How to set the type for the state object in pinia?

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The type of this.previousPieceSelected is inferred from the initial state, and it’s currently initialized to undefined, so it thus has a type of undefined (meaning it can only be assigned a value of undefined).

  1. Use type assertion on the initial undefined value (i.e., the as keyword plus the desired type of Piece | undefined).

  2. Also note that optional parameters can be specified with ?: instead of | undefined. This is just a simpler way of writing it.

export const useStore = defineStore("game", {
  state: () => {
    return {
      moves: [],
      previousPieceSelected: undefined as Piece | undefined, 1️⃣
    }
  },
  actions: {                          2️⃣
    updatePreviousPieceSelected(piece ?: Piece) {
      this.previousPieceSelected = piece
    }
  }
})

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