⚙ Compilation target

es2022

⚙ Library

lib.es2024.object

Missing / Incorrect Definition

The type definitions for Object.groupBy<K, T> state that it returns a Partial<Record<K, T>>. This is useful if K is a union, enum, etc. - the result is is likely Partial, and the result is consistent with a Partial<Record<K, T>> literal, and having Partial helps prevent mistakes from assuming that every key is present in the resulting record. However, it's unnecessary if K is a unrestricted number or string: there's no way that a declared key within the result can have an undefined value, and the result is more consistent with a Record<string | number, T> literal than a Partial<Record<string | number, T>> literal, and any mistakes are more consistently dealt with using TypeScript's noUncheckedIndexedAccess option.

See #56805 (comment) and #56805 (comment) from the PR that originally added types for Object.groupBy.

Sample Code

type Employee = { name: string, role: 'ic' | 'manager' };
const employees: Set<Employee> = new Set();

const byName = Object.groupBy(employees, x => x.name);
for (const [name, nameGroup] of byName) {
  // TypeScript gives an error that nameGroup may be undefined,
  // but it's always defined
  console.log(nameGroup.length);
}

Documentation Link

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/groupBy