locutus/esm/haskell/list/groupBy.js

Locutus other languages' standard libraries to JavaScript for fun and educational purposes

export function groupBy(items, predicate) {
    //  discuss at: https://locutus.io/haskell/list/groupBy/
    // original by: Kevin van Zonneveld (https://kvz.io)
    //      note 1: Groups adjacent elements while predicate(prev, current) is true, like Haskell Data.List.groupBy.
    //   example 1: groupBy([1, 1, 2, 3, 3, 3, 2], (a, b) => a === b)
    //   returns 1: [[1, 1], [2], [3, 3, 3], [2]]
    //   example 2: groupBy(['a', 'A', 'b', 'B'], (a, b) => a.toLowerCase() === b.toLowerCase())
    //   returns 2: [['a', 'A'], ['b', 'B']]
    //   example 3: groupBy([], (a, b) => a === b)
    //   returns 3: []
    if (!Array.isArray(items) || items.length === 0) {
        return [];
    }
    const eq = typeof predicate === 'function' ? predicate : (left, right) => Object.is(left, right);
    const out = [[items[0]]];
    for (let i = 1; i < items.length; i++) {
        const prev = items[i - 1];
        const current = items[i];
        const bucket = out[out.length - 1];
        if (bucket && eq(prev, current)) {
            bucket.push(current);
        }
        else {
            out.push([current]);
        }
    }
    return out;
}