Type Alias OmitIndexSignature<ObjectType>

Omit any index signatures from the given object type, leaving only explicitly defined properties.

This is the counterpart of PickIndexSignature.

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

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

type IndexedResult = Indexed;
//=> '✅ `{}` 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>`';

type KeyedResult = Keyed;
//=> '❌ `{}` is NOT assignable to `Record<\'foo\' | \'bar\', unknown>`'
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Using a mapped type, you can then check for each KeyType of ObjectType...

type OmitIndexSignature<ObjectType> = {
	[KeyType in keyof ObjectType // Map each key of `ObjectType`...
	]: ObjectType[KeyType]; // ...to its original value, i.e. `OmitIndexSignature<Foo> == Foo`.
};
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...whether an empty object ({}) would be assignable to an object with that KeyType (Record<KeyType, unknown>)...

type OmitIndexSignature<ObjectType> = {
	[KeyType in keyof ObjectType
	// Is `{}` assignable to `Record<KeyType, unknown>`?
	as {} extends Record<KeyType, unknown>
		? never // ✅ `{}` is assignable to `Record<KeyType, unknown>`
		: KeyType // ❌ `{}` is NOT assignable to `Record<KeyType, unknown>`
	]: ObjectType[KeyType];
};
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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.