如何在TypeScript中测试/断言变量类型不为any？

Got it, I’ve got a perfect solution that fits all your requirements. We can use TypeScript’s conditional types to create a helper function that only throws a compiler error when you pass an any type—no runtime code needed, and it won’t affect your entire project (no need for --noImplicitAny or similar flags).

Here’s the implementation:

// Define a conditional type that rejects `any`
type NotAny<T> = T extends any 
  ? (any extends T ? never : T) 
  : T;

// Helper function that uses the type
function notAny<T>(x: NotAny<T>): void {}

How it works: #

• The NotAny<T> type leverages TypeScript’s quirky but useful compatibility rules for any: if T is any, then any extends T will evaluate to true (since any is compatible with every type, including itself). In that case, we return never—a type no value can be assigned to.
• When you pass an any variable to notAny, the compiler detects you’re trying to assign any to a parameter expecting never, which triggers an error exactly where you want it.

Test it with your example scenario: #

let foo: any = 'value';
notAny(foo); // ❌ Compiler error: Type 'any' is not assignable to type 'never'

let bar: string = 'hello';
notAny(bar); // ✅ No error at all—works fine for non-any types

Bonus: Friendlier error messages #

If you want the error message to be more explicit, tweak the conditional type to return a string literal instead of never:

type AssertNotAny<T> = T extends any 
  ? (any extends T ? "❌ Invalid: Cannot pass an 'any' type here" : T) 
  : T;

function notAny<T>(x: AssertNotAny<T>): void {}

Now when you pass any, the error will read: Type 'any' is not assignable to type '"❌ Invalid: Cannot pass an 'any' type here"'—much clearer about what’s wrong.

This solution is totally single-use: you only get the check when you call notAny, and it doesn’t require any project-wide configuration changes. The function itself doesn’t need any runtime implementation (you can leave it empty) because we only care about the type checking.

内容的提问来源于stack exchange，提问作者SystematicFrank