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



Question:

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?


Solution:1

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.


Solution:2

The proper way to check if a property exists:

if ('property' in objectVar)  


Solution:3

The .hasOwnProperty can be accessed on the Array's proptotype, if typeof is not idiomatic enough.

    if (Array.prototype.hasOwnProperty('push')) {      // Native array has push property  }    


Solution:4

And it does work fine in Firefox

That's only by coincidence! You can't generally expect a prototype's method to also exist on the constructor function.

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

This was nearly right except you forget new, a perennial JavaScript gotcha. new Array().push, or [].push for short, would correctly check an instance had the method you wanted.


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