TypeScript Error: Property '0' is missing

Question

I have my interface like this

export interface Details {
  Name: [{
    First: string;
    Last: string;
  }];
}   

I have an observable config variable:

Configuration: KnockoutObservable<Details> = ko.observable<Details>();

and I would like to assign it a value in the constructor as follows:

config = {
  Name: [{
    First: "ABC",
    Last: "DEF"
  },
  {
    First: "LMN",
    Last: "XYZ"
  }]
};

this.Configuration(config);

and I am getting an error:

Types of property 'Name' is incompatible and property '0' is missing in type.
Type '{ First:string; Last:string; }[]' is not assignable to 
type '[{ First: string; Last:string; }]'

I don't have control on changing the interface as it is being used elsewhere.
What is the correct way to initialize this config variable ?

Thanks in advance.

Answer 1

I came across the same issue and got around it by changing the interface to:

    interface Details {
        Name: {
            First: string;
            Last: string;
        }[];
    }

I know you may not want the interface changed but hope this helps for anyone that is in this situation.

Answer 2

In this type definition:

interface Details {
  Name: [{
    First: string;
    Last: string;
  }];
}

Name is not an array at compile-time. It's a tuple with one element. Tuples in Typescript are allowed to have extra elements, but they can't have missing elements. As a 1-tuple, Name is essentially an array which must have at least one element.

However, in this value:

const config = {
  Name: [{
    First: "ABC",
    Last: "DEF"
  },
  {
    First: "LMN",
    Last: "XYZ"
  }]
};

Since there is no explicit typing, the Name property here defaults to array type. Arrays can have any number of elements, including zero - which doesn't fit in a 1-tuple. Hence your error.

Your error can be fixed by giving the compiler a hint that your literal is actually a tuple:

const config: Details = { Name: [{...}, {...}] };

If you do need to be able to take in an array of names, you'll have to do some casting, maybe something like this:

if (names.length > 0) {
  const config = {
    Name: names as Details['Name']
  };
  Configuration(config);
}

(You could remove the if check if you can determine that the tuple was simply a mistake by whoever wrote the typings.)

Tuples reference: https://www.typescriptlang.org/docs/handbook/basic-types.html

Answer 3

This error can come from incorrectly typing an array (like I just did):

myArray:[]; //Incorrect, results in error message of `Property '0' is missing in type`

myArray: Array<string>; //Correct

myArray: string[]; //Also correct

The reason is that braces denote a tuple in Typescript, not an array.

The Docs

Answer 4

Updating the interface to following should fix the issue :

 interface Details {
        Name: Array<{
            First: string;
            Last: string;
        }>;
    }