Javascript - Group array of objects by date using lambdas

Question

I have an array of message objects, like this one:

const messages = [
   {
     id: "messageId",
     text: "Hello!",
     date: new Date("2021/01/01")
   },
   {
     id: "messageId",
     text: "Same day!",
     date: new Date("2021/01/01")
   },
   {
     id: "messageId",
     text: "Another day!",
     date: new Date("2021/01/02")
   },
];

As you can see, every object has a native JS Date. How can I group all those objects in an array, by day, using JavaScript lambdas? This is the expected result:

 [
   {
     day: new Date("2021/01/01"),
     data: [
       {
          id: "messageId",
          text: "Hello!",
       },
       {
          id: "messageId",
          text: "Same day!",
       },
     ]
   },
   {
     day: new Date("2021/01/02"),
     data: [
       {
          id: "messageId",
          text: "Another day!",
       },
     ]
   },
];

What do you mean by `native JS Date`? BTW The code block you wrote is invalid javascript :) — a--m, Jan 19 '21 at 21:00
https://stackoverflow.com/questions/46802448/how-do-i-group-items-in-an-array-by-date/46802505 — silverfighter, Jan 19 '21 at 21:06

Nenad Vracar · Accepted Answer · 2021-01-19T21:21:28.657

You could use reduce object to first create object and group by date (year-month-day) and then use Object.values to get an array from that object.

const messages = [
   {
     id: "messageId",
     text: "Hello!",
     date: new Date("2021/01/01")
   },
   {
     id: "messageId",
     text: "Same day!",
     date: new Date("2021/01/01")
   },
   {
     id: "messageId",
     text: "Another day!",
     date: new Date("2021/01/02")
   },
];

const object = messages.reduce((r, { date: day, ...rest }) => {
  const key = `${day.getFullYear()}-${day.getMonth()}-${day.getDate()}`
  if (!r[key]) r[key] = {day, data: [rest]}
  else r[key].data.push(rest)
  return r;
}, {})

const result = Object.values(object)

console.log(result)

Javascript - Group array of objects by date using lambdas

1 Answers1