TypeScript and ...args

Question

TypeScript doesn't show "Expected 1 arguments, but got 0." error, when it should.

type PolyElement = [number, number];

export interface Node {
  element: PolyElement | null;
  next: Node | null;
}

export class Node {
  constructor(element?: PolyElement) {
    this.element = element ?? null;
    this.next = null;
  }
  add(...args: PolyElement[]) {
    for (let i = 0; i < args.length; ++i) {
       //some code
      }
    }
  }

then I invoke method like so:

poly1.add([1,2,3]);

and it outputs: Argument of type '[number, number, number]' is not assignable to parameter of type 'PolyElement'. Types of property 'length' are incompatible. Type '3' is not assignable to type '2'.ts(2345)

also

poly1.add([]);

outputs: Argument of type '[]' is not assignable to parameter of type 'PolyElement'. Type '[]' is missing the following properties from type '[number, number]': 0, 1ts(2345)

etc.

The QUESTION is why:

poly1.add();

DOESN'T output something like: Expected at least 1 arguments, but got 0. ?

Your answer is already in your question: Argument of type '[]'.... Empty array counts as an argument, it is just of the wrong type. — jdaz
– jdaz, Commented Jul 17, 2020 at 21:29
sorry, I made a mistake when copying. I already corrected my question. Now it is ` poly1.add();` — pusiadestroyer42
– pusiadestroyer42, Commented Jul 17, 2020 at 21:32
Does this answer your question? Expect at least one argument for variadic method in TypeScript — Emile Bergeron
– Emile Bergeron, Commented Jul 17, 2020 at 21:51
@EmileBergeron yes, but there is a better reply to this question in this thread that in the other you linked — pusiadestroyer42
– pusiadestroyer42, Commented Jul 17, 2020 at 22:06
Yep, now there is! Ideally, the other questions should be linked to this newer one. — Emile Bergeron
– Emile Bergeron, Commented Jul 17, 2020 at 22:15

Ryan Cavanaugh · Accepted Answer · 2020-07-17 21:52:17Z

5

This syntax

add(...args: PolyElement[]) {

means "Pass me as many arguments (including zero) as you like".

You can write this instead to enforce at least one argument:

add(...args: [PolyElement, ...PolyElement[]]) {

edited Jul 17, 2020 at 21:52

answered Jul 17, 2020 at 21:34

Ryan Cavanaugh

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6 Comments

pusiadestroyer42 Over a year ago

could you help what is the best syntax to pass more than 0 arguments ot type PolyElement?

Emile Bergeron Over a year ago

Maybe add(el: PolyElement, ...args: PolyElement[])? But you'd need to handle el explicitly.

zero298 Over a year ago

In all honesty you got this before me. I just added the bit to enforce at least 1 PolyElement.

Emile Bergeron Over a year ago

That's the thing, you need to use it, if not, it means that you're explicitly ignoring the first parameter.

Emile Bergeron Over a year ago

Now I prefer this answer (still upvoted both). And while looking for a duplicate candidate, didn't see anything similar.

|

zero298 · Accepted Answer · 2020-07-17 21:39:25Z

3

Rest arguments ... means 0 or more. If you want at least 1, then you should explicitly accept a first argument and then accept "the rest".

type NumArr = [number, number];

// Forces at least 1 NumArr
const foo = (a: NumArr, ...b: NumArr[]) => {
    const all = [a, ...b];
    console.log(all);
}

const bar = () => {
    foo([1, 2]); // Good!
    foo([]); // Wrong type, [] is not [number, number]
    foo(); // Expected at least 1 arg
};

answered Jul 17, 2020 at 21:39

zero298

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