流中的条件类型 [英] Conditional types in flow

问题描述

是否可以根据条件在流中键入变量？这样的事情：

Is it possible to type a variable in flow based on a condition? Something like this:

const type = 'xyz';
const a: (type === 'xyz') ? number : string;

推荐答案

简短的回答是否定的。变量必须绑定到单个类型，其中可以包含类似 number |的联合类型串。在条件块内部，Flow可以推断出更专业的类型。例如：

The short answer is no. A variable must be bound to a single type, which can include a union type like number | string. Inside a conditional block Flow can infer a more specialised type. For example:

const a: number | string = "foo";
if(typeof a === "string"){
  a.split(); // OK!
} else {
  a.split(); // Error! a is a number
}

但请注意这些条件分支是静态的，并且它是有保证的在编译时，每个类型内的类型。

But notice that these conditional branches are static, and it's guaranteed at compile time what the type will be inside each.

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