Strings.

Fifth class. Numbers are for math. Messages for humans are text: “500 EUR left”.

You can concatenate with +: "Income: " + income + " EUR". Clearer is a template literal with backticks `: `Income: ${income} EUR`. The expression inside ${...} is evaluated and inserted.

Mini example:

const n = 10; console.log(`Tip: ${n} EUR`);

Common mistake: regular quotes "..." with ${n} inside — then you print the literal ${n}, not the value. You need backticks.