var let const - jellyfish-tom/TIL GitHub Wiki

Variables declared with:

• var:

  • have functional scope
  • get hoisted
  • after hoisting are immediately assigned undefined value
  • can be redeclared (another variable with same name can be declared and assigned within same scope)

• let:

  • have block scope
  • get hoisted
  • after hoisting are NOT assigned with value
  • CAN NOT be redeclared (another variable with same name within same scope CAN NOT be declared)
  • Temporal Dead Zone applies to them
  • redeclaring with let something that has already been declared with var will fail with: Uncaught SyntaxError: Identifier 'x' has already been declared. Same the other way around - redeclaring with var something that was already declared with let throws same error.

• const:

  • have block scope
  • get hoisted
  • after hoisting are NOT assigned with value
  • Temporal Dead Zone applies to them
  • can NOT be redeclared (another variable with same name within same scope can not be declared)
  • need to be immediately assigned with value (in declaration line)
  • can not be reassigned
  • doesnt make its value immutable (ex. you can change value of some key in object)