How can we check if something is undefined?

We can compare it against the variable itself:

var something
something === undefined // true

Fun fact, before ES5, undefined was overwritable so you would use typeof:

var something
typeof something === 'undefined' // true

How about null?

We can do something similar:

var something = null
something === null // true

However, typeof won’t work here:

typeof null // 'object'

String checks are consistent with typeof:

var something = ''
typeof something // 'string'

And we could use it to check if it’s a number as well:

var something = 42
typeof something // 'number'

But there’s a catch:

var something = parseInt('foo') // NaN
typeof something // 'number'

So to check if something’s truly a number:

typeof something && !isNaN(something)

Because type casting happens even during comparison:

1 > '0' // true
1 > '1' // false

Function can be checked with typeof:

var something = function() {}
typeof something // 'function'

So what about an array?

Well there’s always the length property:

var something = []
something.length >= 0 // true

But it will break for undefined and null:

var something
something.length // TypeError

something = null
something.length // TypeError

So can we use typeof to check if a variable is an array?

var something = []
typeof something // object

It seems like it’s not consistent as well.

The best way to check if something is an array:

var something = []
something instanceof Array // true

Alternatively you can use the constructor property as well but you need to make sure it’s not undefined or null:

(
  something && // make sure it's not `undefined` or `null`
  typeof something === 'object' && // make sure it's not a string or number
  something.constructor === Array
)

Now what about an object?

So you might be thinking about using instanceof here:

var something = {}
something instanceof Object // true

However this can be inconsistent as well:

var something = []
something instanceof Object // true

The best bet is to use constructor (be sure to make similar checks like array above):

var something = {}
something.constructor === Object