15 min · up to 70 XP

Variables and data

How programs remember things.

Two lessons ago, Sam gave her reading-list button a brain with one strange line: books = books + 1. It worked. She still doesn't entirely know why.

Tonight her AI tutor names the magic: that line uses a variable, the thing that lets a program remember. Before any definitions, a puzzle. Sam runs three lines: let coffees = 2; then coffees = coffees + 1; then console.log(coffees);

Check your intuition

No theory yet. Three lines: let coffees = 2; → coffees = coffees + 1; → console.log(coffees);, what appears on the screen?

That's the whole idea: a variable is a labelled box the program uses to remember. let total = 2; builds the box, sticks on the label, puts 2 inside. Later, total = 3; swaps the contents, same box, new value.

JavaScript has a second keyword, const, for values that must never change. Sam's weekend project is an order screen for The Beanline, her local café. The editor below is live, go and build her some memory.

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