什么是“as 语法"?tslint 指出的? [英] What is the "as syntax" pointed out by tslint?

问题描述

我升级了 tslint，现在它抱怨:

I upgraded tslint and now it complains about:

ERROR: src/Metronome/JobFetcher.ts[13, 32]: Type assertion using the '<>' syntax is forbidden. Use the 'as' syntax instead.

违规代码如下:

const jobs = <JobConfig[]> <any> await rp(fetchJobsOptions);

as 语法是什么，我为什么要使用它?

What is the as syntax though and why should I use it?

推荐答案

像这样重构你的代码:

const jobs = await rp(fetchJobsOptions) as JobConfig[];

正如 Basarat Ali Syed 在 TypeScript Deep Dive 书中所指出的，它说的是类型转换:

As pointed out in the TypeScript Deep Dive book by Basarat Ali Syed, it says about type casting:

最初添加的语法是.这证明下面:

as foo vs. <foo>

Originally the syntax that was added was <foo>. This is demonstrated below:

var foo: any;
var bar = <string> foo; // bar is now of type "string"

但是使用时语言语法有歧义

However there is an ambiguity in the language grammar when using

<foo> style assertions in JSX:
var foo = <string>bar;
</string>

因此现在建议您只使用 as foo for一致性.

Therefore it is now recommended that you just use as foo for consistency.

不叫类型转换"的原因是转换通常意味着某种运行时支持.然而类型断言纯粹是一种编译时构造，也是一种方式向编译器提供有关您希望代码如何的提示分析.

The reason why it's not called "type casting" is that casting generally implies some sort of runtime support. However type assertions are purely a compile time construct and a way for you to provide hints to the compiler on how you want your code to be analyzed.

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