How to define function return type of a custom object literal

Question

Here is the object, which is returned from a method in a class:

public dbParameters() // HERE
{
    return {
        "values": this.valuesForDb,
        "keys": this.keysForDb,
        "numbers": this.numberOfValues,
    }
}

Could you please advise how to define the type of the function return, in this case ? Or maybe this is not the proper way to do it and I should use another type instead of the object literal ?

Radim Köhler · Accepted Answer · 2016-05-20 09:32:15Z

9

One way could be just a message, that result is a dictinary:

public dbParameters() : { [key: string]: any}
{
    return {
        "values": this.valuesForDb,
        "keys": this.keysForDb,
        "numbers": this.numberOfValues,
    }
}

The other could use some interface

export interface IResult {
    values: any[];
    keys: string[];
    numbers: number[];
}


export class MyClass
{
    public dbParameters() : IResult
    {
        return {
            values: this.valuesForDb,
            keys: this.keysForDb,
            numbers: this.numberOfValues,
        }
    }
}

With interface we have big advantage... it could be reused on many places (declaration, usage...) so that would be the preferred one

And also, we can compose most specific setting of the properties result

export interface IValue {
   name: string;
   value: number;
}
export interface IResult {
    values: IValue[];
    keys: string[];
    numbers: number[];
}

Play with that here

answered May 20, 2016 at 9:32

Radim Köhler

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2 Comments

Robert Brax Over a year ago

Superb answer thanks, I will go with the interface solution. Do you suggest putting interfaces into a single file for entire project, or on per sub apps of the project ?

Radim Köhler Over a year ago

Yes, exactly.. Interfaces should be extracted (not only one file, but could be even more - per logical structure) ... so, we do have few of them for example - and we try to keep them grouped by modules, types...

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