JS对可迭代对象的聚合处理

enumerable_group.js

function frequencies_by(enumerable, key_fn) {
    key_fn = key_fn || (x => x);
    const result = {};
    enumerable.forEach(e => {
        const key = key_fn(e);
        result[key] = result[key] || 0;
        result[key] += 1;
    });
    return result;
}

function group_by(enumerable, key_fn, value_fn) {
    value_fn = value_fn || (x => x);
    const groups = {};
    enumerable.forEach(e => {
        const group = key_fn(e);
        groups[group] = groups[group] || [];
        groups[group].push(value_fn(e));
    });
    return groups;
}