Adding dynamic fields to a Typescript type/interface?

Question

I have a Typescript interface that is just a set of fields, e.g.

export interface Data {
   date_created: string;
   stamp: string;
 }
let myData: Data;

However, my case requires me to add "dynamic" fields that I can not hard-code before runtime, so I should be write something like

const dynamicFieldname = getDynamicFieldNameFromSomeDataSource(); // type is string. 
mydata[dynamicFieldname] = dynamicFieldvalue;

when I write this, I get a Typescript Error:

Error: TS7017: Element implicitly has an 'any' type because type 'Data' has no index signature.

How can I achieve the possibility to have these dynamic fields in my Typescript object, such as, how can I add the required 'index signature' to an interface?

I think this should answer your question.

Robert Kossendey
– Robert Kossendey

2020-09-03 12:13:20 +00:00
Commented Sep 3, 2020 at 12:13 — Robert Kossendey
– Robert Kossendey, Commented Sep 3, 2020 at 12:13
You can go let myData: Data & { [k: string]: any };

hackape
– hackape

2020-09-03 12:35:40 +00:00
Commented Sep 3, 2020 at 12:35 — hackape
– hackape, Commented Sep 3, 2020 at 12:35

Zer0 · Accepted Answer · 2020-09-03 12:14:02Z

2

I think what you're looking for will look like this:

interface data {
    [key: string]: string;
}

NOTE: this won't allow you to keep the date_created and stamp fields, as those are also strings (basically if you use [key: string]: T, you can't use other hardcoded properties of type T anymore).

answered Sep 3, 2020 at 12:14

Zer0

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7 Comments

hackape Over a year ago

let myData: Data & { [k: string]: any }; can keep the fields.

Zer0 Over a year ago

Indeed, but you should avoid any types at all costs

onkami Over a year ago

@hackape can I declare a new type like this?

Zer0 Over a year ago

yes using type extendedData = Data & { [k: string]: any };, however as I previously said: the any type should be avoided @AskarIbragimov

hackape Over a year ago

At all cost? That I’d disagree "at all cost". How could dynamicValue be typed then, if it’s indeed dynamic, like, from user input whatsoever. any is a type too, you just need to understand what you’re doing with that code.

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