How to determine if Native JavaScript Object has a Property/Method?
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.
scunliffe
Software Developer. Mainly Web & Mobile Applications. Also an avid AutoCAD user/developer. Twitter: http://twitter.com/scunliffe Blogs: Software Design Blog, Indie Game Dev Blog
Updated on July 27, 2022Comments
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scunliffe almost 2 years
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?