Group same elements in JS array, but only when consecutive

Question

I have an array of objects representing a series of 'messages' in a chat. I want to group messages that are consecutively created by the same 'user', which is stored in each array item. Here's a simplified version of what I'm trying to do:

[
    { message: "One", user: "Bob" },
    { message: "Two", user: "Bob" },
    { message: "Three", user: "Bob" },
    { message: "Hello", user: "Sam" },
    { message: "Hello", user: "Bob" },
    { message: "Hello", user: "Sam" },
    { message: "Hello", user: "Sam" }
]

should be turned into:

[
    [
        { message: "One", user: "Bob"},
        { message: "Two", user: "Bob" },
        { message: "Three", user: "Bob" }
    ],
    [
        { message: "Hello", user: "Sam" }
    ],
    [
        { message: "Hello", user: "Bob" }
    ],
    [
        { message: "Hello", user: "Sam" },
        { message: "Hello", user: "Sam" }
    ]
]

Is there any easy way to do this? I couldn't find an answer/algorithm anywhere.

KARASZI István · Accepted Answer · 2020-06-07 20:46:08Z

12

You can do that with the reduce method.

const data = [
  { message: "One", user: "Bob" },
  { message: "Two", user: "Bob" },
  { message: "Three", user: "Bob" },
  { message: "Hello", user: "Sam" },
  { message: "Hello", user: "Bob" },
  { message: "Hello", user: "Sam" },
  { message: "Hello", user: "Sam" }
];

const result = data.reduce((acc, value) => {
  // compare the current value with the last item in the collected array
  if (acc.length && acc[acc.length - 1][0].user == value.user) {
    // append the current value to it if it is matching
    acc[acc.length - 1].push(value);
  } else {
    // append the new value at the end of the collected array
    acc.push([value]);
  }

  return acc;
}, []);
  
console.log(result);

edited Jun 7, 2020 at 20:46

answered Nov 18, 2018 at 17:44

KARASZI István

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Comments

Jordan Soltman · Accepted Answer · 2018-11-18 17:42:43Z

2

You could do something like this:

const messages = [
    { message: "One", user: "Bob" },
    { message: "Two", user: "Bob" },
    { message: "Three", user: "Bob" },
    { message: "Hello", user: "Sam" },
    { message: "Hello", user: "Bob" },
    { message: "Hello", user: "Sam" },
    { message: "Hello", user: "Sam" }
]

let currentUser;
let groupedMessages = [];

for (message of messages) {

  if (message.user !== currentUser) {
    groupedMessages.push([]);
    currentUser = message.user;
  }
  groupedMessages[groupedMessages.length - 1].push(message)
}

console.log(groupedMessages);

answered Nov 18, 2018 at 17:42

Jordan Soltman

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Comments

Nina Scholz · Accepted Answer · 2018-11-18 17:54:36Z

1

You could check the last element and if it has not the same user, then create a new array.

var data = [{ message: "One", user: "Bob" }, { message: "Two", user: "Bob" }, { message: "Three", user: "Bob" }, { message: "Hello", user: "Sam" }, { message: "Hello", user: "Bob" }, { message: "Hello", user: "Sam" }, { message: "Hello", user: "Sam" }],
    grouped = data.reduce((r, o, i, a) => {
        if (!i || a[i - 1].user !== o.user) {
            r.push([]);
        }
        r[r.length - 1].push(o);
        return r;
    }, []);
    
console.log(grouped);

edited Nov 18, 2018 at 17:54

answered Nov 18, 2018 at 17:43

Nina Scholz

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Comments

Akrion · Accepted Answer · 2018-11-18 18:51:03Z

1

You can solve this with just Array.reduce and using its parameters for index of the array and the actual array:

const data = [{ message: "One", user: "Bob" }, { message: "Two", user: "Bob" }, { message: "Three", user: "Bob" }, { message: "Hello", user: "Sam" }, { message: "Hello", user: "Bob" }, { message: "Hello", user: "Sam" }, { message: "Hello", user: "Sam" } ]

const group = d => d.reduce((r,c,i,a) => (a[i].user == (a[i-1] && a[i-1].user) 
 ? r[r.length-1].push(c) : r.push([c]), r), [])

console.log(Object.values(group(data)))

answered Nov 18, 2018 at 18:51

Akrion

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