chapter-03.md

Variable

A variable is like a box or container that can hold any kind of value you put into it, but one at a time. Every programming language had its own way of creating variables. For JavaScript, it is simply using the let keyword.

The best way to learn about variables is by doing it, so let's create chapter-03.js file with the following content:

let myVariable = 10;
console.log(myVariable);

Now run the script. What do you see? The variable myVariable is printed out! But why does it print 10? That's because we have assigned that number into our variable! The equal sign = is known as assignment operator, and it is used to assign value to variables.

Let's try another one.

let newVariable = 'Hello, World!';
console.log(newVariable);

The Console will now print out Hello, World! when you run the file.

You can reassign a variable by typing out the variable name followed by another assignment operator. Here is an example of reassigning a variable:

let myVariable = 10;
myVariable = 'Hello, World!';
console.log(myVariable);

When you reassign a variable, the old value is discarded and replaced by the new value.

So try making some variables now. you can even do it the old fashioned math way like let x = 10 and let y = 5 and then do arithmetic operation on it:

let x = 10;
let y = 5;
console.log(x + y);
console.log(x / y);

A variable is used to hold value, and that variable can be used later in your code when you need it. If a variable has more than one word, you can follow the convention and use the camelCase style of writing. You've used it before:

let firstName = 'Nathan';

Constant variable

Sometimes, you might need to declare a variable that doesn't need to change its value for the entire execution of the program. A constant variable is a variable in which the value never changes after it has been declared. To create a constant variable in JavaScript, you can use the const keyword:

const COUNTRY = 'United States';

To further differentiate between constant and regular variable, a constant is usually written in UPPERCASE format rather than the camelCase format. If the constant name has more than one word, use underscore to join them together:

const MY_COUNTRY = 'United States';

Why do we need constant? Good question! It's because sometimes in code, you need arbitrary value that never changes. An example of this is the Pi number in math, which is used to count the area of a circle:

const PI = 3.14;

But then again, you might wonder why not just use let? That's because a good programming language doesn't want you to accidentally change the constant down the road and causing errors.

The var keyword

In older versions of JavaScript code base, you might encounter the keyword var used instead of let.

var firstName = 'Nathan';

The var keyword is used in the same manner as let, which is to create mutable variables. So why JavaScript create the new let keyword to replace var? You can find out more about it at the var keyword extra chapter.

Code exercise

Write a program with three variables, each with the following value:

1. The first variable contain your name
2. The second variable contain your age
3. The third variable contain your occupation

Then print the variables by using three console.log() statements. Example output:

Nathan Sebhastian
27
Software Developer