Partition by predicate in TypeScript

Splits an array in two by a predicate, narrowing the type when the predicate is a guard.

function partition<T>(
  items: T[],
  predicate: (item: T) => boolean,
): [T[], T[]] {
  const pass: T[] = [];
  const fail: T[] = [];
  for (const item of items) {
    (predicate(item) ? pass : fail).push(item);
  }
  return [pass, fail];
}

Keywords and builtins used here

• T
• boolean
• const
• fail
• for
• function
• of
• return

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Step 5 of 11 in Generic helpers, step 13 of 20 in Generics.

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