Partition an array based on a condition

Split items into [matching, not matching] in one pass.

JavaScriptNode.js & browsers Version 1.0.0 · MIT · .md

function partition(values, predicate) { return values.reduce((groups, value) => (groups[predicate(value) ? 0 : 1].push(value), groups), [[], []]); }

Usage

partition([1, 2, 3, 4, 5], n => n % 2); // → [[1, 3, 5], [2, 4]]
const events = [{ to: 'ana@example.com', status: 'delivered' }, { to: 'rui@example.com', status: 'bounced' }];
const [delivered, other] = partition(events, event => event.status === 'delivered');
other; // → [{ to: 'rui@example.com', status: 'bounced' }]

Lines marked → were executed in Node.js 22 and in headless Chromium, and each result was compared with the value shown. Lines marked e.g. are illustrative.

Know the limits

• The predicate’s result is tested for truthiness: n % 2 puts odd numbers first.
• Object.groupBy (ES2024) is the built-in for more than two groups.

The function above is the code the usage lines run. It does not validate its arguments; add checks where it receives untrusted input.

Keep exploring

• Clone an array — Create a new array without copying the objects inside it.
• Remove duplicate values — Keep the first occurrence of each value, in its original order.
• Flatten a nested array — Turn a nested array into one sequence of values.