How to combine two javascript array into one and group by date

Question

Hello I'am new to programming and I stumble upon on grouping array data by date from two arrays.

here is my arrays:

header = [
    {"2019-04-22": "Sun, Apr 22, 2019"},
    {"2019-04-21": "Sat, Apr 21, 2019"},
]

body = [
    {"2019-04-22": "doing customer support”},
    {"2019-04-22": "reply to emails"},
    {"2019-04-21": "send message to customers"},
]

How do I group the arrays into one array as example below

combinearray = {
    "2019-04-22": [
        "Sun, Apr 22, 2019",
        "doing customer support",
        "reply to emails",
    ],
    "2019-04-21": [
        "Sat, Apr 21, 2019",
        "send message to customers",
    ],
}

Grouping two array data by date seems completely not easy for me I'm a beginner to javascript programming. I would appreciate any answers.

combinearray should be obj wrapped in {} instead of [] — Maheer Ali
– Maheer Ali, Commented Apr 27, 2019 at 4:07
The desired output seems like a slight smell to me. Is there any reason that each date is an array of objects, rather than just "2019-04-21": [ "Sat, Apr 21, 2019", "send message to customers"]? You repeat the key for every single object - what is the point of an object if the only portion you care about is the string? — Tyler Roper
– Tyler Roper, Commented Apr 27, 2019 at 4:14
I'm asking why you want an object structure that nests and repeats data where it doesn't seem to be at all necessary. I understand what you want, I just think you should consider it a bit. If you have a good reason for it, then sure, just pointing out that it seems redundant. You've made an array of key-value pairs where the key (seemingly) serves no purpose. — Tyler Roper
– Tyler Roper, Commented Apr 27, 2019 at 4:18

Maheer Ali · Accepted Answer · 2019-04-27 04:25:57Z

You can do that in following steps:

First use concat() to combine both arrays i.e header and body
Then use reduce() on that. And pass empty object as second argument(the initial value of accumulator).
In inside reduce() callback use Object.keys()[0] to get date.
Check if the date if date is not already key of accumulator set it to empty [].
Use push() to add the elements to the array.

Note: This will not remove reference to the real object in header and body.

const header = [ {"2019-04-22": "Sun, Apr 22, 2019"}, {"2019-04-21": "Sat, Apr 21, 2019"} ]
const body = [ {"2019-04-22": "doing customer support"}, {"2019-04-22": "reply to emails"}, {"2019-04-21": "send message to customers"}, ]

const res = header.concat(body).reduce((ac,a) => {
  let key = Object.keys(a)[0];
  ac[key] = ac[key] || [];
  ac[key].push(a)
  return ac;
},{})
console.log(res)

However as mentioned in the comments there is no need to have object with keys. Just simple array of the values of that key are enough. For that push() a[key] instead of a.

const header = [ {"2019-04-22": "Sun, Apr 22, 2019"}, {"2019-04-21": "Sat, Apr 21, 2019"} ]
const body = [ {"2019-04-22": "doing customer support"}, {"2019-04-22": "reply to emails"}, {"2019-04-21": "send message to customers"}, ]

const res = header.concat(body).reduce((ac,a) => {
  let key = Object.keys(a)[0];
  ac[key] = ac[key] || [];
  ac[key].push(a[key])
  return ac;
},{})
console.log(res)

Code Maniac · Accepted Answer · 2019-04-27 04:22:08Z

You can use combine arrays then use reduce

used spread syntax to merge arrays
use reduce to build an object in desired format
Object.entries to get date and it's respective value
Check if the date is already present as key on object or not, if it's already present push the value to it else create a new key

let header = [{"2019-04-22": "Sun, Apr 22, 2019"},{"2019-04-21": "Sat, Apr 21, 2019"},]

let body = [{"2019-04-22": "doing customer support"},{"2019-04-22": "reply to emails"},{"2019-04-21": "send message to customers"},]


let final = [...header,...body].reduce((op,inp) => {
  let [key,value] = Object.entries(inp)[0]
  op[key] = op[key] || []
  op[key].push(value)
  return op
},{})

console.log(final)

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