const isType = <T>(type: string) => (obj: unknown): obj is T =>  toString.call(obj) === `[object ${type}]`

I know it's a function about judging types, but it uses a lot of arrows and I don't understand what the arrows at each stage mean

const isType = <T>(type: string) => (obj: unknown): obj is T 

The first part indicates that the function accepts a parameter and returns a boolean value of type judgment

=>  toString.call(obj) === `[object ${type}]`

what the latter part means

CodePudding user response：

Consider this simplified example:

function isType<T,>(type: string) {
    return function (obj: unknown): obj is T {
        return toString.call(obj) === `[object ${type}]`
    }
}

const foo = isType('hello')([])

isType is a function <T>(type: string) => which returns another function (obj: unknown): obj is T. THis another function is a typescript custom typeguard

However, toString is not a typeguard itself, hence, I' m not sure if this partial function/typeguard is useful

CodePudding user response：

Let's give it a try. Just to start with, I don't think this does what is supposed to do.

The function is returning another function, that will act as a type guard. That means that TS can use it to narrow types.

You can consider isType as being a factory function. You'll need to make 2 calls for this to actually work

1. one to get a type guard
2. to actually call the type guard