Identifying Primitive and Reference Types in Javascript

5 Primitive Types: Number, Boolean, String, Null, Undefined

typeof “abc” => string
typeof “true” => boolean
typeof will work for mostly all of above except null.
typeof null will return “object”.
So to verify “null” just compare to null. (value == null)

6 Reference Types: Function, Object, Array, Date, Error, RegExp

typeof object will be “object”
typeof function will be “function”

for all the rest use instanceof to identity

items instanceof Array

Note that items instanceof Object will also return true since instanceof can identity inherited types.

For Array, if you are using frames and communicating values between frames then instanceof may not provide accurate results if you are checking instanceof. Its recommended to use Array.isArray(items) in that case.