JavaScript之纯数组按指定特征分组、map、reduce、typeof、concat、length

数组分组，将传入的数据按指定特征进行分组
输入：数组值，函数或属性
输出：对象 key:数组所有元素经过处理后的值 value:相同key的数组元素集合

let arrFloatingPointNumber = [7.7, 5.2, 7.3],
    arrLength = ['one', 'two', 'weet', 'thren'];

function groupBy(arr, fn) {
    return arr.map(item => {
        if (typeof fn === 'function') {
            return fn(item);
        } else {
            return item[fn];
        }
    }).reduce((pre, cur, i) => {
        if (!pre[cur]) {
            pre[cur] = [arr[i]];
        } else {
            pre[cur] = pre[cur].concat(arr[i]);
        }
        return pre;
    }, {});
}

console.log('arrFloatingPointNumber:', groupBy(arrFloatingPointNumber, Math.floor));
// arrFloatingPointNumber: {5: Array(1), 7: Array(2)}
console.log('arrLength:', groupBy(arrLength, 'length'));
// arrLength: {3: Array(2), 4: Array(1), 5: Array(1)}