How to return sum of last month values from array of objects

Question

I have an array of 50 objects with value and ISOString date format.

const totalIncome = [ 
  {value: '5419.59',
   date: '2019-11-20T00:03:09.496Z'},
  {value: '1242.41',
   date: '2019-12-08T03:08:30.016Z'},
  {value: '3242.41',
   date: '2019-12-08T03:08:30.016Z'},
      ...
];

How can I return sum of all values that are from last month (all values from December)? I want to do it in vanilla JS.

By 'last month' do you mean in the last 30 days, or from the previous month, e.g. all values from November if you're in December? — ldtcoop, Dec 31 '19 at 16:22
This question has been asked and answered many times on Stack Overflow. Did you find them in your research? — Heretic Monkey, Dec 31 '19 at 16:28

ed' · Accepted Answer · 2019-12-31T16:55:44.847

Set the month and year you care about
Filter the array down to objects inside that month & year
Reduce the array down to the sum of all remaining value

Keep in mind that the below is just for demonstration purposes, new Date may not always parse date strings in the way you expect and there are timezones to consider. If you have full ISO strings and consider last month to respect the local timezone of the device running this script, then it might be ok.

const totalIncome = [{
    value: '5419.59',
    date: '2019-11-20T00:03:09.496Z'
  },
  {
    value: '1242.41',
    date: '2019-12-08T03:08:30.016Z'
  },
  {
    value: '3242.41',
    date: '2019-12-08T03:08:30.016Z'
  },
];

const today = new Date();
let lastMonth = today.getMonth() - 1;
let relevantYear = today.getYear();

if (lastMonth === -1) {
  relevantYear--;
  lastMonth = 11;
}

const total = totalIncome.filter(i => {
  const date = new Date(i.date);

  return date.getMonth() === lastMonth && date.getYear() === relevantYear;
}).reduce((prev, curr) => prev + parseFloat(curr.value), 0);

console.log(total);

score 0 · Answer 2 · answered Dec 31 '19 at 16:30

0

var sum = lastMonthIncome.reduce((accVal, crntObj) => {
  if (crntObj.date.includes("2019-11")) {
    return accVal + crntObj.value;
  } else {
    return accVal;
  }
}, 0);

here you can change includes("2019-11") as per your required month.

answered Dec 31 '19 at 16:30

Ayyappa Gollu

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How to return sum of last month values from array of objects

2 Answers2