Angular: empty array as property of typed object

Question

so I have a model:

export interface Clan {
  name: string;
  members: [string];
  invitees: [string];
}

When the user creates a new clan it does not has yet invitees, only one member.

newClan = {
   name: form.value.name;
   members: [user.id];
   invitees: [];
}

And this gives the an error

Property '0' is missing in type '[]' but required in type '[string]'.

Of course I could use values as null or undefined, also or just define invitees in the model as not required:

export interface Clan {
  name: string;
  members: [string];
  invitees?: [string];
}

But is there a way to have an invitees empty array on the creation of a new clan?

Just an FYI. [string] is how you declare a tuple that will contain a string in typescript, at least the start of a tuple. Usually, you will have more than one type in a tuple. string[] is how you declare an array of type string. — R. Richards
– R. Richards, Commented Jul 20, 2019 at 12:18
Now I understand, Thx @R.Richards. Went back to read about tuples. Your answer is actually the correct one! — quasi
– quasi, Commented Jul 20, 2019 at 13:15

pzaenger · Accepted Answer · 2019-07-20 15:47:46Z

6

I guess, it might be reasonable to use string[] instead of [string], because [string] is an array with exact one string in it.

export interface Clan {
  name: string;
  members: string[];
  invitees: string[];
}

const newClan: Clan = {
   name: form.value.name;
   members: [user.id];
   invitees: [];
}

Edit: @R. Richards comment offers a good explanation. Read here more about tupels in TypeScript:

Tuple types allow you to express an array with a fixed number of elements whose types are known, but need not be the same.

edited Jul 20, 2019 at 15:47

answered Jul 20, 2019 at 12:03

pzaenger

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6 Comments

quasi Over a year ago

I prefer R. Richards explication. Because it explains why I still can have multiple values in what I though was an array. (see comments of question) Thank you!

pzaenger Over a year ago

That's true and a good point. Therefore I added a link to the accordant TypeScript part about tupels.

quasi Over a year ago

Strange they define it with "fixed number of elements" as I still can push new elements into it as long as I'm using the same type. Or I am misinterpreting the definition?

pzaenger Over a year ago

Mmh. If I do something like const bar: [number] = [1, 1];, which is one element too many, I get the following error: Type '[number, number]' is not assignable to type '[number]'. Types of property 'length' are incompatible.

pzaenger Over a year ago

However, string[] is not a tuple but an array. Therefore you can push element by element.

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