javascript - How to split array by duplicate values? - Stack Overflow

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I have an array of objects, and I have to split it by duplicate values on index property. For a sample array of objects like the next one:

[
  {index: 0, value: 3},
  {index: 0, value: 3},
  {index: 0, value: 3},
  {index: 1, value: 3},
  {index: 1, value: 3},
  {index: 2, value: 3},
  {index: 2, value: 3}
]

The expected result should be:

{
  0: [
    {index: 0, value: 3},
    {index: 0, value: 3},
    {index: 0, value: 3}
  ],
  1: [
    {index: 1, value: 3},
    {index: 1, value: 3}          
  ],
  2: [
    {index: 2, value: 3},
    {index: 2, value: 3}
  ]
}

I have an array of objects, and I have to split it by duplicate values on index property. For a sample array of objects like the next one:

[
  {index: 0, value: 3},
  {index: 0, value: 3},
  {index: 0, value: 3},
  {index: 1, value: 3},
  {index: 1, value: 3},
  {index: 2, value: 3},
  {index: 2, value: 3}
]

The expected result should be:

{
  0: [
    {index: 0, value: 3},
    {index: 0, value: 3},
    {index: 0, value: 3}
  ],
  1: [
    {index: 1, value: 3},
    {index: 1, value: 3}          
  ],
  2: [
    {index: 2, value: 3},
    {index: 2, value: 3}
  ]
}

• javascript
• arrays
• object

Share Improve this question Follow edited Oct 17, 2019 at 21:37 Shidersz 17.2k22 gold badges2727 silver badges5151 bronze badges asked Jan 19, 2019 at 19:12 a8hoka8hok 30811 gold badge33 silver badges1212 bronze badges 1

• You claim you need to split on duplicate values, but it looks like you're splitting on duplicate indices. Also, why bother using an object with ascending numerical keys from 0 to n? An array seems more appropriate for the task. Please clarify your intent and post what you've attempted so far. – ggorlen Commented Jan 19, 2019 at 19:47

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4 Answers 4

Sorted by: Reset to default 5

Here you have one solution using reduce() and Object.assign():

const input = [
  {index: 0, value: 3},
  {index: 0, value: 3},
  {index: 0, value: 3},
  {index: 1, value: 3},
  {index: 1, value: 3},
  {index: 2, value: 3},
  {index: 2, value: 3},
  {index: 0, value: 3}
];

let obj = input.reduce((res, curr) =>
{
    if (res[curr.index])
        res[curr.index].push(curr);
    else
        Object.assign(res, {[curr.index]: [curr]});

    return res;
}, {});

console.log(obj);

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The previous example will make groups of all objects with equal index property. However, if you are looking to separate the objects by streaks that have equal indexes you can do this:

const input = [
  {index: 0, value: 3},
  {index: 0, value: 3},
  {index: 0, value: 3},
  {index: 1, value: 3},
  {index: 1, value: 3},
  {index: 2, value: 3},
  {index: 2, value: 3},
  {index: 0, value: 3},
  {index: 0, value: 3},
  {index: 0, value: 3},
  {index: 2, value: 3},
  {index: 2, value: 3}
];

let obj = input.reduce((res, curr) =>
{
    if (curr.index === res.last)
    {
        res.r[res.idx - 1].push(curr);
        return res;
    }

    Object.assign(res.r, {[res.idx]: [curr]});
    res.last = curr.index;
    res.idx++;
    return res;

}, {r: {}, idx: 0, last: null});

console.log(obj.r);

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You can use a simple reduce to get this:

const input=[{index:0,value:3},{index:0,value:3},{index:0,value:3},{index:1,value:3},{index:1,value:3},{index:2,value:3},{index:2,value:3}]

/* If index exists in the accumulator object, use that.
  else, create the index and point to an empty array
  push the item in context to the array
*/
const output = input.reduce((acc, v) => {
  acc[v.index] = acc[v.index] || [];
  acc[v.index].push(v);
  return acc;
}, {});

console.log(output)

Here's a short version:

const input=[{index:0,value:3},{index:0,value:3},{index:0,value:3},{index:1,value:3},{index:1,value:3},{index:2,value:3},{index:2,value:3}]

const output = input.reduce((a,v) => ((a[v.index] = a[v.index] || []).push(v), a), {});

console.log(output)

Do a simple reduce operation:

const data = [{
    index: 0,
    value: 3
  },
  {
    index: 0,
    value: 3
  },
  {
    index: 0,
    value: 3
  },
  {
    index: 1,
    value: 3
  },
  {
    index: 1,
    value: 3
  },
  {
    index: 2,
    value: 3
  }, {
    index: 2,
    value: 3
  }
];

const modified = data.reduce((val, acc) => {
  if (acc[val.index]) {
    acc[val.index].push(val);
  } else {
    acc[val.index] = [{
      index: [val.index],
      value: [val.value]
    }];
  }
  return acc;
}, {});

console.log(modified);

You can use simple for loop

const input = [
    {index: 0, value: 3},
    {index: 0, value: 3},
    {index: 0, value: 3},
    {index: 1, value: 3},
    {index: 1, value: 3},
    {index: 2, value: 3},
    {index: 2, value: 3}
];

const newArray = {}
for(var i of input) {
  if(newArray[i.index]) {
    newArray[i.index].push(i)
  } else {
    newArray[i.index] = [i]
  }
}

console.log(newArray)

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