Type that is a generic object derived from the keyof keyword

Question

Type that is a generic object derived from the keyof keyword

I am trying to create a generic function that will only update values of type object within a map, and ignore anything else.

Is it possible to achieve this? I have looked for a solution but haven't found one yet.

enum SKey {
  key1 = "key1",
  key2 = "key2",
}

type SItem = {
  [SKey.key1]: boolean;
  [SKey.key2]: {
    attr1: string;
    attr2: boolean;
  };
};

const mapDB: SItem = {
  [SKey.key1]: true,
  [SKey.key2]: {
    attr1: "hello",
    attr2: true,
  },
};

export function updateDbItem<K extends keyof SItem>(
  key: K,
  newFields: Partial<SItem[K]>
) {
  const item: SItem[K] = mapDB[key];
  mapDB[key] = { ...item, ...newFields };
}

However, I am encountering the following error:

error TS2698: Spread types may only be created from object types.

mapDB[key] = { ...item, ...newFields };
              ~~~~~~~~

Would appreciate any help or suggestions on how to resolve this issue. Thank you!

typescript object generics types keyof

Answer 1

Answer №1

To create a union of keys with object-type values, you can utilize a mapped type.

In the provided code snippet, only SKey.key2 is included based on the current entries in SItem. Adding more entries to SItem would result in additional keys meeting the criteria being included.

Here is an example:

TS Playground

type SItemObjectKey = keyof {
  [
    K in keyof SItem as SItem[K] extends Record<string, unknown>
      ? K
      : never
  ]: unknown;
};

export function updateDbItem<K extends SItemObjectKey>(
  key: K,
  newFields: Partial<SItem[K]>
) {
  const item: SItem[K] = mapDB[key];
  mapDB[key] = { ...item, ...newFields }; // Ok now!
}

For reference, check out the utility type Record<Keys, Type>

Answer 2

To create a union of keys with object-type values, you can utilize a mapped type.

In the provided code snippet, only SKey.key2 is included based on the current entries in SItem. Adding more entries to SItem would result in additional keys meeting the criteria being included.

Here is an example:

TS Playground

type SItemObjectKey = keyof {
  [
    K in keyof SItem as SItem[K] extends Record<string, unknown>
      ? K
      : never
  ]: unknown;
};

export function updateDbItem<K extends SItemObjectKey>(
  key: K,
  newFields: Partial<SItem[K]>
) {
  const item: SItem[K] = mapDB[key];
  mapDB[key] = { ...item, ...newFields }; // Ok now!
}

For reference, check out the utility type Record<Keys, Type>

Type that is a generic object derived from the keyof keyword

Answer №1

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