Question

162

How to determine if Native JavaScript Object has a Property/Method?

rated 0 times [ 169] [ 7] / answers: 1 / hits: 49162 / 15 Years ago, fri, february 27, 2009, 12:00:00

I thought this would be as easy as:

if(typeof(Array.push) == 'undefined'){

  //not defined, prototype a version of the push method

  // Firefox never gets here, but IE/Safari/Chrome/etc. do, even though

  // the Array object has a push method!

}

And it does work fine in Firefox, but not in IE, Chrome, Safari, Opera, they return all properties/methods of the native Array object as 'undefined' using this test.

The .hasOwnProperty( prop ) method only works on instances... so it doesn't work, but by trial and error I noticed that this works.

//this works in Firefox/IE(6,7,8)/Chrome/Safari/Opera

if(typeof(Array().push) == 'undefined'){

  //not defined, prototype a version of the push method

}

Is there anything wrong with using this syntax to determine if a property/method exists on a Native Object / ~JavaScript Class~, or is there a better way to do this?

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brianaclaras

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4 Years ago

answered 15 Years ago gavenmekhio · Accepted Answer

First of all, typeof is an operator, not a function, so you don't need the parentheses. Secondly, access the object's prototype.

alert( typeof Array.prototype.push );

alert( typeof Array.prototype.foo );

When you execute typeof Array.push you are testing if the Array object itself has a push method, not if instances of Array have a push method.