locutus/esm/kotlin/collections/groupBy.js

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

export function groupBy(values, selector) {
    //  discuss at: https://locutus.io/kotlin/collections/groupBy/
    // original by: Kevin van Zonneveld (https://kvz.io)
    //      note 1: Groups elements using selector(value), similar to Kotlin's groupBy.
    //   example 1: groupBy([1, 2, 3, 4], (n) => (n % 2 === 0 ? 'even' : 'odd'))
    //   returns 1: {odd: [1, 3], even: [2, 4]}
    //   example 2: groupBy(['ant', 'bear', 'cat'], (s) => s.length)
    //   returns 2: {'3': ['ant', 'cat'], '4': ['bear']}
    //   example 3: groupBy([], () => 'x')
    //   returns 3: {}
    if (!Array.isArray(values) || typeof selector !== 'function') {
        return {};
    }
    const out = {};
    for (let i = 0; i < values.length; i++) {
        const value = values[i];
        const key = String(selector(value, i, values));
        const bucket = out[key];
        if (bucket) {
            bucket.push(value);
        }
        else {
            out[key] = [value];
        }
    }
    return out;
}