Variables and Operators

If the embed above does not work here is a link to the full version of the video

Declaring variables

There are three different keywords that allow us to declare variables in JavaScript:

var

var allows us to declare a variable. This variable’s value and type can be changed after assignment. The scope of the variable is not restricted to the local block

var a = 6;
var myName = "martin";

let

let allows us to declare a variable. This variable’s value and type can be changed after assignment. The scope of the variable is limited to the block in which it is declared

let a = 6;
let myName = "martin";

const

const allows us to declare a constant variable. The value of this variable cannot be changed once assigned (this is sometimes referred to as being immutable)

const PI = 3.14;
const myName = "Martin";

Naming variables

Variable names must start with a letter, an underscore (_) or a dollar sign ($). We normally useCamelCase for naming variables in JavaScript (as opposed to Python where snake_case_is_more_common). We cannot use reserved words for variable names.

Scope

The scope of a variable refers to the regions of code in which a variable is valid - the places in the code where the variable exists and can be read from or assigned to. If a variable is out of scope then it cannot be used; if it is in scope then we can refer to it by name and access the value it contains.

In this example, myName is out of scope when it is used by console.log, as it is declared using let so is only valid inside the block of code where it is declared.

if (true) {
    let myName = "Martin";
}

// will throw a ReferenceError because myName is out of scope
console.log(myName);

In this example, myName is in scope when it is used by console.log, as it is declared using var so it is valid both inside and outside the block of code where it is declared.

if (true) {
    var myName = "Martin";
}

// will throw a ReferenceError because myName is out of scope
console.log(myName);

Data types in JavaScript

Typing

JavaScript is dynamically typed and weakly typed. We do not need to tell JS what type a variable will be when we declare it, and we can change the type of data stored in a variable once it has been declared. JavaScript will convert types as it needs to during script execution.

Operators

Equality

JavaScript has two ways of comparing equality, == and ===.

== and != only consider value

=== and !== consider both value and type

Code examples

There are a set of code examples that accompany this course showing the use of JavaScript. (Right click and open in a new window/tab) if you’re viewing this on Learning Central.

Next: Branching and Boolean Expressions