MEPX
Chapter 3 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 3 of 16

Variables and scope

let, const, var and where each one lives.

Three keywords, two behaviours

let and const only exist inside the block they are declared in. var ignores blocks and belongs to the whole function, which is why it surprises people inside loops and if statements.

  • Default to const, use let when the value must change, and avoid var in new code.
  • const stops you reassigning the binding, not changing what it points at.

Hoisting

Declarations are processed before the code runs. A var is created early and holds undefined, while let and const exist but cannot be touched until the line that declares them, which throws a ReferenceError.