TypeScript type predicates All In One

TypeScript type predicates All In One

类型谓词 / 类型断言

Narrowing / 类型收窄

type predicates

https://www.typescriptlang.org/docs/handbook/2/narrowing.html#using-type-predicates

To define a user-defined type guard, we simply need to define a function whose return type is a type predicate:

https://www.typescriptlang.org/docs/handbook/advanced-types.html#using-type-predicates

Type Guards

A type guard is some expression that performs a runtime check that guarantees the type in some scope.

To define a type guard, we simply need to define a function whose return type is a type predicate:

function isFish(pet: Fish | Bird): pet is Fish {
  return (pet as Fish).swim !== undefined;
}

pet is Fish is our type predicate in this example.
A predicate takes the form parameterName is Type, where parameterName must be the name of a parameter from the current function signature.

Any time isFish is called with some variable, TypeScript will narrow that variable to that specific type if the original type is compatible.

// both calls to 'swim' and 'fly' are now okay.
let pet = getSmallPet();

if (isFish(pet)) {
  pet.swim();
} else {
  pet.fly();
}

tsc

# local 👍
# vscode 🚀
$ npm install -D typescript
$ npm install -D ts-node

$ npx tsc -v
# Version 5.2.2

# ts-node ✅
$ yarn dev
$ npm run dev

demos

export {};

// is 类型谓词
// function isString(str: any): str is string {
// function isString(str: unknown): str is string {
// ❓ Parameter 'str' implicitly has an 'any' type, but a better type may be inferred from usage.ts(7044)
function isString(str): str is string {
  return typeof str === 'string';
}

function isNumber(num: number): boolean {
  return typeof num === 'number';
}

function test(x: unknown) {
  if (isString(x)) {
    console.log(`string ✅`);
  }
  // ❌ Argument of type 'unknown' is not assignable to parameter of type 'number'.ts(2345)
  // if (isNumber(x)) {
  //   console.log(`number ✅`);
  // }
  if (isNumber(x as number)) {
    console.log(`number ✅`);
  }
}

test(`str`);
test(2023);

predicate

英 ['predɪkət] 美 ['predɪkət]
释义:

1. vt. 断言，断定; 宣布，宣讲; 使基于
2. vi. 断言，断定
3. n. 谓语; 述语

https://www.iciba.com/word?w=predicate

主谓宾 / 主语 谓语 宾语

我在 看 书
我在 钓 鱼
I'm fishing

主语 谓语 宾语

functions overload

函数重载

https://www.typescriptlang.org/docs/handbook/2/functions.html#overload-signatures-and-the-implementation-signature

refs

https://stackoverflow.com/questions/40081332/what-does-the-is-keyword-do-in-typescript

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