How can I test whether a Javascript object has a method with a given name?

Question

Consider the following sample:

var Container = function(param) {
    this.member = param;
    var privateVar = param;
    if (!Container.prototype.stamp) {  // <-- executed on the first call only
        Container.prototype.stamp = function(string) {
            return privateVar + this.member + string;
        }
    }
}

var cnt = new Container();

Is there any way to determine whether the object cnt has a method named stamp without knowing that it is instantiated from Container ?

Another Example

I think Visual Studio Watcher is broken :) Because it worked on Firebug. — uzay95
– uzay95, Commented Oct 14, 2010 at 13:18

Tomas · Accepted Answer · 2010-10-14 11:34:32Z

2

You can test for the existence of stamp with:

if (cnt.stamp) ...

or you can check whether it is a function with

if (typeof cnt.stamp === 'function') ...

answered Oct 14, 2010 at 11:34

Tomas

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2 Comments

Tomas Over a year ago

it does work if you test it seperately in a javascript console: var a = { b: function() {} }; typeof a.b . Maybe f_SetEk is not a function? It sais it's an undefined identifier in your image...

uzay95 Over a year ago

No it is function but in visual studio Watch window it doesn't says it is function. But you are right it is working well in browser.

AutomatedTester · Accepted Answer · 2010-10-14 11:33:53Z

2

You can use hasOwnProperty

o = new Object();  
o.prop = 'exists';  
o.hasOwnProperty('prop');             // returns true  
o.hasOwnProperty('toString');         // returns false  
o.hasOwnProperty('hasOwnProperty');   // returns false

answered Oct 14, 2010 at 11:33

AutomatedTester

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1 Comment

uzay95 Over a year ago

I saw hasOwnProperty in some examples that shows Array.hasOwnProperty('push') . But in my example it doesn't work!

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