Why do Typescript function type signatures include parameter names

Question

New to typescript, so maybe I'm missing something here...

I was trying to write a simple filter function on a container I'd made

class Container<T> {
    filter(predicate: (T) => boolean): Container<T> {
        for(const element of this.contents) {
            if(predicate(element))

and tslint gave me an error about starting variables with capital letters (which is a rule I have on purpose). I wasn't sure what it meant at first, but apparently it's taking the T in (T) => boolean to be the name of the parameter, and not the type. After googling around for some typescript callback examples, I saw everyone typing a function signature as

(paramName: ParamType) => ReturnType.

But it seems like the paramName here is pointless. I'm not declaring the function here, I'm just giving its signature. Why is this valid Typescript?

A similar question has been handled in this q&a.

Sefe
– Sefe

2018-12-19 21:41:25 +00:00
Commented Dec 19, 2018 at 21:41 — Sefe
– Sefe, Commented Dec 19, 2018 at 21:41

Community · Accepted Answer · 2020-06-20 09:12:55Z

3

The official reason appears to be to "help with readability"

A function’s type has the same two parts: the type of the arguments and the return type. When writing out the whole function type, both parts are required. We write out the parameter types just like a parameter list, giving each parameter a name and a type. This name is just to help with readability. ...

-- TypeScript Handbook - Functions

You are correct that the names are not used.

edited Jun 20, 2020 at 9:12

CommunityBot

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answered Dec 19, 2018 at 21:32

Alexander O'Mara

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