Variables and Constants

Variables are name storage for the data used by our applications. When this data isn’t going to change we can mark the variable as a constant.

Table of Contents

  1. Variables
  2. Variable Names
  3. Constants
  4. Further Reading

Variables

Variables are defined in Javascript using the let keyword.

let warning = "There are too many goats in the kitchen!";

Javascript is a dynamically typed language so we do not specify the data type when defining a variable.

let number = 42;
let e = 2.71828;
let name = "Wally";

We can also separate the definition of a variable from its assignment.

let greeting; // The greeting variable is current undefined.
// Let's assign greeting a value:
greeting = "Greetings and salutations.";

Variable Names

Variable names in Javascript:

  • Must only contain letters, numbers, or the symbols $ and _.
  • Must start with a letter, $, or _.
  • Are case sensitive.
  • Should be written in camelCase if they comprise multiple words.

Constants

Sometimes it is handy to mark a variable as “never changing”. We do this in Javascript using the const keyword.

const PI = 3.1415926;

The const keyword marks a variable as immutable. If we attempt to change the value of a constant our code will throw an error.

PI = 3; // Uncaught TypeError: invalid assignment to const 'PI'

Further Reading