Extract 工具類型八個(gè)使用技巧
作者：阿寶哥
本文我將介紹 Extract 工具類型的 8 個(gè)使用技巧，掌握這些技巧之后，在工作中你就能更好地利用 Extract 工具類型來滿足不同的使用場景。
Extract 是 TypeScript 中內(nèi)置的工具類型，它用于從聯(lián)合類型中提取出符合某個(gè)條件的類型，生成一個(gè)新的類型。這個(gè)工具類型在日常開發(fā)中非常有用，它能夠幫助我們編寫類型安全的代碼和更好地實(shí)現(xiàn)代碼復(fù)用。
/**
* Extract from T those types that are assignable to U.
* typescript/lib/lib.es5.d.ts
*/
type Extract<T, U> = T extends U ? T : never;
type T0 = Extract<"a" | "b" | "c", "a" | "f">
// type T0 = "a"本文我將介紹 Extract 工具類型的 8 個(gè)使用技巧，掌握這些技巧之后，在工作中你就能更好地利用 Extract
1.提取指定的基本數(shù)據(jù)類型
type MyTypes = string | number | boolean;
type StringOrNumber = Extract<MyTypes, string | number>;
let uid: StringOrNumber = "semlinker" // Ok
uid = 2024 // Ok
uid = false // Error
// Type 'boolean' is not assignable to type 'StringOrNumber'.2.提取指定的字符串字面量類型
type Color = 'red' | 'green' | 'blue' | 'yellow';
type PrimaryColors = Extract<Color, 'red' | 'green' | 'blue'>;
const primaryColor: PrimaryColors = 'blue'; // Ok
const secondaryColor: PrimaryColors = 'yellow'; // Error
// Type '"yellow"' is not assignable to type 'PrimaryColors'.3.提取可調(diào)用的函數(shù)類型
type Value = string | number | (() => void);
type CallableFn = Extract<Value, Function>;
const fn1: CallableFn = () => console.log('semlinker'); // Ok
const fn2: CallableFn = 'semlinker'; // Error
// Type 'string' is not assignable to type '() => void'.4.提取兩個(gè)聯(lián)合類型的共有成員
type TaskStatus = "Todo" | "InProgress" | "Done" | "Archived";
type ModuleHandledStatus = "Todo" | "Done" | "OnHold";
type ModuleSpecificStatus = Extract<TaskStatus, ModuleHandledStatus>;
// type ModuleSpecificStatus = "Todo" | "Done"5.提取含有特定屬性的子類型
Animal 聯(lián)合類型，包含了多種動物的描述對象，我們想從中提取出含有 "legs" 屬性的子類型。
type Animal =
| { type: 'dog', legs: number }
| { type: 'cat', legs: number }
| { type: 'fish', fins: number };
type AnimalsWithLegs = Extract<Animal, { legs: number }>;
const dog: AnimalsWithLegs = { type: 'dog', legs: 4 }; // Ok
const cat: AnimalsWithLegs = { type: 'cat', legs: 4 }; // Ok
const fish: AnimalsWithLegs = { type: 'fish', fins: 6 }; // Error
// Type '"fish"' is not assignable to type '"dog" | "cat"'.6.提取特定的事件類型
type EventTypes = MouseEvent | KeyboardEvent | TouchEvent;
type OnlyMouseEvents = Extract<EventTypes, MouseEvent>;
function handleMouseEvent(event: OnlyMouseEvents) {
console.log('Handling mouse event:', event.clientX, event.clientY);
}
document.addEventListener('click', (event) => {
handleMouseEvent(event); // OK
});
document.addEventListener('keydown', (event) => {
handleMouseEvent(event); // Error
// Argument of type 'KeyboardEvent' is not assignable to parameter of type 'MouseEvent'.
});7.在類型守衛(wèi)中使用 Extract
使用 Extract 可以在類型守衛(wèi)中精確地過濾類型，使得在條件分支中可以安全地使用過濾后的類型。
type Pet = { type: 'dog', bark: () => void } | { type: 'cat', meow: () => void };
function isDog(pet: Pet): pet is Extract<Pet, { type: 'dog' }> {
return pet.type === 'dog';
}
const pet1: Pet = { type: 'dog', bark: () => console.log('Woof!') }
const pet2: Pet = { type: "cat", meow: () => console.log("Meow!") }
console.log(`pet1 is dog: ${isDog(pet1)}`) // "pet1 is dog: true"
console.log(`pet2 is dog: ${isDog(pet2)}`) // "pet2 is dog: false"8.在函數(shù)重載中使用 Extract
在處理 API 請求的場景中，我們需要根據(jù)不同的請求類型（如 GET、POST、DELETE）處理不同類型的數(shù)據(jù)。為了增強(qiáng)類型安全和確保每種請求類型都正確地處理其數(shù)據(jù)，我們可以利用 TypeScript 的函數(shù)重載和 Extract 工具類型。
type RequestType = 'GET' | 'POST' | 'DELETE';
type RequestData = {
GET: undefined;
POST: { body: string };
DELETE: { resourceId: number };
};
// Function overloading, based on the request type, accepts matching data types
function sendRequest<T extends RequestType>(type: 'GET', data: Extract<RequestData[T], undefined>): void;
function sendRequest<T extends RequestType>(type: 'POST', data: Extract<RequestData[T], { body: string }>): void;
function sendRequest<T extends RequestType>(type: 'DELETE', data: Extract<RequestData[T], { resourceId: number }>): void;
function sendRequest<T extends RequestType>(type: T, data: RequestData[T]): void {
console.log(`Sending ${type} request with data:`, data);
}
sendRequest('GET', undefined); // Ok
sendRequest('POST', { body: "semlinker" }); // Ok
sendRequest('DELETE', { resourceId: 2024 }); // Ok
sendRequest('POST', { body: 2024 }); // Error
// Type 'number' is not assignable to type 'string'.
sendRequest('DELETE', undefined); // Error
// Argument of type 'undefined' is not assignable to parameter of type '{ resourceId: number; }'.責(zé)任編輯：姜華
來源：
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