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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);
}
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
I'm working on a big project and I simplified what it matters here. This is the code:
a = new Thing(/*sayHi + sayHey*/);
function sayHi() {
alert("hi");
}
function sayHey() {
alert("hey");
}
function Thing (callback) {
callback();
}
I'd like to, with just the callback parameter, call both the sayHi() and the sayHey() function, at the order I put them. Is it possible? How would I do it? Thank you.
Pass an anonymous function that calls both of them sequentially:
a = new Thing(function() {
sayHi();
sayHey();
});
function sayHi() {
alert("hi");
}
function sayHey() {
alert("hey");
}
function Thing (callback) {
callback();
}
Alternatively to #Barnar's answer, create and pass a regular named function. If the callback logic gets heavier, you might want that anyway.
function hiHeyCallback() {
sayHi();
sayHey();
}
a = new Thing(hiHeyCallback);
I would like to pass a function with parameters to another function and have it run on an event, like this:
var main_object = function () {
this.main_function = function (function) {
document.addEventListener('click',function);
}
this.passed_function = function (variable) {
alert(variable);
}
}
var main_object = new main_object();
main_object.main_function(main_object.passed_function(3));
In modern JavaScript engines, you can bind the function:
mainObject.main_function(main_object.passed_function.bind(main_object, 3));
The first argument to bind will be this when the function executes and any remaining arguments to bind will be leading arguments in the call to the function.
If I understand you right,
main_object.main_function(function() { main_object.passed_function(3) });
For what you're talking about, you could just use bind. In your case, you would do:
main_object.main_function(main_object.passed_function.bind( main_object, 3 ));
function mainfunc(func) {
alert(func);
}
function callBackFn(a) {
alert(a);
}
mainfunc("arg1", callBackFn("javaScritFnParameter")); //call this in load
For sure it works no need to worry... but callbackFn will execute first and next only "arg1" will execute.
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);
}