1.3 Type Checking

JavaScript only checks types in run-time. Dynamic type checking uses one of two operators:

typeof(value) – Returns a string with the primitive type of a value.

value instanceof Object – Checks if a value has a specified object type.

>>>

typeof(Object.notDefined)

"undefined"

>>>

typeof(null)

"object"

This is a common pitfall. Take note.

>>>

typeof(true)

"boolean"

>>>

typeof(42)

"number"

>>>

typeof("")

"string"

>>>

typeof({ key: 'value' })

"object"

>>>

typeof([1, 2, 3])

"object"

The typeof operator returns "object" for all object types.

>>>

[1, 2, 3] instanceof Array

true

>>>

[1, 2, 3] instanceof Object

true

All object type values are naturally also instances of Object

>>>

42 instanceof Number

false

Primitive types, like number, are not the same as their wrapper object types (e.g. Number).

>>>

typeof(function () {})

"function"

Functions are really object types, but the typeof operator isn't quite consistent with the type model...

>>>

(function () {}) instanceof Object

true

>>>

Need more magic? Check out the other developer tools on this site.

Console Learner — An interactive JavaScript course.