Intro to Variables in JavaScript

Variables allow computers to store and manipulate data. You can think of a variable as a simple and shorter name that we use to represent the data we want to refer to, kind of like the x and y in mathematics.

Computer variables, however, can store not only numbers, but different values at different times. JavaSript has seven different data types: undefined, null, boolean, string, symbol, number and object. Any of the above seven data types can be stored in a variable.

Declare JavaScript Variables

In JavaScript, we can create or declare a variable by using the magical keyword var in front of it:

1var myName;
2var myPhoneNumber;

The code above creates two variables: myName and myPhoneNumber.

Notice also how each statement, a list of instruction that we give to a computer to execute, ends with a semicolon.

To declare a variable, you can use numbers, letters, and $ or _, but not spaces or make a variable start with a number.

Storing Values with the Assignment Operator

Once we have declared a variable, we can store a value in it thanks to the assignment operator =.

1myPhoneNumber = 17;

Here, the Number value 17 is assigned to the variable myPhoneNumber. Always keep in mind that assignment goes from right to left.

Everything standing on the right of the = operator is resolved before the value is assigned to the variable on the left of the operator.

1myVar = 17;
2myNum = myVar;

The code above assigns the Number 17 to myVar, then resolves myVar to 17, and finally assigns the same value to myNum.

Initializing Variables with the Assignment Operator

Although you can declare a variable and subsequently assign a value to it, it is common to initialize a variable to an initial value at the moment when it is declared.

1var myNumber = 0;

The code above creates a new variable called myNumber and assigns it an initial value of 0.

Understanding Uninitialized Variables

When working with variables, it's important to keep in mind that, when JavaScript variables are declared, they have an initial value of undefined.

For this reason, if we try to perform any mathematical operation with an undefined variable, we will get as a result NaN ("Not a Number").

1var a = 16;
2var b = 'My name is ';
3
4a = a + 1;
5b = b + ' Enea!';
6
7console.log(a); // 17
8console.log(b); // "My name is Enea!"

Understanding Case Sensitivity in Variables

In JavaScript capitalization matters! In fact, all variables and function names are case sensitive. For instance, if we take into consideration the following three variable names: MYVAR, MyVar and myvar, they all represent three different variables.

Even if the three name above are all valid names for a variable in JavaScript, it's best practice to write variable names in camelCase. Thus, if we have to deal with a multi-word variable name, we write the first word in lowercase and the first letter of each subsequent word as capitalized.

Examples:

1var myPhone;
2var myPhoneNumber;
3var myPersonalPhoneNumber;