MEPX
Chapter 4 of 16All chapters
  1. 1Introduction
  2. 2Syntax and statements
  3. 3Variables and scope
  4. 4Data types
  5. 5Comparison and coercion
  6. 6Strings
  7. 7Numbers and maths
  8. 8Arrays
  9. 9Objects
  10. 10Functions
  11. 11Conditions and loops
  12. 12Classes
  13. 13The DOM
  14. 14Events
  15. 15Promises and async
  16. 16Modules

Chapter 4 of 16

Data types

Primitives, objects and typeof.

The primitives

There are seven: string, number, boolean, null, undefined, symbol and bigint. Everything else, arrays and functions included, is an object.

  • typeof reports the type, with one famous exception: typeof null returns "object".
  • That is a bug from the first version of the language, kept because too much code depends on it.

undefined and null

undefined means a value was never set. null means someone set it to nothing on purpose. Keeping that distinction makes intent readable later.