Variables: Labelled Boxes for Information
How computers store and remember things
The first idea in every programming language: variables. A variable is just a labelled box. You put something in the box, give the box a name, and then you can refer to that thing by the name instead of the value.
Variables are like labelled jars in a kitchen
You have jars on your kitchen shelf labelled "sugar", "salt", "flour". You don't need to know exactly how many grams of sugar is in the jar — you just say "use the sugar jar." The name (sugar) points to whatever is currently inside. If you refill the sugar jar, the name still works. In programming, a variable is the label. The contents can change.
The same concept in every language
The syntax changes — the idea doesn't. Here's storing someone's name across four languages:
Same variable, four languages
JavaScript: let name = "Sarah" Python: name = "Sarah" Java: String name = "Sarah"; SQL: DECLARE @name VARCHAR(50) = 'Sarah'
Notice what changes and what stays the same
In every case: (1) we choose a name for our box, (2) we put a value in it, (3) we can use the name later. The words "let", "String", "DECLARE" are just that language's way of saying "create a box." The concept is identical.
Types: what kind of thing is in the box
Not all boxes hold the same kind of thing. Programming languages distinguish between types of data:
- Text (String) — Letters, words, sentences. Always in quotes. "Hello", "Sarah", "42 High Street".
- Number (Integer, Float) — Numbers. 42, 3.14, -7. No quotes. Used for maths.
- Yes/No (Boolean) — Only two possible values: true or false. Is the user logged in? Is the form valid?
- Lists (Array) — Multiple things in one box, in order. ["red", "green", "blue"]. A shopping list.
- Objects — A box of boxes — a name paired with a set of properties. {name: "Sarah", age: 28, active: true}.
Different types in JavaScript
let name = "Sarah" // Text (String)
let age = 28 // Number (Integer)
let isLoggedIn = true // Boolean
let colours = ["red", "blue", "green"] // Array (list)
let user = { // Object (box of boxes)
name: "Sarah",
age: 28,
active: true
}Try this
Think of a person you know. What information would you store about them in a contact book? Write out 5 pieces of information (name, phone, age, city, active member: yes/no) and decide what TYPE each one is. That's how a database thinks about a person.