method pointer/ref in javascript

Question

Here is an example of my current code :

var myObject = new Obj();

if(something)
    myObject.method1(arg1, arg2);
else
    myObject.method2(arg1, arg2);

and how I declared my obj :

function Obj() { }

Obj.prototype.method1 = function(a, b) { }
Obj.prototype.method2 = function(a, b) { }

Since i'm doing this kind of test a bunch of time, I was wondering if it was possible to do something like that :

if(something)
    var method = method1;
else
    var method = method2;

myObject.method(arg1, arg2);

T.J. Crowder · Accepted Answer · 2013-10-26 16:27:10Z

2

Yes, functions are first-class objects in JavaScript, so you can store function references in variables and then call the function via the variable. In your case, you need to do that in a special way to ensure that this is myObject within the call:

var method;
if(something)
    method = myObject.method1;
else
    method = myObject.method2;

method.call(myObject, arg1, arg2);

Note the use of call there at the end: That allows you to call a function and control what this is within the function call.

If the condition really is that short, you can do this:

var method = something ? myObject.method1 : myObject.method2;
method.call(myObject, arg1, arg2);

Or even:

var method = something ? "method1" : "method2";
myObject[method](arg1, arg2);

More to explore:

Function#call (in the spec | on MDN)
Function#apply (in the spec | on MDN)
You must remember this (on my blog)
Mythical methods (on my blog)

answered Oct 26, 2013 at 16:27

T.J. Crowder

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2 Comments

Simon-Okp Over a year ago

just for curiosity, why did you write method = myObject.method1, it seems a bit redundant since myObject is already mentioned in call(). Wouldn't method = Obj.prototype.method1 make more sense ? But thanks, that's exactly what I wanted ^^

T.J. Crowder Over a year ago

@user1278743: I seem to have missed that question. We may not know that myObject doesn't have a special method1. If it doesn't, then myObject.method1 and Obj.prototype.method1 refer to the same function and the latter is just more typing, but if it does, we'd get the wrong method. :-) It also might not be clear to people reading the code that myObject is backed by Obj.prototype so clarity would suffer.

Rahul Tripathi · Accepted Answer · 2013-10-26 16:27:30Z

1

You use a function reference in a variable like this:-

var method;
if(something)
    var method = myObject.method1;
else
    var method = myObject.method2;

method.call(myObject, arg1, arg2);

answered Oct 26, 2013 at 16:27

Rahul Tripathi

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Comments

Adassko · Accepted Answer · 2013-10-26 16:56:59Z

0

you can just do:

var method = myObject.method1;

then call it like:

method.call(myObject);

but you can also bind method to an object:

var method = myObject.method1.bind(myObject);

then you can call it normally:

method();

and this inside will be myObject

you can replace bind with simple:

var method = function() { myObject.method1(); }

if you want to support older browsers

answered Oct 26, 2013 at 16:56

Adassko

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