locutus/esm/haskell/list/nubBy.js

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

export function nubBy(items, eq) {
    //  discuss at: https://locutus.io/haskell/list/nubBy/
    // original by: Kevin van Zonneveld (https://kvz.io)
    //      note 1: Keeps the first occurrence of each element using a custom equality predicate, like Haskell Data.List.nubBy.
    //   example 1: nubBy([1, 2, 2, 3, 1], (a, b) => a === b)
    //   returns 1: [1, 2, 3]
    //   example 2: nubBy(['aa', 'ab', 'ba'], (a, b) => a.charAt(0) === b.charAt(0))
    //   returns 2: ['aa', 'ba']
    //   example 3: nubBy([], (a, b) => a === b)
    //   returns 3: []
    if (!Array.isArray(items)) {
        return [];
    }
    const equals = typeof eq === 'function' ? eq : (left, right) => Object.is(left, right);
    const out = [];
    for (const item of items) {
        const duplicate = out.some((existing) => equals(item, existing));
        if (!duplicate) {
            out.push(item);
        }
    }
    return out;
}