Why bother with variables

01 / 06

What is painful about typing values in

Type the same value in over and over and, when you want to change it, you have to hunt down every one of them. Miss one and you have a bug.

With a variable there is only ever one place to edit.

let price = 300;console.log(price * 2);console.log(price * 5);

Result

600
1500

02 / 06

The name does the explaining

console.log(300 * 8) tells you nothing about what is being worked out. price * count tells you just by being read.

You spend longer reading code than writing it. A name is a note to your future self.

let price = 300;let count = 8;console.log(price * count);

Result

03 / 06

A worked-out result can have a name too

It is not only values you typed that go into a variable. The result of a sum goes in just the same.

Name the halfway results and you can break a long sum into pieces.

let price = 300;let count = 8;let total = price * count;console.log(total);

Result

04 / 06

Build variables out of variables

You can use what is in one variable to make another.

Breaking it small and stacking it up like this is how readable code gets written.

let price = 300;let count = 8;let total = price * count;let tax = total / 10;console.log(total + tax);

Result

05 / 06

One place to edit

A variable lets you keep the initial value in one place in the source.

Change let price = 300; below to let price = 400; and run the program again. The total becomes 3200 and the tax becomes 320. Running again recalculates each line in order.

let price = 300;let count = 8;let total = price * count;console.log(total);console.log(total / 10);

Result

2400
240

06 / 06

How to name things

A name should say what is inside.

✗ a, x, data … no idea what is in there
○ price, userName, total … you read it and you know

Clear beats short. A longer name is fine, as long as you in three days can read it.

let a = 300;let price = 300;console.log(a * 8);console.log(price * 8);

Result

2400
2400