Is it possible in TypeScript to declare type of the properties in an object if those are dynamically added?

Question

I have a method like this.

private getStuff(data: string[]) : CertainType[] { ... }

I decided that it makes more sense to have the output in form of a dictionary (C#'ish) than an array. As far my google-foo tells me, I should use an object and put in stuff into it with the ID as the fields' names. So instead of:

const things: Thing[]; ...
things = [];
output.push(thingy);
console.log(things[3]);

I'm doing this:

const stuff: any; ...
stuff = { };
stuff[3] = stuffy;
console.log(things[3]);

So the signature of the method changed to this.

private getStuff(data: string[]) : any { ... }

It all works just dandy but. I like the semi-hard typed variables in TypeScript and coming from C# I strongly believe in avoiding a lot of issues by being (seemingly) nit-picky in advance (without getting within range of PO. of course).

Since the number of properties isn't known in advance, nor are their IDs, I wonder if any is the closest type setting or if there's something corresponding to the syntax of C# as:

Dictionary<string, Thing> dicky;

Titian Cernicova-Dragomir · Accepted Answer · 2019-04-23 07:46:27Z

2

If you need an object that can be indexed by any string but all values in the object are of a certain type you need a type with an index signature.

This can be written as

const stuff: Record<string, Thingy> = {};

Or, more verbosely, but equivalent:

const stuff: { [index: string]: Thingy } = {};

answered Apr 23, 2019 at 7:46

Titian Cernicova-Dragomir

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5 Comments

Konrad Viltersten Over a year ago

Like they say in Amazing Grace: "Oh, I was blind but now I C#"... Perfect! I'm going refactor me some code right away.

Titian Cernicova-Dragomir Over a year ago

@KonradViltersten glad to hear, don't forget to upvote and mark as answered :)

Konrad Viltersten Over a year ago

Don't be so quick with the reply, then. I was hitting "accept" but it told me to wait for 59 seconds. You are simply too helpful. :)

Konrad Viltersten Over a year ago

Right, as we're waiting for the time block to accept - in the first sample you're assuming that I have a class called Record, right? If I don't have a specialized class for it, I have to use the verbose, syntax. Is that correctly understood?

Titian Cernicova-Dragomir Over a year ago

@KonradViltersten no, Record is predefined, you can use it. They are the same thing essentially Record<string, Thingy> == { [index: string]: Thingy }

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