readonly 属性

对于 TypeScript，属性也可以标记为 readonly。虽然它不会在运行时改变任何行为，但在类型检查期间无法写入标记为 readonly 的属性。

Properties can also be marked as readonly for TypeScript. While it won't change any behavior at runtime, a property marked as readonly can't be written to during type-checking.

interface SomeType {
  readonly prop: string;
}

function doSomething(obj: SomeType) {
  // We can read from 'obj.prop'.
  console.log(`prop has the value '${obj.prop}'.`);

  // But we can't re-assign it.
  obj.prop = "hello";
}

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interface Home {
  readonly resident: { name: string; age: number };
}

function visitForBirthday(home: Home) {
  // We can read and update properties from 'home.resident'.
  console.log(`Happy birthday ${home.resident.name}!`);
  home.resident.age++;
}

function evict(home: Home) {
  // But we can't write to the 'resident' property itself on a 'Home'.
  home.resident = {
    name: "Victor the Evictor",
    age: 42,
  };
}

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interface Person {
  name: string;
  age: number;
}

interface ReadonlyPerson {
  readonly name: string;
  readonly age: number;
}

let writablePerson: Person = {
  name: "Person McPersonface",
  age: 42,
};

// works
let readonlyPerson: ReadonlyPerson = writablePerson;

console.log(readonlyPerson.age); // prints '42'
writablePerson.age++;
console.log(readonlyPerson.age); // prints '43'

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