Different types for Typescript index signature

Question

I've the following interface.

interface IDetails {
  name: string;
  age: number;
}

interface IConfig {
  appName: string;
  userDetails: IDetails;
  [key: string]: string | IDetails
}

const details: IConfig = {
  appName: 'test',
  userDetails: {
    name: 'xyz',
    age: 67,
  },
}

const t: string = 'userDetails'
const name: string = details[t].name
//                              ^^^^ 
// throws the following error
//
// Property 'name' does not exist on type 'string | IDetails'.
//  Property 'name' does not exist on type 'string'.

When I try to assign multiple types/interfaces to the key signature, I get this error. I know I can use [key: string]: any. But, I don't want to generalize it. Is there any possible way to make this work?

Link to TypeScript Playground

Why do you have [key: string]: string | IDetails? without it what you're trying would just work. Are you expecting more properties? — Nitzan Tomer
– Nitzan Tomer, Commented Dec 17, 2016 at 13:18
@NitzanTomer This is a simple use case. The t is the example will be dynamic. So, if I need to access the properties, I need to do this. — Pranesh Ravi
– Pranesh Ravi, Commented Dec 17, 2016 at 13:21

Nitzan Tomer · Accepted Answer · 2016-12-17 13:30:09Z

4

If you're going to dynamically access the properties of details, then you better use type guards:

function isDetails(obj: string | IDetails): obj is IDetails {
    return Object.keys(obj).length === 2 && typeof obj["name"] === "string" && typeof obj["age"] === "number";
}

Then you can do:

if (isDetails(details[t])) {
    // details[t].name should be fine with the compiler
}

answered Dec 17, 2016 at 13:30

Nitzan Tomer

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