Remove any index signatures from the given object type, so that only explicitly defined properties remain.

Use-cases:

• Remove overly permissive signatures from third-party types.

This type was taken from this StackOverflow answer.

It relies on the fact that an empty object ({}) is assignable to an object with just an index signature, like Record<string, unknown>, but not to an object with explicitly defined keys, like Record<'foo' | 'bar', unknown>.

(The actual value type, unknown, is irrelevant and could be any type. Only the key type matters.)

const indexed: Record<string, unknown> = {}; // Allowed

const keyed: Record<'foo', unknown> = {}; // Error
// => TS2739: Type '{}' is missing the following properties from type 'Record<"foo" | "bar", unknown>': foo, bar

Instead of causing a type error like the above, you can also use a conditional type to test whether a type is assignable to another:

type Indexed = {} extends Record<string, unknown>
	? '✅ `{}` is assignable to `Record<string, unknown>`'
	: '❌ `{}` is NOT assignable to `Record<string, unknown>`';
// => '✅ `{}` is assignable to `Record<string, unknown>`'

type Keyed = {} extends Record<'foo' | 'bar', unknown>
	? "✅ `{}` is assignable to `Record<'foo' | 'bar', unknown>`"
	: "❌ `{}` is NOT assignable to `Record<'foo' | 'bar', unknown>`";
// => "❌ `{}` is NOT assignable to `Record<'foo' | 'bar', unknown>`"

Using a mapped type, you can then check for each KeyType of ObjectType...

import type {RemoveIndexSignature} from 'type-fest';

type RemoveIndexSignature<ObjectType> = {
	[KeyType in keyof ObjectType // Map each key of `ObjectType`...
	]: ObjectType[KeyType]; // ...to its original value, i.e. `RemoveIndexSignature<Foo> == Foo`.
};

...whether an empty object ({}) would be assignable to an object with that KeyType (Record<KeyType, unknown>)...

import type {RemoveIndexSignature} from 'type-fest';

type RemoveIndexSignature<ObjectType> = {
	[KeyType in keyof ObjectType
		// Is `{}` assignable to `Record<KeyType, unknown>`?
		as {} extends Record<KeyType, unknown>
			? ... // ✅ `{}` is assignable to `Record<KeyType, unknown>`
			: ... // ❌ `{}` is NOT assignable to `Record<KeyType, unknown>`
	]: ObjectType[KeyType];
};

If {} is assignable, it means that KeyType is an index signature and we want to remove it. If it is not assignable, KeyType is a "real" key and we want to keep it.

import type {RemoveIndexSignature} from 'type-fest';

type RemoveIndexSignature<ObjectType> = {
	[KeyType in keyof ObjectType
		as {} extends Record<KeyType, unknown>
			? never // => Remove this `KeyType`.
			: KeyType // => Keep this `KeyType` as it is.
	]: ObjectType[KeyType];
};