Why does lodash's _.sum() function fail Vuejs objects

Question

I'm using Vuejs and Lodash and I have a computed property that simply sums a property of a collection. I've previously used _.sum() for this but when I use that with a vuejs vm it seems to just concatenate the string [object Object]!

I've made a jsfiddle with the following code:

new Vue({
  el: "#app",
  data: {
    todos: [
      { text: "Learn JavaScript", time: 5 },
      { text: "Learn Vue", time: 10 },
      ]
  },
  computed: {
    additup() { return _.sum(this.todos, todo => todo.time ) },
    additup2() { return _.sum(this.todos, function (todo) {
      return parseInt(todo.time);
    })} },
    additup3() { 
      var t = 0; 
      _.each(this.todos, function(todo) { t+=todo.time; }); 
      return t; 
    }
  }
})

The output is:

Method 1 gives: [object Object][object Object]
Method 2 gives: [object Object][object Object]
Method 3 gives: 15

Is there a way to fix _.sum()? Or a way to fix my understanding of why it's not working ;-)

Answer 1

Use sumBy instead of sum:

This method is like _.sum except that it accepts iteratee which is invoked for each element in array to generate the value to be summed. The iteratee is invoked with one argument: (value).