Variables

let

In JavaScript the word let declares a variable. A variable is a reference to another piece of code. So, instead of typing out 'A teacher and web developer based in the SF Bay Area' you can use a reference like jeremyBio.

let jeremyBio = 'A teacher and web developer based in the SF Bay Area';
console.log(jeremyBio);

Reserved Words

There are special words in JavaScript. These reserved also known as keywords tell are program to do something specific. For instance, one reserved words is typeof. Here is a list of other keywords.

Naming Variables

Naming Rules

Here are the rules for naming variables:

1. Reserved words cannot be used as variable names.
2. Variable names must begin with a letter, an underscore (_) or a dollar sign ($).
3. Variable names can only contain letters, numbers, underscores, or dollar signs.

Naming Conventions

Camel Casing

A convention when naming variables is to use lower camel casing.

Camel casing is when each word is capitalized. For upper camel case the first word starts with a capital letter and for lower camel case the first word is lowercase. Typically when programmers just say camel case they are referring to lower camel case.

ThisIsWhatUpperCamelCaseLooksLike
thisIsWhatLowerCamelCaseLooksLike

Good Variable Names

There are certain conventions for variables. Math variables for instance are often named x, y, z, a, or b. Variables that are repeated are often named i or j, which we will get to in the iteration section. Commonly variable names are a balance between being short and descriptive.

// Using math variables
let x = 3;
let y = 2;
let resultOfAdd = x + y;
// Descriptive but short
let favFruit = 'Mango';

Declaring Variables

To create a variable we use the let keyword followed by a space then the variable name. This is also known as declaring a variable.

let favPizza; // Declaring the variable favPizza

Assigning Variables

To tell the variable the piece of data it is referencing we use the equal sign.

let favPizza; // Declare the variable
favPizza = 'pepperoni'; // Assign the variable
// We can also declare and assign the variable on the same line
let favGuitarist = 'Jimi Hendrix'; // Declare and assign the variable

Reassigning Variables

The reason variables are called variables is because you can change what the variable is referencing.

let favPizza = 'pepperoni'; // declare and assign the variable
console.log(favPizza); // prints 'pepperoni'
favPizza = 'mushroom'; // resassign the variable
console.log(favPizza); // prints 'mushroom'

Data Types

We will be talking about three primitive data types: strings, numbers, and booleans.

Strings

Strings are typically words or phrases although a single character or several random characters put together also are be considered a string.

Strings start and end with a quote. The quote can be a single quote or a double quote, but the first quote and last quote must match.

'This is a proper string';
"This is also a string";
"This will not run because the quotes are not matching';

Numbers

Numbers are a second data type. These numeric values include numbers with and without decimals. Note that numbers wrapped in quotes would be considered part of the string data Types.

8; // a number
8.342 // another number

Boolean

There are two boolean values - true and false.

This data type is useful for creating logical programs. Imagine you are checking to see if an email is valid. If there is an @ sign you may want to pass the user onto the next screen, but if there is not you may want to have the user check their email address.

true; // one of two boolean values
false; // the second boolean value

typeof

The typeof operator tells you the data type. The syntax is either typing typeof then a space then your code you want check or you can wrap the code in parenthesis.

typeof true; // boolean
typeof 9; // number
typeof '9'; // string
typeof 'true'; // string
typeof 'Hellllo'; // string
typeof(true); // boolean