Variables

From W3Schools.

Declare with var, let, and const (or nothing)

  • The var keyword was used in all JavaScript code from 1995 to 2015.
  • The let and const keywords were added to JavaScript in 2015.
  • You cannot re-declare a variable declared with let or const.
  • You cannot re-assign a variable declared with const.
  • Variables defined with let must be declared before use.
  • Variables defined with let have Block Scope.
let person = "John Doe", carName = "Volvo", price = 200;
let carName;  // Will have undefined as value
  • $ is a valid character in a variable name. Programmers often use it as an alias for the main function in a JavaScript library.
  • "" is a valid character in a variable name. A convention among programmers is to prefix private/hidden variables with "".

Block Scope and variables

  • Before ES6 (2015), JavaScript had only Global Scope and Function Scope.
  • let and const provide Block Scope in JavaScript.

Variables declared inside a { } block cannot be accessed from outside the block: { let x = 2; } // x can NOT be used here //