带部分类型注释的TypeScript函数参数解构

Question

Let's consider example:

function func1(num: number) {
  return num * 2;
}

function func2(by: number, num = func1(1)) {
  return num / by;
}

It works perfectly fine. In func2 I don't have to manually provide a type for num argument since it is provided from func1 .

But when I write func2 in form of argument object destructuring I have to manually provide a type for num in func3 :

function func3({ by, num = func1(1) }: { by: number, num: number }) {
  return num / by;
}

How to write func3 without retyping type for num ? Something like this (code below doesn't work):

function func3({ by, num = func1(1) }: { by: number }) {
  return num / by;
}

Answer 1

There is currently no better way to do this unfortunately. Ideas about improving this have been floating around for a while but the compiler team has not found a good one yet. This issue is the latest proposal, @RyanCavanaugh says they are giving it another try, well see what comes of it. For now you have to write the names again in the type annotation