javascript - Add self executing init method to constructor function? -

let’s have constructor function not have access to. constructor function want inject self executing init method gets executed whenever new instance gets created constructor.

for example: let’s there cat constructor, unfortunatly not have access it:

function cat() {     // ...code not have access     // ...maybe comes external file or something? } 

and can of course create new cats:

var coolcat = new cat(); 

all , have new cat instance.

but want (if actaully had access cat constructor function body, of course not!) this:

function cat() {   this.roaroninit = function() {     alert(’roooaaar!’);   };   this.roaroninit(); } 

…so when this:

var coolcat = new cat(); 

…i cool roar-alert box!

i understand how add method roaroninit cat constructor (cat.prototype.roaroninit = function …) there way can add call method (which gets executed whenever cat instance created) constructor body?

this seems such trivial thing, , it’s super easy, can’t seem figure out afternoon! bearing me.

update

thanks answers far! did forget 1 important thing, , not know in advance constructor function be, or it's name etc. because i'm running through function accepts constructor parameter, , returns constructor (with it's original name/prototype) back.

let's start definition of cat:

function cat(name){ this.name=name; } cat.prototype.meow=function(){alert(this.name)} 

now, can overwrite new constructor returns regular cat, after running our script:

var oldcat = cat; function cat(name){     var self=new oldcat(name);     self.roaroninit=function(){alert("rooooaaarrr")};     self.roaroninit();     return self; } 

we can new cat("muffin"), , roar, , we'll still have access properties on original cat prototype chain. show in example snippet:

// safe, define original oldcat()    function oldcat(name){    this.name=name;  }  oldcat.prototype.meow=function(){alert(this.name)}      //var oldcat = cat;  function cat(name){    var self=new oldcat(name);    self.roaroninit=function(){alert("rooooaaarrr")};    self.roaroninit();    return self;  }    var coolcat = new cat("muffin");  coolcat.meow();

now, if want abstract accept function, it's not hard. have bit of work constructor, pass arguments. javascript - create instance array of arguments

function injecttoconstructor(c){     return function(){         var self = new (c.bind.apply(c,[c].concat([].slice.call(arguments))))();         console.log("object initiated:");         console.log(self);         return self;     }; } 

then can like:

cat = injecttoconstructor(cat); var coolcat = new cat("muffin"); // logs 2 items coolcat.meow();