javascript: best way of checking if object has a certain element or property?

Question

Suppose I have this:

var person = { "name": "John Doe", "email": "[email protected]" };

This object only has two elements called name and email. Some persons also have an element age, but this particular person doesn't. What's the best way to check this?

• if (person.age) { ... }
• if (person.age != undefined) { ... }
• if (person.age !== undefined) { ... }
• if (typeof(person.age) != 'undefined') { ... }
• if (person.hasOwnProperty('age')) { ... }

I know all these don't do the same, e.g. if (person.age) would also fail if age does exist but it's false or null or '' or 0. And I wonder if some aren't just flat out wrong.

Note that person is known to be an existing object here, but person.age may or may not exist.

Answer 1

Let's check the reliability of these ways of checking if object has a certain element or property:

This can fail if Boolean(person.age) is false

if (person.age) { ... }

This can fail if person.age is null or undefined

if (person.age != undefined) { ... }

These can fail if person.age is undefined

if (person.age !== undefined) { ... }
if (typeof(person.age) != 'undefined') { ... }

On the other hand, the hasOwnProperty() method returns a boolean indicating whether the object has the specified property as own (not inherited) property. So It does not depend on the value of person.age property. So it is the best way here

if (person.hasOwnProperty('age')) { ... }

If you want to go further and check if an object has a property on it that is iterable(all properties including own properties as well as the inherited ones) then using for..in loop will give you the desired result.