Why are these two function types in typescript different?

Question

Using typescript, I'm finding why two different functions assigned to different local variables result in different signatures. I thought one was just more explicit.

let a: (number)=>number =
  function(x: number): number {return 42;};

let z = function(x:number): number { return 42; };

> .type a
let a: (number: any) => number
> .type z
let z: (x: number) => number

I thought a was just a more explicit version of writing z, but somehow it gets typed more liberally as accepting any.

Using Typescript version 2.5.2

Answer 1

let a: (number)=>number

The parameter name is required. This is exactly equivalent to:

let a: (number: any)=>number

In other words, the first number here defines a parameter named "number"

What you need is,

let a: (x: number)=>number =
  function(x: number): number {return 42;};

The name, x, doesn't matter.