Compiler option to switch lib .d.ts anys to unknown

[JoshuaKGoldberg]

🔍 Search Terms

compiler option flag lib.d.ts noImplicitAny unknown any

✅ Viability Checklist

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• This isn't a request to add a new utility type: https://github.com/microsoft/TypeScript/wiki/No-New-Utility-Types
• This feature would agree with the rest of our Design Goals: https://github.com/Microsoft/TypeScript/wiki/TypeScript-Design-Goals

⭐ Suggestion

A few built-in (lib*.d.ts) types include any, even though unknown would be safer. Most notably:

• JSON.parse():

  TypeScript/src/lib/es5.d.ts

  Line 1144 in 56a0825

  parse(text: string, reviver?: (this: any, key: string, value: any) => any): any;

• .json(): https://github.com/microsoft/TypeScript/blob/56a08250f3516b3f5bc120d6c7ab4450a9a69352/src/lib/dom.generated.d.ts#L19416
• Storage: https://github.com/microsoft/TypeScript/blob/56a08250f3516b3f5bc120d6c7ab4450a9a69352/src/lib/dom.generated.d.ts#L22262

These are kept as any for legacy support reasons: it would be a massive breaking change to restrict them to unknown. But, for newer projects and those that don't rely on those anys, switching them to unknown would be much safer long-term.

Proposal: could we have a compiler option to switch the lib definition anys to unknowns? Maybe, strictLibDefinitions? useUnknownInLibDefinitions? (I'm not convinced of those specific names)

📃 Motivating Example

Using this compiler option will prevent many of the common anys from sneaking into projects that use built-ins such as JSON.parse and Response.

For example, this code does not have a type error by default, but would with the new compiler option enabled:

const data = JSON.parse(`"clearly-a-string"`);
//    ^? any (today)
//    ^? unknown (with this compiler option)

console.log(data.some.property.that.does.not.exist);

💻 Use Cases

I'm not sure that this could even be enabled with strict anytime soon. It might introduce a lot of type errors in even many projects that are already in strict mode but happen to use JSON.parse et al.

This is not the same as #27265. That issue tracks a flag to handle implicit anys differently. This is for a flag to change the definitions in TypeScript's built-in .d.ts files.

I don't think this is the same as #26188. Per #60899 (comment): I'd interpreted that one as suggesting making the change always - with varying levels of compiler-option-orientation in the comments.

The implementation details of this might be tricky. The dom lib generator could theoretically produce two .d.ts outputs for each of today's files: one with any and one with unknown. Or, if two files aren't doable, there could be an intrinsic declared that switches between any and unknown.

[MartinJohns]

Duplicate of #26188.

[JoshuaKGoldberg]

Ah, I meant to mention that in the OP, thanks. I'd interpreted that one as suggesting making the change always - with varying levels of compiler-option-orientation in the comments (e.g. #26188 (comment) against it).