❌ **Don't** ever use the types `Number`, `String`, `Boolean`, `Symbol`, or `Object`

These types refer to non-primitive boxed objects that are almost never used appropriately in JavaScript code.

Instead of `Object`, use the non-primitive `object` type ([added in TypeScript 2.2](../release-notes/typescript-2-2.html#object-type)).

❌ **Don't** ever have a generic type which doesn't use its type parameter.

See more details in [TypeScript FAQ page](https://github.com/Microsoft/TypeScript/wiki/FAQ#why-doesnt-type-inference-work-on-this-interface-interface-foot--).

❌ **Don't** use `any` as a type unless you are in the process of migrating a JavaScript project to TypeScript. The compiler _effectively_ treats `any` as "please turn off type checking for this thing". It is similar to putting an `@ts-ignore` comment around every usage of the variable. This can be very helpful when you are first migrating a JavaScript project to TypeScript as you can set the type for stuff you haven't migrated yet as `any`, but in a full TypeScript project you are disabling type checking for any parts of your program that use it.

}

❌ **Don't** use optional parameters in callbacks unless you really mean it:

}

❌ **Don't** write separate overloads that differ only on callback arity:

❌ **Don't** put more general overloads before more specific overloads:

❔ **Why:** TypeScript chooses the _first matching overload_ when resolving function calls.

When an earlier overload is "more general" than a later one, the later one is effectively hidden and cannot be called.

❌ **Don't** write several overloads that differ only in trailing parameters:

x.diff("something", true ? undefined : "hour");

❌ **Don't** write overloads that differ by type in only one argument position: