Typescript 中的可选类成员

Optional class members in Typescript

提问by angularJsNewbie

Is there a way to specify type-safe optional members in Typescript classes?

有没有办法在 Typescript 类中指定类型安全的可选成员？

That is, something like...

也就是说，像...

class Foo {
    a?: string;
    b?: string;
    c: number;
}

....

foo = new Foo();
...
if (foo.a !== undefined) { ... (access foo.a in a type-safe string manner) ... }

In case you are familiar with OCaml/F#, I am looking for something like 'string option'.

如果您熟悉 OCaml/F#，我正在寻找诸如“字符串选项”之类的东西。

采纳答案by basarat

The following works in TypeScript 3.x:

以下在 TypeScript 3.x 中有效：

class Foo {
  a?: string;
  b?: string;
  c: number = 123;
}

Note that you need to initialise any members that are not optional (either inline as shown or in the constructor).

请注意，您需要初始化任何非可选成员（如图所示或在构造函数中内联）。

回答by Fredrik_Macrobond

Optional class properties was added as a feature in Typescript 2.0.

可选的类属性作为 Typescript 2.0 中的一项功能添加。

In this example, property bis optional:

在本例中，属性b是可选的：

class Bar {
  a: number;
  b?: number;
}

Typescript 2.0 release notes - Optional class properties

Typescript 2.0 发行说明 - 可选的类属性

回答by Distagon

In some use cases you can accomplish it with Parameter properties:

在某些用例中，您可以使用Parameter 属性来完成它：

class Test {
    constructor(public a: string, public b: string, public c?: string)
    {
    }
}

var test = new Test('foo', 'bar');

playground

操场

回答by Mentor

Optional properties and methods can now be declared in classes, similar to what is already permitted in interfaces:

可选的属性和方法现在可以在类中声明，类似于接口中已经允许的：

class Bar {
    a: number;
    b?: number;
    f() {
        return 1;
    }
    g?(): number;  // Body of optional method can be omitted
    h?() {
        return 2;
    }
}

When compiled in --strictNullChecks mode, optional properties and methods automatically have undefined included in their type. Thus, the b property above is of type number | undefined and the g method above is of type (() => number) | undefined. Type guards can be used to strip away the undefined part of the type:

在 --strictNullChecks 模式下编译时，可选属性和方法会自动在其类型中包含 undefined 。因此，上面的 b 属性的类型为 number | undefined 并且上面的 g 方法是类型 (() => number) | 不明确的。类型保护可用于去除类型的未定义部分：

function test(x: Bar) {
    x.a;  // number
    x.b;  // number | undefined
    x.f;  // () => number
    x.g;  // (() => number) | undefined
    let f1 = x.f();            // number
    let g1 = x.g && x.g();     // number | undefined
    let g2 = x.g ? x.g() : 0;  // number
}

Optional class properties

可选的类属性