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Variables

The declaration of a variable is possible by using one of the keywords:

  • const
  • let
  • var

The next step is to enter the variable name. To assign a value to a variable, we use the equal sign =. In case of using const, we need to make this assignment during the declaration. However, using the keywords let orvar, we can do it later.

The variables in JS are dynamically typed. This means that during the declaration we do not specify the variable type and we can assign an object of any type, e.g.: 

var variableExample = 10;

In the example above, the variable named variableExample is of the numeric type, but there is nothing to prevent you from assigning a new value of a different type to it, e.g.:

variableExample = 'And now I am a string woohoo! Can't do this in Java';

Now the variable variableExample is of typestring. In JavaScript, you can assign any type of value ** to a variable at any time.

const

We use the const keyword to denote variables that are constants. We cannot declare another constant with the same name, but the most important feature of such a "variable" is the fact that we cannot assign a different value to a constant once assigned. In turn, it is possible to change the content of the object to which such a reference points, for example:

const iAmConstantObject = {
  foo: '1',
  bar: '2'
}

iAmConstantObject.bar = '3';

//iAmConstantObject = 'cannot do it';

let and var

The keywords let andvar denote variables to which we can assign new values after declaring the variable. There are, however, important differences between var andlet:

  • you cannot re-declare a variable let with the same name,var allows it
var foo = 10;
let bar = 20;

var foo = 15;
let bar = 30; // here we will have an error: Uncaught SyntaxError: Identifier 'bar' has already been declared

Moreover:

  • let offers [scope] (scope_variable_range.md) block, whilevar scope is functional
  • let is not hoisted.

NOTE: Given the choice of var andlet, you'd better declare variables with let.

Variable naming

JS, like other languages, has certain variable naming conventions. They are similar to those in Java, i.e .:

  • the variable name should start with a lowercase letter
  • subsequent words are written together, starting each next we use a capital letter - this is the so-called camelCase notation
  • variable name cannot start with a digit (0-9)
  • variable name cannot contain spaces
  • variable name cannot contain Polish letters
  • variable name cannot be a keyword reserved by JavaScript.

Also keep in mind that in JS there is a distinction between uppercase and lowercase letters.

var foo; 
var Foo; // these are two different variables