locutus/esm/elixir/Enum/frequencies_by.js

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

export function frequencies_by(values, selector) {
    //      discuss at: https://locutus.io/elixir/frequencies_by/
    // parity verified: Elixir 1.18
    //     original by: Kevin van Zonneveld (https://kvz.io)
    //          note 1: Counts items by selector key, similar to Elixir Enum.frequencies_by/2.
    //       example 1: frequencies_by(['a', 'aa', 'b'], (value) => value.charAt(0))
    //       returns 1: {a: 2, b: 1}
    //       example 2: frequencies_by([1, 2, 3, 4], (value) => (value % 2 === 0 ? 'even' : 'odd'))
    //       returns 2: {odd: 2, even: 2}
    //       example 3: frequencies_by([], (value) => String(value))
    //       returns 3: {}
    if (typeof selector !== 'function') {
        throw new TypeError('frequencies_by(): selector 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(selector(value, i, values));
        out[key] = (out[key] ?? 0) + 1;
    }
    return out;
}