Javascript Extend a Function

Question

The main reason why I want it is that I want to extend my initialize function   Something like this      main js  window onload   init    function init         doSomething          extend js  function extends init        doSomethingHereToo        So I want to extend a function like I extend a class in PHP   And I would like to extend it from other files too  so for example I have the original init function in main js and the extended function in extended js

User · Answer

Another option could be   var initial   function         console log   initial function         var iWantToExecuteThisOneToo   function          console log   the other function that i wanted to execute         function extendFunction  oldOne  newOne         return  function             oldOne            newOne                 var extendedFunction   extendFunction  initial  iWantToExecuteThisOneToo

User · Answer

The other methods are great but they don t preserve any prototype functions attached to init  To get around that you can do the following  inspired by the post from Nick Craver     function          var old prototype   init prototype      var old init   init      init   function              old init apply this  arguments              Do something extra            init prototype   old prototype

User · Answer

This is very simple and straight forward  Look at the code  Try to grasp the basic concept behind javascript extension   First let us extend javascript function   function Base props        const  props   props     this getProps        gt   props         We can make method private by not binding it to this object          Hence it is not exposed when we return this      const privateMethod        gt   do internal stuff        return this     You can extend this function by creating child function in following way  function Child props        const parent   Base props      this getMessage        gt   Message is   parent getProps              You can remove the line below to extend as in private inheritance          not exposing parent function properties and method      this prototype   parent     return this     Now you can use Child function as follows   let childObject   Child  Secret Message   console log childObject getMessage           logs  Message is Secret Message  console log childObject getProps             logs  Secret Message    We can also create Javascript Function by extending Javascript classes  like this   class BaseClass       constructor props            this props   props            You can remove the line below to make getProps method private              As it will not be binded to this  but let it be         this getProps   this getProps bind this             getProps             return this props           Let us extend this class with Child function like this   function Child props        let parent   new BaseClass props      const getMessage        gt   Message is   parent getProps          return      parent  getMessage     I have used spread operator       Again you can use Child function as follows to get similar result   let childObject   Child  Secret Message   console log childObject getMessage           logs  Message is Secret Message  console log childObject getProps             logs  Secret Message    Javascript is very easy language  We can do almost anything  Happy JavaScripting    Hope I was able to give you an idea to use in your case

User · Answer

With a wider view of what you re actually trying to do and the context in which you re doing it  I m sure we could give you a better answer than the literal answer to your question  But here s a literal answer  If you re assigning these functions to some property somewhere  you can wrap the original function and put your replacement on the property instead     Original code in main js var theProperty   init   function init         doSomething          Extending it by replacing and wrapping  in extended js theProperty    function old        function extendsInit             old            doSomething               return extendsInit     theProperty    If your functions aren t already on an object  you d probably want to put them there to facilitate the above  For instance     In main js var MyLibrary         init  function init                  In extended js  function         var oldInit   MyLibrary init      MyLibrary init   extendedInit      function extendedInit             oldInit call MyLibrary      Use  call in case  init  uses  this          doSomething                 But there are better ways to do that  Like for instance  providing a means of registering init functions     In main js var MyLibrary    function         var initFunctions           return           init  function init                 var fns   initFunctions              initFunctions   undefined              for  var index   0  index  lt  fns length    index                    try   fns index       catch  e                                       addInitFunction  function addInitFunction fn                if  initFunctions                       Init hasn t run yet  remember it                 initFunctions push fn                 else                       init  has already run  call it almost immediately                    but  asynchronously   so the caller never sees the                    call synchronously                  setTimeout fn  0                                         Here in 2020  or really any time after  2016   that can be written a bit more compactly     In main js const MyLibrary         gt        let initFunctions           return           init                 const fns   initFunctions              initFunctions   undefined              for  const fn of fns                    try   fn      catch  e                                       addInitFunction fn                if  initFunctions                       Init hasn t run yet  remember it                 initFunctions push fn                 else                       init  has already run  call it almost immediately                    but  asynchronously   so the caller never sees the                    call synchronously                  setTimeout fn  0                      Or   Promise resolve   then      gt  fn                          Not   then fn   just to avoid passing it an argument

User · Answer

2017  solution The idea of function extensions comes from functional paradigm  which is natively supported since ES6  function init        doSomething          extend js  init    f   gt  u   gt    f u      doSomethingHereToo       init    init     As per  TJCrowder s concern about stack dump  the browsers handle the situation much better today  If you save this code into test html and run it  you get test html 3 Uncaught ReferenceError  doSomething is not defined     at init  test html 3      at test html 8     at test html 12  Line 12  the init call  Line 8  the init extension  Line 3  the undefined doSomething   call  Note  Much respect to veteran T J  Crowder  who kindly answered my question many years ago  when I was a newbie  After the years  I still remember the respectfull attitude and I try to follow the good example

User · Answer

There are several ways to go about this  it depends what your purpose is  if you just want to execute the function as well and in the same context  you can use  apply     function init      doSomething      function myFunc      init apply this  arguments     doSomethingHereToo        If you want to replace it with a newer init  it d look like this   function init      doSomething        anytime later var old init   init  init   function       old init apply this  arguments     doSomethingHereToo

User · Answer

Use extendFunction js  init   extendFunction init  function args      doSomethingHereToo          But in your specific case  it s easier to extend the global onload function   extendFunction  onload   function args      doSomethingHereToo          I actually really like your question  it s making me think about different use cases   For javascript events  you really want to add and remove handlers - but for extendFunction  how could you later remove functionality  I could easily add a  revert method to extended functions  so init   init revert   would return the original function  Obviously this could lead to some pretty bad code  but perhaps it lets you get something done without touching a foreign part of the codebase

[javascript] Javascript: Extend a Function

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