How to write typescript declaration file for this object?

Question

Here is the javascript code:

const obj = {
  result: {},
  add(key, value) {
    this.result[key] = value;
  }
};

obj.add('test', 1);

How to write a typescript declaration for obj to let vscode know that now obj.result has a property named "test" whose type is number?

kaznovac · Accepted Answer · 2020-04-20 14:23:45Z

1

you probably can't (i'm unaware of constructs that can achieve this in v3.8), there is not ts-type information in runtime (if you want to determine type of result property based on calls of add function).

what you can do is create generic definition of obj that ties value argument type to result property structure.

type MyType<T> = {
  result: Record<string, T>;
  add: (key: string, value: T) => void;
}

answered Apr 20, 2020 at 14:23

kaznovac

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