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);
}
I have a (asynchronous) function that gets the ID of a logged in user in Chrome. I'm trying to return the value of the ID with a callback but it keeps returning 'undefined'.
Before someone tries to mark this a duplicate, I used the code from here (and tried other places too): How to return value from an asynchronous callback function? But it didn't work:
function getGaia(callback) {
chrome.identity.getProfileUserInfo(function(userInfo){
var userId = userInfo.id;
callback(userInfo.id);
});
}
getGaia(function(id){
return id;
});
var gaiaId = getGaia();
I get the following error:
'callback' is a not a function
What exactly am I doing wrong / what is the correct code?
That is because you are not providing a callback.
function doSomethingLater(callback) {
setTimeout(callback, 1000);
}
console.log('This is before the callback');
doSomethingLater(function() {
console.log('This is the callback')
});
So when you are calling var gaiaId = getGaia(); you are not passing in a callback function
[Edit] This is what your code would need to look like:
function getGaia(callback) {
chrome.identity.getProfileUserInfo(function(userInfo){
var userId = userInfo.id;
// This will call the function that you pass in below
//and pass in userInfo.if as a parameter
callback(userInfo.id);
});
}
var gaiaId = getGaia(function (id) {
// id === userInfo.id from above
// Do something with the id that you pass in
});
You can think of functions like variables in JavaScript,
So you can assign a function to a variable like this:
var foo = function () { ... }
This means that you can pass this into functions like normal variables. When you pass the function in as a parameter, you are assigning the function to the name that you specify in the parameters:
var foo = function () { ... }
function hasCallback(callback) {
// The following two line do exactly the same thing:
callback(); // Using the function that you passed in
foo(); // Using the function directly
}
hasCallback(foo);
All I have done above is, instead of creating the variable foo I just created the function inline:
var foo = function () { ... }
function hasCallback(callback) {
// The following two line do exactly the same thing:
callback(); // Using the function that you passed in
foo(); // Using the function directly
}
hasCallback(foo);
// Becomes:
function hasCallback(callback) {
callback(); // Using the function that you passed in
}
hasCallback(function () { ... });
I'm dealing with a stubborn API, and I need to stop a function executing on the first call, and then execute normally afterwards.
Currently I'm doing it like this:
var counter = 0;
function notFirstTime() {
counter++;
if (counter > 2) {
return;
}
}
Is there a better way of doing this?
You can use function that "returns function" for such cases:
function skipFirstCall(fun) {
var first_called = true;
return function() {
if (!first_called) {
fun.apply(this, arguments);
}
first_called = false;
}
}
var myFunc = skipFirstCall(function(){
console.log("I was executed!");
});
myFunc();
myFunc(); // I was executed!
myFunc(); // I was executed!
myFunc(); // I was executed!
You can pass any function to skipFirstCall function as argument to skip its first call:
var myAnotherFunc = skipFirstCall(function(){
console.log("myAnotherFunc was executed!");
});
myAnotherFunc();
myAnotherFunc(); // myAnotherFunc was executed!
myAnotherFunc(); // myAnotherFunc was executed!
Yes! there's a simple and elegant solution - you can redefine the function on the first call, like so:
var foo = function() {
foo = function() {
console.log("i'm after the 1st call");
}
}
The first time you call foo(), it rewrites the foo variable and sets it to a new function.
Is it possible to pass a callback function that does not exist yet? My goal is to have a common function that will wait for another callback function to exist, when it does exist, it should execute it. This is what I have so far, but I can't figure out how to pass the function in that doesn't exist as a function yet.
function RunTemplateFunction(callback, userInfo) {
if ($.isFunction(callback)) {
callback(userInfo);
} else {
var myInterval = setInterval(function () {
if ($.isFunction(callback)) {
clearInterval(myInterval);
callback(userInfo);
}
}, 200);
}
}
I run the function like this:
RunTemplateFunction(MyFunctionToRun, GetUserInfo());
I get MyFunctionToRun is undefined for obvious reasons, I also tried the workaround of passing the function as a string and then convert the string to a function using eval(). But that throws the same error. I also thought of using the new function(), but that actually creates a new function.
Any help is appreciated. thank you.
If you call RunTemplateFunction by undefined there is no way we can see, is callback is defined or not, as we don't have reference to anything.
If you can modify the declaration to accept object as below, we can achieve what we want
function RunTemplateFunction(options, userInfo) {
if ($.isFunction(options.callback)) {
console.log('called1',userInfo);
options.callback(userInfo);
} else {
var myInterval = setInterval(function () {
if ($.isFunction(options.callback)) {
console.log('Called dynamically!!');
clearInterval(myInterval);
options.callback(userInfo);
}
}, 200);
}
}
var options = {}
RunTemplateFunction(options,{user:122});
options.callback = function(){
console.log("I'm called!!");
}
This will print
Called dynamically!!
I'm called!!
EDIT:
We can also call callback function in following way without setInterval, it will look different but options.callback variable is replaced by template.callMe function and its instantaneous also.
function TemplateRunner(userInfo){
this.callMe = function(cb){
this.templateFunction(cb);
}
this.templateFunction = function(callback){
callback(userInfo);
}
}
var template = new TemplateRunner({user:100})
template.callMe(function(user){
console.log('call me1',user);
});
template.callMe(function(user){
console.log('call me2',user);
})
This will print
call me1 {user: 100}
call me2 {user: 100}
I want to call a function that is in another function.
example for the functions:
function funcOne() {
function funcTwo() { // i want to call to this function
//do something
}
}
I need to call to funcTwo function, when I click on a button which is outside of these two functions
how can i do it?
No, You can't call unless you return that function.
Function2 is private to function1.
you use
function funcOne() {
return {
funcTwo :function() { // i want to call to this function
//do something
}
}
}
EDIT: Structuring code
function funcOne() {
var funcTwo = function() { // private function
//do something
}
return {
funcTwo : funcTwo
}
}
Now you can call it as:
funcOne().funcTwo()
As you have it defined in your example, you can't. funcTwo is scoped inside of funcOne, so it can only be called from inside funcOne. You can assign funcTwo to a variable that is scoped outside of funcOne and that would work:
var funcRef;
function funcOne() {
funcRef = function funcTwo() {
}
}
In this case, funcRef would hold a reference and could be used, but that reference is only set once funcOne has been executed.
Reading some Douglas Crockford may help you understand...
Try recoding as:
function funcOne() {
this.funcTwo = function() {
}
}
I think you'd have to declare an instance of a funcOne object and then call the funcTwo method of that object. I'm a bit busy at the moment so I can't refine this answer at the moment.
It is not possible as the second function will be created just when the first function is called. it is not existent prior to that.
You would have to define it outside the first function like so:
function funcOne() {
}
function funcTwo() { // i want to call to this function
//do something
}
Or you could also call the first function and return the second function like this:
function funcOne() {
function funcTwo() { // i want to call to this function
//do something
}
return functTwo;
}
And then call it like this:
var f = funcOne();
f();