Generic group by in TypeScript

Group by a key function, with the return type derived from the key.

function groupBy<T, K extends string>(
  items: T[],
  keyFn: (item: T) => K,
): Record<K, T[]> {
  const groups = {} as Record<K, T[]>;
  for (const item of items) {
    const key = keyFn(item);
    (groups[key] ??= []).push(item);
  }
  return groups;
}

Keywords and builtins used here

• T
• as
• const
• extends
• for
• function
• of
• return
• string

The run, in numbers

Lines

11

Characters to type

235

Tokens

88

Three-star pace

105 tpm

At the three-star pace of 105 tokens a minute, this run takes about 50 seconds.

Type this snippet

Step 2 of 11 in Generic helpers, step 10 of 20 in Generics.

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