What is the “as syntax” pointed out by tslint?

问题:

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.

The offending code looks like:

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

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

回答1:

Refactor your code like this:

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

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

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"

However there is an ambiguity in the language grammar when using

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

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

Type Assertion vs. Casting

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.