Typescript how to type the rest of parameters in object

Question

  function test(data){
      console.log(data)
  }

  test({comments: 'hi', eat: true, sleep: true})

In the test function, I am sure of the comment argument will appear, where as for other arguments, the properties might be dynamic but the value will be of boolean type,

  test({comments: 'hi', drink: true, sleep: true})

considering this situation, how should I correctly type data? I have tried something like this, but it seems wrong

function(data: {data: {comments: string, [key: string]: boolean})

Is the question, then, "what type do I give to the parameter data, which should be an object with at least one property named comments of type string, but which could have lots of other properties that I don't care about"? — Ray Toal
– Ray Toal, Commented Nov 26, 2020 at 9:43
If you don't care about the other properties, then type DataType = { comments: string; [key: string]: unknown; }; will work. Ref: stackoverflow.com/a/33860616 — Yoshi
– Yoshi, Commented Nov 26, 2020 at 9:50
Right, right, your question is indeed different (and pretty cool). Looks like intersection types have potential, but fall short. More info here: github.com/microsoft/TypeScript/issues/20597. — Ray Toal
– Ray Toal, Commented Nov 26, 2020 at 10:05

Николай Гольцев · Accepted Answer · 2020-11-26 10:55:08Z

1

I can suggest you some kind of workaround:

function test<T extends {
  [key: string]: any;
}>(data: { comments: unknown } & { [key in keyof T]: key extends "comments" ? string : boolean }) {
  const comments = data.comments as string;
  console.log(comments);
  console.log(data.eat);
  console.log(data.sleep);
}

test({comments: "hi"}); // works
test({comments: "hi", eat: true}); // works
test({comments: true}); // doesn't works
test({comments: 5}); // doesn't works
test({comments: "hi", eat: "true"}); // doesn't works
test({comments: "hi", eat: 5}); // doesn't works

It typed well outside of the function's body, but in function's body to type data.comments correctly you should add some narrowing.

answered Nov 26, 2020 at 10:55

Николай Гольцев

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