind
test(callback_b.bind(null, "appropriate value here"));
...but it's their problem, not yours.
Side note: If they pass you callback_b and you call it without any arguments, you won't get an error. JavaScript allows you to call a function with fewer arguments than it expects, or more. How the function handles that is up to the author of the function.

You can pass an anonymous function as the callback that will itself return your desired callback function with parameters.
test(function() { return callback_b(' how are you'); });
see this working snippet that will first use callback_a, then callback_b (with parameter) as the callback:
function callback_a(){
alert('a');
}
function callback_b(p){
alert('b says'+ p);
}
function test(callback){
if(true){
callback();
}
}
test(callback_a);
test(function() { return callback_b(' how are you'); });

You can pass the parameter while calling the callback
function test(callback){
if(condition){
callback();
}
else if(other condition){
callback("b");
}
}
test(callback_b);
You can write your callback function like
function callback_a_b(){
if(arguments.length){
var arg = [].slice.call(arguments);
alert('b says'+ arg[0])
}
else{
alert('a');
}
}

You can pass array of parameters as second param of test function or in ES6 use spread operator read more here
function test(callback, params){
if(condition){
if (params === undefined){
callback();
} else {
callback.apply(null, params); //params must be array
//ES6: callback(...params);
}
}
}
test(callback_a);
test(callback_b, [" whatever"]);

I've just checked in my browser (ffox 51.0.1) that the following works:
function test(callback,other_args){if(condition){callback(other_args);}}
results:
condition=true
test(callback_a)
=> shows the alert with 'a'
condition=false
test(callback_a)
=> doesn't show anything
condition=true
test(callback_b,"pepe")
=> shows the alert with 'b sayspepe'
condition=false
test(callback_b,"pepe")
=> doesn't show anything

can we give a callback with parameters?

function abc(arg1,arg2, callback){
console.log(arg1 + arg2) // I am doing something really interesting
if(callback){
callback(arg1+ arg2)
}
}
function add(){
console.log("from call back " + arguments[0])
}
var a =10;
abc(1,2, add)
this works fine, but if we need to send some additional arguments to the call back, what should we do??
Here, Apart from (arg1+ arg2) I need some other arguments to be set from the caller of abc to the callback
And, what is the difference between abc(1,2,add) and abc(1,2,add()) ??
Thanks :)

abc(1,2,add) => Giving the function argument as "function type". This is like giving a pointer to the function for later invoking it.
abc(1,2,add()) => Calling the add() function and giving its return value as argument.
Do you need that the callback support more than an argument? Since JavaScript is a dynamic language, just call the same callback function with more arguments:
callback(1,2,3,4)
callback(1,2,3,4,5)
callback(1,2,3,4,5,6).
JavaScript isn't strict with function signatures: functions have as many arguments as the caller gives to them.
For example, if you've a callback function like this:
function(a, b) {
}
And later you call it this way:
function("hello"); // Where's the second argument??
JavaScript won't complain. Simply b will be undefined.

the difference between add and add():
add is a function and add() is the return value of the function.
if you want to get more arguments of the function. use the arguments object
every function has an arguments object
arguments[0] == 1 arguments[1] == 2 arguments[2] == add.

You can call your callback with parameters from your original function and you can also provide the parameters for the callback inside your function call:
function f1 (args, callback, params) {
callback.apply(this, params); // Params needs to be an array
callback.apply(this, [args]); // This would call callback with 1 param - args
callback.call(this, args); // This is the same as the line above
}
function f2 () {
for (item in arguments) {
console.log(item);
}
}
f1('arg', f2, [1, 2, 3]);
If you call your function with a function call inside the parameters then it would immediately evaluate and it would not be executed as a callback.

The difference between abc(1,2,add) and abc(1,2,add()) is that in the second case, it's not abc that calls add. The consequence is that add executes much sooner than expected and without any arguments.
The usual way to pass extra parameters to a callback is to create an anonymous function that closes over the parameters:
var a=10;
var b=20;
abc(a, b, function(text){
add(a, b);
console.log(text); //logs "abc"
});

overriding a javascript function's return statement

Assume I have a js function. From some other point in the program, I want to run its code, but not its return statement. In its place, I would like to run some other return statement that references the variables in the scope of the original function.
Is there a way to do this, other than loading up the function source, replacing the return, and using eval on the result? Minimal modification of the original is possible, though it should not affect the original's performance by adding e.g. an extra function call.

You could try something like this, but I'm not sure it meets your conditions.
Edit: Fixed to work in jsfiddle
// Modified to set all "shared" variables as "members" of the function.
var test = function() {
test.val = "one";
test.val2 = "two";
return 1;
}
// Using different result
function test2() {
test();
return test.val2;
}

Unless you're able to restructure your methods to accommodate a callback or introduce some other parameter-based logic-flow (not an option for 3rd party code), you're out of luck.
Here's a callback sample (fiddle, credit to dzejkej's answer)
function foo(callback) {
var x = 2;
// pass your values into the callback
return callback ? callback.call(this, x) : x * 2;
}
document.write(foo());
document.write("<hr/>");
// specify the parameters for your callback
document.write(foo(function(x){ return x * 4;}) );

You can introduce a callback function that will get called if available otherwise "standard" value will be returned.
function test(callback) {
// ...
return callback ? callback.call(this) : /* original value returned */ "xyz";
}
test(function() { /* "this" is same as in test() */ });
EDIT:
If you want to pass variables inside callback then you just list them in the .call() function.
Example:
function test(callback) {
var a = 4;
var b = 2;
// ...
return callback ? callback.call(this, a, b) : a * b;
}
test(); // 8
test(function(a, b) { return a + b; }); // 6
See this fiddle.

Provided that you would keep variables of the outer scope function within a single object, you could try something like the following:
function original(a, b, c, rep) {
var data = {};
// Do some fancy stuff but make sure to keep everything under data
data.a = a.replace(/foo/, 'bar');
...
if ( Object.prototype.toString.call(rep) === '[object Function]' )
return rep.call(data);
return data;
}
function replacement() {
return 'foo' + this.a;
}
// Now let's make use of both the original and the replacement ...
console.log(original('foo', x, y)); // => {a: "bar", b: ...}
console.log(original('foo', x, y, replacement)); // => {a: "foobar", b: ...}
Hope, it's what you where asking for.
cheers

I think you really misunderstand the concept of return statement. The return statement of a function will simply return a value, or an object, or undefined if there is no return parameter specified.
If all you're trying to do is execute a function but "not its return statement" than you would just invoke the function and not do anything with the returned value/object:
However, if what you mean is that you would like to execute a function but not execute the "parameter" to its return statement then that literally means to selectively execute an arbitrary portion of the body of a function. And as far as I know that is not possible (without using reflection to get the function definition, modify the definition, and then dynamically invoking the modified version - which you said you didn't want to do).