keyof/indexed access types (intro)
Use keyof to create unions of property names and indexed access types (T[K] / T["prop"]) to read property types safely.
keyof/indexed access types (intro) is a free TypeScript Academy lesson on CoddyKit. This is lesson 3 of 3. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the TypeScript Academy learning path, and your progress syncs across the web and the CoddyKit app. The TypeScript Academy course includes 3 lessons in total.
Intro
Goal: Build safer helpers using keyof (property-name unions) and indexed access (T[K]) to read property types.
keyof
keyof produces a union of property names of a type (e.g., "id" | "name" | "email").
interface User {
id: number;
name: string;
email?: string;
}
type UserKeys = keyof User; // "id" | "name" | "email"keyof typeof
Use keyof typeof to get keys from a value (object) for parameter typing.
const SETTINGS = {
theme: "dark",
lang: "en",
} as const;
type SettingKey = keyof typeof SETTINGS; // "theme" | "lang"Indexed access
Indexed access returns the type of a property: User["name"] is string.
interface User {
id: number;
name: string;
email?: string;
}
type NameType = User["name"]; // string
getProperty helper
Combine keyof with K extends keyof T to restrict keys and return the precise T[K] type.
function getProperty<T, K extends keyof T>(obj: T, key: K): T[K] {
return obj[key];
}
const u = { id: 1, name: "Ada" };
const v1 = getProperty(u, "name"); // string
const v2 = getProperty(u, "id"); // number
console.log(v1.toUpperCase(), v2.toFixed(0));Tips
Tips:
- Prefer keyof typeof for keys from objects.
- Use T[K] to reflect property types without duplicating models.
- Avoid stringly-typed keys; lean on unions for safety.
Indexed access check
Quick check: What does T[K] represent?
Recap
Recap: keyof gives key unions; keyof typeof derives keys from values; T[K] reads property types—perfect for safe helpers.
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Frequently Asked Questions
Is the “keyof/indexed access types (intro)” lesson free?
Yes — the full text of “keyof/indexed access types (intro)” is free to read here on the web. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the TypeScript Academy course, upgrade to CoddyKit PRO. The TypeScript Academy course includes 3 lessons in total.
What will I learn in “keyof/indexed access types (intro)”?
Use keyof to create unions of property names and indexed access types (T[K] / T["prop"]) to read property types safely. You practise TypeScript Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start TypeScript Academy?
No prior experience is required. TypeScript Academy on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 3 of 3.
How long does the “keyof/indexed access types (intro)” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this TypeScript Academy lesson?
Yes. Every TypeScript Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- Generic functions & constraints (extends, default params)
- Partial, Readonly, Record, Pick, Omit
- keyof/indexed access types (intro)