// before
let objArr = [
    {
        date: '01/01/2018'
        total: 1
    },
    {
        date: '01/01/2018'
        total: 2
    },
    {
        date: '01/02/2018'
        total: 3
    },
    {
        date: '01/02/2018'
        total: 4
    },
    ...
]

// final result
let finalArr = [
    {
        date: '01/01/2018'
        total: 3
    },
    {
        date: '01/02/2018'
        total: 7
    },
    ...
]

objArr.reduce((acc, obj) => {
      acc.set(obj.date, (acc.get([obj.date]) || 0) + obj.total);
      return acc;
    }, new Map())

// bad output
badArray = [
    ...,
    {
        date: '01/02/2018'
        total: 4
    },
    {
        date: undefined
        total: NaN
    },
    {
        date: undefined
        total: NaN
    }
]