Typescript: push not available on custom typed array

Question

Using Javascript with Typescript (v1.4.1) I have a custom type as follows:

interface WordDataForQuestion {
    question : {
        id : number;
        vocab : string;
        comment : string;
    };
    possibleAnswers : {
        [index : number] : {
            id : number;
            vocab : string;
            comment : string;
        }
    };
}

Now I want to push an element in the array possibleAnswers

nextWordData.possibleAnswers.push(
    {
        id : vocabs[index].id,
        vocab : vocabs[index].voc_t,
        comment : vocabs[index].com_t
    }
);

but it gives me the following error:

exercise.ts(69,42): error TS2339: Property 'push' does not exist on type '{ [index: number]: { id: number; vocab: string; comment: string; }; }'.

I don't understand what is wrong - possibleAnswers should be a JavaScript array here which supports the push-operation. Am I not right?

Take a look at your interface definition -- you're defining an array-like object, not an actual array. — Qantas 94 Heavy
– Qantas 94 Heavy, Commented Jan 24, 2015 at 13:26

Dick van den Brink · Accepted Answer · 2015-01-24 13:27:05Z

3

You are almost there. You did define it as an object with an indexer. Not as an array, see example below

possibleAnswers : {
        id : number;
        vocab : string;
        comment : string;
}[];

answered Jan 24, 2015 at 13:27

Dick van den Brink

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2 Comments

Mathias Bader Over a year ago

Thanks al lot - it is working now. I'm really wondering because I didn't see this information on typescriptlang.org/Handbook#interfaces-array-types, there they do not put the [] after the interface definition ...

Qantas 94 Heavy Over a year ago

@mbader: it's actually under "basic types": typescriptlang.org/Handbook#basic-types-array

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