overloading indexer in typescript with string key and numeric key

Question

I am trying to add some types to some old code.

There is a transitionData object, that might have objects added by index like this:

this.transitionData[id] = transition

where id is a number and transition is a Transition type.

Or it might be:

transitions[t].timer.stop()

Where t is a string and timer is of type Timer.

I would like this interface:

export interface TransitionData {
  [index: number]: Transition
  [key: string]: {timer: Timer}
}

but typescript complains:

Numeric index type 'Transition' is not assignable to string index type '{ timer: Timer; }

Karol Majewski · Accepted Answer · 2019-03-02 17:33:06Z

1

Create two index signatures and merge them together.

interface TransitionMap {
    [index: number]: Transition
}

interface TimerMap {
    [index: string]: { timer: Timer }
}

declare const transitionData: TransitionMap & TimerMap;

TypeScript will understand what goes where.

transitionData["1"] // Timer
transitionData[1]   // Transition

answered Mar 2, 2019 at 17:33

Karol Majewski

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1 Comment

Titian Cernicova-Dragomir Over a year ago

The reason TS is not happy about this is that the way indexing works in JS, transitionData["1"] === transitionData[1] (ex: typescript-play.js.org/#code/…)

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