Question

It seems to me that there are four different ways I can determine whether a given object (e.g. foo) has a given property (e.g. bar) defined:

1. if (foo.hasOwnProperty(bar)) {
2. if ('bar' in foo) {
3. if (typeof foo.bar !== 'undefined') {
4. if (foo.bar === undefined) {

To determine if there is a property named “bar” in the object foo, are all three of those statements equivalent? Are there any sublte semantics I don’t know that makes any of these three statements different?

Answer 1

No they are totally different. Example:

foo = {bar: undefined};
Object.prototype.baz = undefined;
Object.prototype.bing = "hello";

Then:

(typeof foo.bar != "undefined")  === false
('bar' in foo)                   === true
(foo.hasOwnProperty('bar'))      === true


(typeof foo.baz != "undefined")  === false
('baz' in foo)                   === true
(foo.hasOwnProperty('baz'))      === false


(typeof foo.bing != "undefined") === true
('bing' in foo)                  === true
(foo.hasOwnProperty('bing'))     === false

Logic-wise:

• foo.hasOwnProperty('bar') implies 'bar' in foo
• typeof foo.bar != "undefined" implies 'bar' in foo
• But those are the only inferences you can draw; no other implications are universally true, as the above counterexamples show.