Pass a JavaScript function as parameter

Question

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

User · Answer

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

User · Answer

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       gt   hello world     look at this

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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  amp  amp   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

User · Answer

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

User · Answer

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       lt button className  img-submit  type  button   onClick    onSubmit  gt Upload Image lt  button gt    MADE A NEW FUNCTION upload and called passed onSubmit and event as arguments   lt button className  img-submit  type  button   onClick  this upload bind this event onSubmit   gt Upload Image lt  button gt   upload event fn       custom codes are done here   fn event

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

Here it s another approach       function a first second                return  second  first                      a  Hello  function e  alert e    world            gt  Hello world

User · Answer

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

[javascript] Pass a JavaScript function as parameter

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