RefinementWithIndex - API reference

API reference › @evolu/common › Types › RefinementWithIndex

type RefinementWithIndex<A, B> = (a: A, index: number) => a is B;

Defined in: packages/common/src/Types.ts:160

A type guard function that refines type A to a narrower type B at a given index.

Useful for callbacks that need both the element and its position while maintaining type narrowing.

Indexed refinement

import { partitionArray, type RefinementWithIndex } from "@evolu/common";

type Item = {
  readonly type: "number" | "string";
  readonly value: unknown;
};
type NumberItem = Item & { readonly type: "number" };

const isNumberItem: RefinementWithIndex<Item, NumberItem> = (
  item,
  index,
): item is NumberItem => index > 0 && item.type === "number";
const items: ReadonlyArray<Item> = [
  { type: "number", value: 1 },
  { type: "number", value: 2 },
];
const [numbers, others] = partitionArray(items, isNumberItem);

expectTypeOf(numbers).toEqualTypeOf<ReadonlyArray<NumberItem>>();
expect(numbers[0]?.value).toBe(2);
expect(others[0]?.value).toBe(1);