locutus/esm/elixir/Enum/group_by.js

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

export function group_by(values, keySelector, valueSelector) {
    //      discuss at: https://locutus.io/elixir/group_by/
    // parity verified: Elixir 1.18
    //     original by: Kevin van Zonneveld (https://kvz.io)
    //          note 1: Groups values by key selector, similar to Elixir Enum.group_by/3.
    //       example 1: group_by(['one', 'two', 'three'], (value) => value.length)
    //       returns 1: {3: ['one', 'two'], 5: ['three']}
    //       example 2: group_by([1, 2, 3, 4], (value) => (value % 2 === 0 ? 'even' : 'odd'))
    //       returns 2: {odd: [1, 3], even: [2, 4]}
    //       example 3: group_by([1, 2, 3], (value) => (value % 2 === 0 ? 'even' : 'odd'), (value) => value * 10)
    //       returns 3: {odd: [10, 30], even: [20]}
    if (typeof keySelector !== 'function') {
        throw new TypeError('group_by(): keySelector must be a function');
    }
    if (valueSelector !== undefined && typeof valueSelector !== 'function') {
        throw new TypeError('group_by(): valueSelector must be a function');
    }
    if (!Array.isArray(values)) {
        return {};
    }
    const out = {};
    for (let i = 0; i < values.length; i++) {
        const value = values[i];
        const key = String(keySelector(value, i, values));
        const mapped = valueSelector ? valueSelector(value, i, values) : value;
        const bucket = out[key] ?? [];
        bucket.push(mapped);
        out[key] = bucket;
    }
    return out;
}