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          


        

Answer 1

The proper way to check if a property exists:

if ('property' in objectVar)

Answer 2

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.

Answer 3

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.

Answer 4

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
}