python's hasattr in javascript?

Question

in javascript:

d={one: false, two: true}
d.one
d.two
d.three

I want to be able to differentiate between d.one and d.three. By default they both evaluate to false, but in my case they should not be treated the same.

Answer 1

You can do

"one" in d // or "two", etc

or

d.hasOwnProperty("one")

You probably want hasOwnProperty as the in operator will also return true if the property is on the an object in the prototype chain. eg.

"toString" in d // -> true

d.hasOwnProperty("toString") // -> false

Answer 2

The values aren't strictly false:

js> d={one: false, two: true}
[object Object]
js> d.one == false
true
js> d.three == false
false
js> d.three === false
false    
js> d.three === undefined
true
js> 'three' in d
false
js> 'one' in d
true

Also, see comments by olliej and Ken below.

Answer 3

Well, d.one is false and d.three is undefined.

var d={one: false, two: true};
alert("one: " + d.one + "\nthree: " + d.three);
  // Output:
  // one: false
  // three: undefined

Try it out with this jsFiddle

Javascript does have some funky true false evaluation at times, but this isn't one of those situations:

alert(d.three == false);                                          // alerts false

---

To check for undefined you can use typeof

if (typeof something  == "undefined") 

Or you can check if three is a property of d

if (d.hasOwnProperty("three"));