Typescript error on using array that gets elements in loop

Question

A basic use-case that illustrates what I'm doing:

class Foo {
  public constructor(arg1: number, arg2: number) {
    console.log(arg1);
    console.log(arg2);
  }
}

function foo(test: number): number[] {
  let args: number[] = [];
  for (let i = 0; i < 2; i++) {
    args.push(i);
  }
  if (args.length !== 2) {
    throw new Error('Invalid argument length');
  }
  return new Foo(...args);
}

console.log(foo(1));

When trying to run this, I get:

error TS2556: Expected 2 arguments, but got 0 or more.

16   return new Foo(...args);

Which I'm absolutely unable sure where to go to fix this.

basarat · Accepted Answer · 2019-04-28 21:08:37Z

1

Here is the error you are getting :

class Foo {
  public constructor(arg1: number, arg2: number) {
  }
}

declare let args: number[];
new Foo(...args); // expected 2 but got 0 or more.

why

Because number[] can be an array of length 0 or more than 2. So it doesn't match [number,number] i.e. an array of exactly length 2.

Fix

Either annotate as a tupple [number,number]:

declare let args: [number,number];
new Foo(...args); // Ok

Or get the values from the array your self:

new Foo(args[0],args[1]);

answered Apr 28, 2019 at 21:08

basarat

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4 Comments

MasterOdin Over a year ago

So if I have some number of different classes that all take a variable amount of arguments, there's no real way to use variable expansion here for the arguments as I'd always have to annotate it as a tuple of the appropriate length?

basarat Over a year ago

Or annotate as any. It is okay to use in a controlled manner when you want to do localized magic coding 👍

Matt Browne Over a year ago

Casting or annotating as any doesn't seem to make a difference. I guess casting to a tuple of a specific length is the only way, other than using Reflect.construct() (which doesn't work in IE).

Matt Browne Over a year ago

I figured it out—you can cast the class itself to any: new (Foo as any)(...args);

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