Steve Fenton

TypeScript NotFunction type

I have been working out how to create a TypeScript NotFunction type for a while, in response to a Stack Overflow question. With the arrival of conditional types, I think there may be a way. It’s not perfect – but it does work.

The type works by taking all possible types and converting the type to never if it extends the Function type.

type NotFunction<T> = T extends Function ? never : T;

You can use this type to enforce “anything except functions” as shown below:

const x: NotFunction<typeof myVariable> = myVariable;

Here is a full working example:

type NotFunction<T> = T extends Function ? never : T;

const aFunction = (input: string) => input;
const anObject = { data: 'some data' };
const aString = 'data';

// Error: Functions aren't assignable to "never"
const x: NotFunction<typeof aFunction> = aFunction;

// OK
const y: NotFunction<typeof anObject> = anObject;
const z: NotFunction<typeof aString> = aString;

You also get compiler assistance if you accidentally use a different type on the left-hand and right-hand side of the expression (which is possible due to having to repeat yourself):

// Error: string and function aren't assignable
const x: NotFunction<typeof aString> aFunction;