Unable to use indexOf on a Typed Array due to TypeScript error

Question

I have defined an Interface, created an array of type Interface and am now attempting to use .indexOf, a method of an Array, and I am getting IDE error complaints that make no sense to me. Hoping someone here may be able to lend a thought to solve this pickle.

Interface

export interface IAddress {
  name: string,
  registrationId: number
}

Code

let friends: IAddress[];

// assume friends has a few elements...

let index = friends.indexOf((friend: IAddress) => {
  return !!(friend.name === 'some name');
});

TypeScript Errors:

Argument of type '(friend: IAddress) => boolean' is not assignable to parameter of type 'IAddress'.
Type '(friend: IAddress) => boolean' is missing the following properties from type 'IAddress': registrationId

If I were to remove the :IAddress from the typed def next to friend: I see this error instead.

Argument of type '(friend: any) => boolean' is not assignable to parameter of type 'IAddress'.
Type '(friend: any) => boolean' is missing the following properties from type 'IAddress': registrationId

@Dale Burell -- Would like to know why I can't have a formal gesture of thanks in a question? It's just a bit of nice etiquette and attitude towards fellow developers. — Pixxl
– Pixxl, Commented Jan 2, 2019 at 2:29

Yosvel Quintero · Accepted Answer · 2019-01-02 02:48:38Z

9

Array.prototype.indexOf() receives a parameter searchElement and a second optional parameter fromIndex.

Here is the API reference.

Updated answer based on @Pixxl comment to use Array.prototype. findIndex() to get index variable:

const friends: IAddress[];

// assume friends has a few elements...
const index = friends.findIndex((friend: IAddress) => friend.name === 'some name');

edited Jan 2, 2019 at 2:48

answered Jan 2, 2019 at 2:36

Yosvel Quintero

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2 Comments

Pixxl Over a year ago

Awesome, I didn't realize index and indexOf are meant for primitive values whereas find and findIndex are meant for more object-like values. I'd also like to suggest changing this answer to one that uses findIndex instead as it accomplishes the same task without needing to declare an index var outside the scope.

Yosvel Quintero Over a year ago

Great comment @Pixxl, thank you.. Edidting the answer to use Array.prototoype.findIndex()...

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