countBy_.countBy(list,iteratee,[context]) 
Sorts a list into groups and returns a count for the number of objects in each group. Similar to groupBy,but instead of returning a list of values,returns a count for the number of values in that group.

_.countBy([1,2,3,4,5],function(num) {
  return num % 2 == 0 ? 'even': 'odd';
});
=> {odd: 3,even: 2}

自己例子：

const ck = _.countBy(this.handlingUnitMaterials,function (hu) {
      return hu.checked == true ? 'ischecked' : 'nochecked';
    });