Item 32: Avoid Including null or undefined in Type Aliases

Things to Remember

• Avoid defining type aliases that include null or undefined.

Code Samples

function getCommentsForUser(comments: readonly Comment[], user: User) {
  return comments.filter(comment => comment.userId === user?.id);
}

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---

type User = { id: string; name: string; } | null;

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---

interface User {
  id: string;
  name: string;
}

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---

type NullableUser = { id: string; name: string; } | null;

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function getCommentsForUser(comments: readonly Comment[], user: User | null) {
  return comments.filter(comment => comment.userId === user?.id);
}

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---

type BirthdayMap = {
  [name: string]: Date | undefined;
};

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type BirthdayMap = {
  [name: string]: Date | undefined;
} | null;

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