i have this 2 array,

const firstArray = [
{
 key: 'a',
 value: 'a'
},
{
 key: 'b',
 value: 'b'
},
{
 key: 'c',
 value: 'c'
}
]

const secondArray = [
{
 key: 'b',
 value: 'd'
},
{
 key: 'c',
 value: 'e'
}
]

i want to merge these 2 array, and if a same key found on second array the second array value will replace first array value

the new array should look like this

const expectedArray = [
{
 key: 'a',
 value: 'a'
},
{
 key: 'b',
 value: 'd'
},
{
 key: 'c',
 value: 'e'
}
]

CodePudding user response：

As I see lodash in your tags, I assume you want to use it.

You can use something like this:

var merged = _(firstArray)
  .concat(secondArray)
  .groupBy("key")
  .map(_.spread(_.merge))
  .value();

It basically groups all objects with the same key into an array with groupBy("key"), then flattens out each array of values merging them and taking the last one with .map(_.spread(_.merge)).

I suggest commenting out the final likes and see all the intermediate steps if you want to fully understand the process, it's interesting!

const firstArray = [{
    key: 'a',
    value: 'a'
  },
  {
    key: 'b',
    value: 'b'
  },
  {
    key: 'c',
    value: 'c'
  }
]

const secondArray = [{
    key: 'b',
    value: 'd'
  },
  {
    key: 'c',
    value: 'e'
  }
]

var merged = _(firstArray)
  .concat(secondArray)
  .groupBy("key")
  .map(_.spread(_.merge))
  .value();

console.log(merged);

<script src="https://cdn.jsdelivr.net/lodash/4.13.1/lodash.min.js"></script>

<iframe name="sif1" sandbox="allow-forms allow-modals allow-scripts" frameborder="0"></iframe>

CodePudding user response：

Solution without lodash is to iterate through the second array and just look for matches.

If there's no match, push the item to the first array.

If there's a match, replace the item in the first array with the one from the second array.

const mergedArray = [...firstArray]

secondArray.forEach(newItem => {
  const matchIndex = mergedArray.findIndex(prevItem => prevItem.key === newItem.key)

  if (matchIndex === -1) {
    mergedArray.push(newItem)
  } else {
    mergedArray[matchIndex] = newItem
  }
})

CodePudding user response：

const mergedArray = [...secondArray, ...firstArray];
const lengthOfUniqueArray = new Set(mergedArray.map((item) => item.key)).size;
const expectedArray = newArray.slice(0, lengthOfUniqueArray);

expectedArray will be what you want.

CodePudding user response：

You can concat, reverse, can all unique elements by key, and then reverse back:

const { flow, concat, reverse, uniqBy } = _

const mergeArrays = flow(
  concat,
  reverse,
  arr => uniqBy(arr, 'key'),
  reverse
)

const firstArray = [{"key":"a","value":"a"},{"key":"b","value":"b"},{"key":"c","value":"c"}]
const secondArray = [{"key":"b","value":"d"},{"key":"c","value":"e"}]

var merged = mergeArrays(firstArray, secondArray)

console.log(merged)

<script src="https://cdn.jsdelivr.net/lodash/4.13.1/lodash.min.js"></script>

<iframe name="sif2" sandbox="allow-forms allow-modals allow-scripts" frameborder="0"></iframe>