CODEWITHSUNDEEP
typescriptcastingtslintany
twinlakes
twinlakes

Reputation: 10228

How to block implicitly casting from `any` to a stronger type

TypeScript allows implicitly casting from any to a stronger type. Considering deserialized JSON is of type any, this behavior allows many type errors and breaks typings.

What compiler or linting options can I use to block implicitly casting from any to a stronger type?

Repro This operation should be flagged for allowing casting from any to a stronger type.

const o = JSON.parse("");
const s: string = o.twinlakes;

Upvotes: 6

Views: 1884

Answers (2)

Michael Haephrati
Michael Haephrati

Reputation: 4215

In additiona to Josh's answer, since the type check takes place during runtime, you can try using the 'as' keyword to cast the value from 'any' to the type you want.

const o = JSON.parse("{\"twinlakes\":\"Value\"}");

const s: string = (o as any).twinlakes;

How that works

Upvotes: 0

Josh
Josh

Reputation: 3420

If you haven't yet, try enabling the TypeScript --noImplicitAny compiler flag to stop declarations from accidentally forgetting to mark types of things that can't be inferred. This'll stop code such as function takesData(data) { /* ... */ } from allowing their data variables to "implicitly" be type any.

Edit January 2023: the correct answer is now to use typescript-eslint:

Additionally, there are several ESLint rules from typescript-eslint you can use for anys that can sneak in even with --noImplicitAny:

You can enable those rules individually in your ESLint config to use them in your project. Alternately, they're all enabled if you extend from the plugin:@typescript-eslint/strict configuration.

Finally, as Titian mentions in the comments, consider using unknown in your code in any place you'd have wanted to use any. unknown is similar to any but TypeScript will enforce that you properly type narrow unknown values before using them unsafely.

The answer below is outdated because TSLint is deprecated. Please see above for the answer with typescript-eslint.

Additionally, there are a couple TSLint rules that help here:

  • no-any: Disallows usages of any as a type declaration. Use this to stop declarations from including any in them, e.g. let data: any = {/*...*/}.
  • no-unsafe-any: Disallows accidentally using any in an unsafe way. This is different from no-any in that you might still be using any types without knowing it: for example, let data = JSON.parse("{}");.

Upvotes: 9

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