Question

I’ve seen many different ways of checking for available properties or methods in javascript.

if(typeof window.somePropOrMethod​ !== "undefined"){ }

if(window.hasOwnProperty("somePropOrMethod")){ }

if("somePropOrMethod" in window){ }

if(!!window.somePropOrMethod) { }

Which one should I use, and why?
Is it all up to personal preference or are there subtle differences between them?

Answer 1

It depends on the situation, and how stringent you want your test to be.

1. Doesn’t check that the somePropOrMethod attribute is defined on the specific object; it only checks it’s in the inheritance chain. Normally this won’t matter; especially when checking on window. It does however, specifically check the “somePropOrMethod” attribute is not undefined

   function Foo() {
   
   }
   Foo.prototype.bar = 4;
   Foo.prototype.baz = undefined;
   
   var x = new Foo();
   
   typeof x.bar​ !== "undefined"; // true, but "x" doesn't actually have a bar attribute; it's in the inheritance chain
   typeof x.baz​ !== "undefined"; // false
   

2. Checks the whether the specific object holds the attribute and excludes the inheritance chain from it’s search; but it doesn’t check the somePropOrMethod isn’t undefined.

   window.foo = undefined;
   window.hasOwnProperty("foo"); // true; but it's undefined
   
   // any properties on window.__proto__ will return false
   

3. The same as the first, except it doesn’t check against undefined; it just checks the object has some member (which could be undefined).

4. This checks the object, and it’s inheritance chain (i.e. #1 and #3 territory), but is only checking for a truthy value;

   window.foo = false;
   window.bar = 0;
   window.baz = null;
   
   !!window.foo; // false
   !!window.bar; // false
   !!window.baz; // false
   

---

Of course, what you should be doing is if you want a method, you should check it’s a function;

if(typeof window.someMethod​ !== "function"){ } // specifically check for a function

and you should do the same for other attributes (string, number etc, or even use the instanceof keyword).