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Lesson 14 — Generics
Types as parameters
A generic function has a type parameter in angle brackets, usually called T. T is a placeholder that gets filled in at each call: call it with a number and T is number, with a string and T is string. You get one function that works for any type and still keeps exact types.
function echo<T>(value: T): T { return value; }
const n = echo(5); // n: number
const s = echo("hey"); // s: string
Why it matters: arrays, Promises and React's useState are all generic — this is how libraries stay type-safe.
Want it another way?
Task
Make wrap generic so it accepts any type, and return the value inside an array. Expect [ 5 ], then [ 'hi' ].
Run your code — when the output matches the task, you will see a green check. Then mark the lesson complete.
Test your knowledge
Test your knowledgeA quick 3-question quiz on this lesson — instant feedback.