Proposal: PartialRecord

[twrayden]

type PartialRecord<K extends keyof any, T> = {
  [P in K]?: T;
};

In the scenario below PartialRecord is more typesafe then Record:

Record:

type User = {
  id: string;
  isCool: boolean;
};

const usersById: Record<string, User> = {};

function loadUsers() {
  const users = [{ id: 'bob', isCool: true }]; // from API or other source
  users.forEach(user => {
    usersById[user.id] = user;
  });
}
loadUsers();

const user = usersById['sam']; // Type is User, user is undefined.
user.isCool // Runtime error, user is not defined.

PartialRecord:

const usersById: PartialRecord<string, User> = {};

// **same as above**

const user = usersById['sam']; // Type is User | undefined, user is undefined.
user.isCool // Typescript compile error, user is not defined.

By using PartialRecord you are forced to handle the undefined case.

You can technically achieve the same thing with Partial<Record<string, User>>. But remembering to wrap Record in Partial feels cumbersome to me and is a bit of an eyesore. There is also the potential for a better shorter name.

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[twrayden]

Found the tsconfig option noUncheckedIndexedAccess which removes the need for this.

microsoft/TypeScript#39560