流类型问号? [英] flow type question mark?

问题描述

对使用'?'感到困惑在流动. AFAIK(要感谢流类型问号在参数之前还是之后?):

So confused on using '?' in flow. AFAIK (thanks to flow type question mark before or after param?):

1. 何时为'?'在':'之前，表示bar是可选的，可以是字符串或未定义: bar?: string

1. When '?' before ':', means bar is optional, can be string or undefined: bar?: string

何时为'?'在':'之后，表示bar可能是类型，可以是字符串，未定义或null. bar: ?string

When '?' after ':', means bar is maybe type, can be string, undefined, or null. bar: ?string

我的问题是:在哪种情况下，第一种选择应该优先于第二种选择? bar?: ?string怎么样?

My question is: In which circumstance we should the first option over the second? How about bar?: ?string ?

流很难...

推荐答案

可选表示该属性可以省略.看这个例子:

Optional means that the property can be omitted. Have a look at this example:

type Foo = {
  optional?: string
}

const foo: Foo = {}; // OK

type Bar = {
  maybe: ?string;
}

const bar: Bar = {}; // Error: `maybe` is missing in object literal

关于可选和可能的组合-允许将null分配给可选属性:

Regarding combination of optional and maybe - it allows assigning null to optional property:

type Baz = {
  maybeAndOptional?: ?string;
}

let baz: Baz = {}; // OK
baz = { maybeAndOptional: null }; // OK

type Foo = {
  optional?: string
}

let foo: Foo = {}; // OK
foo = { optional: null } // Error: null is incompatible with string

这篇关于流类型问号?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！