Typescript: string is assignable to {} Type

Issue

Why is this:

const test: {} = "test" // shouldn't this raise "Type 'string' is not assignable to type '{}'." error?
console.log(test)

A valid Typescript code?
Just a moment ago this bit of code caused me to send wrong API calls, because I had something like this:

const getAPIData = async (id: string, filters: NetworkFilters | {} = {})

What I wanted to achieve is the filters prop to have default value of object with no properties, but by my mistake I passed string there and this broke my API calls. Why is it possible? It should raise an error.

Solution

This is sadly one of the edge cases in TS where it's probably a good idea to explain what assignable actually means.

{} is an object without properties, but as such, any object is assignable to it and string is not exception. (string also has methods f.e. toLowerCase).

If you for example declared const test: { toLowerCase: () => string } = "test", you will also not get an error, because toLowerCase is a method of string.

If you said const test: { notAStringMethod: () => string } = "test", then this fails, because notAStringMethod is not a method of string and thus isn't assignable.

Answered By - Mike S.