ts-02-groupby

1.000

6/6 tests· typing

Proposed solution

```ts
export function groupBy<T, K extends string | number>(
  items: T[],
  keyFn: (item: T) => K,
): Record<K, T[]> {
  const result: Partial<Record<K, T[]>> = {};
  
  for (const item of items) {
    const key = keyFn(item);
    if (!result[key]) {
      result[key] = [];
    }
    result[key]?.push(item);
  }
  
  return result as Record<K, T[]>;
}
```

Test output (stdout)

TAP version 13
# Subtest: groups by string key, preserving order
ok 1 - groups by string key, preserving order
  ---
  duration_ms: 0.584773
  type: 'test'
  ...
# Subtest: empty input yields empty record
ok 2 - empty input yields empty record
  ---
  duration_ms: 0.06379
  type: 'test'
  ...
# Subtest: groups by numeric key
ok 3 - groups by numeric key
  ---
  duration_ms: 0.065971
  type: 'test'
  ...
# Subtest: groups objects by a property
ok 4 - groups objects by a property
  ---
  duration_ms: 0.07055
  type: 'test'
  ...
# Subtest: single group when keyFn is constant
ok 5 - single group when keyFn is constant
  ---
  duration_ms: 0.056781
  type: 'test'
  ...
# Subtest: does not mutate the input array
ok 6 - does not mutate the input array
  ---
  duration_ms: 0.04701
  type: 'test'
  ...
1..6
# tests 6
# suites 0
# pass 6
# fail 0
# cancelled 0
# skipped 0
# todo 0
# duration_ms 76.610561

System prompt

You are an expert programmer. Solve the task exactly as specified. Output your solution as fenced code blocks using the required file name(s) and the exact function/type signatures requested. Prefer correctness; do not include prose outside code unless asked.