TypeScript Foundations·Types that pay for themselves

Generics that stay readable

Write one typed function for many value types without losing safety.

Intermediate 15 min read Lesson 9 of 300 Challenge included

What you will be able to do

Use T for a value type and K extends keyof T for property keys.
Partial, Pick, Omit and Record are built-in transformations.
If a generic is used once and unconstrained, a concrete type is usually clearer.

In plain English

A generic is a type parameter. It lets a function preserve the relationship between an input and an output. Constraints narrow what the parameter can be, while utility types derive new shapes from existing ones.

collections.tsTypeScript

function first<T>(items: T[]): T | undefined {
  return items[0]
}

function byId<T extends { id: string }>(items: T[], id: string): T | undefined {
  return items.find((item) => item.id === id)
}

const score = first([88, 91, 74])
const lesson = byId([{ id: 'html', title: 'HTML' }], 'html')

Your turn

Build a typed picker

Medium

Write pick<T, K extends keyof T>(items: T[], key: K): T[K][] that extracts one property from each item.

TS · starter

function pick(items, key) {
  return []
}

Check your understanding before moving on.

Knowledge checkQuestion 1

What does K extends keyof T guarantee?

Select an answer to continue

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Next: A strict project you can trust

Lesson 9 of 300 · 0% of course