الدرس 11 · Advanced and in practice

Utility types

لم يُترجم بعد؛ يُعرض باللغة الإنجليزية

A utility type is a generic type built into TypeScript that makes a new type from one you already have. Follow one User interface through Partial, Readonly, Pick and Omit, watch tsc reject a key User does not have, and open up Partial to find the mapped type inside.

11 دقيقة9 خطوات5 أسئلةBeginner

شكل 11 · تفاعلي

جارٍ تحميل الرسم…

أو ← →

اختبر نفسك

5 أسئلة سريعة

اختر إجابة لتعرف السبب. تُحفَظ نتيجتك مع تقدّمك.

01What is Partial<User>?

Partial marks every field optional with ?, so Partial<User> is { id?: number; name?: string; email?: string }.

02Which type is { name: string; email: string }, given interface User { id: number; name: string; email: string }?

Omit keeps every field except the keys you name, so Omit<User, "id"> drops id. Pick<User, "name" | "email"> gives the same type.

03Why does Pick<User, "age"> fail?

Pick is declared as Pick<T, K extends keyof T>. keyof User is "id" | "name" | "email", so tsc reports error TS2344: Type '"age"' does not satisfy the constraint 'keyof User'.

04What happens when code assigns locked.name = "Bo" and locked has type Readonly<User>?

Readonly marks every field readonly, so tsc reports error TS2540: Cannot assign to 'name' because it is a read-only property. It is a compile-time check only.

05Which mapped type is the same as Partial<User>?

TypeScript defines Partial<T> as { [P in keyof T]?: T[P] }: P visits each key of T, and the ? makes that field optional.

المصطلحات الأساسية

المسرد

Utility type: A generic type that ships with TypeScript, such as Partial or Pick, that builds a new type from an existing one.
Generic type: A type that takes other types as arguments in angle brackets, like Partial<User>.
Optional property: A field marked with ? that a value may leave out, as in id?: number.
readonly: A modifier that lets code read a field but makes assigning to it a compile error.
keyof: An operator that gives the union of a type's key names: keyof User is "id" | "name" | "email".
Union type: A type that is one of several, written with |, such as "id" | "name".
Mapped type: A type built by going over keys, { [P in keyof T]: ... }, to make one field per key.
Constraint: A rule on a type argument, written with extends: in Pick<T, K extends keyof T>, K must be keys of T.

تفاصيل إضافية

الملاحظات والمصادر

Partial, Readonly, Pick and Omit are declared in lib.es5.d.ts, the type library tsc loads by default. They exist only while checking: like every type, they are erased and add nothing to the JavaScript.
Omit does not check its keys: it is declared as Omit<T, K extends keyof any>, so Omit<User, "age"> is allowed and simply removes nothing. Pick requires K extends keyof T, which is why the typo is caught.
readonly is checked only by tsc. At run time the object can still be changed by code that is not type-checked; Object.freeze is the run-time lock.
There are many more utility types, such as Required (the opposite of Partial), Record, ReturnType and NonNullable; the TypeScript Handbook lists them all.

اختبار يتاح بعد الخطوة الأخيرة.