Find the Difference Between Two Arrays (Javascript Algorithm) + Funky Use of the Spread Operator - Stack Overflow

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Question

Compare two arrays and return a new array with any items only found in one of the two given arrays, but not both. In other words, return the symmetric difference of the two arrays.

Note You can return the array with its elements in any order.

Professional Developers Answer

function diffArray(arr1, arr2) {
  return [...diff(arr1, arr2), ...diff(arr2, arr1)];

  function diff(a, b) {
    return a.filter(item => b.indexOf(item) === -1);
  }
}

Question

Compare two arrays and return a new array with any items only found in one of the two given arrays, but not both. In other words, return the symmetric difference of the two arrays.

Note You can return the array with its elements in any order.

Professional Developers Answer

function diffArray(arr1, arr2) {
  return [...diff(arr1, arr2), ...diff(arr2, arr1)];

  function diff(a, b) {
    return a.filter(item => b.indexOf(item) === -1);
  }
}

My Question

I don't understand how this code operates. In particular I have never seen the spread operator used like this. Please could you explain how this works?

• javascript
• algorithm
• ecmascript-6

Share Improve this question Follow asked Apr 27, 2020 at 15:58 AndrewNeedsHelpAndrewNeedsHelp 41555 silver badges1616 bronze badges 2

• 2 it is just concatenating two arrays together – epascarello Commented Apr 27, 2020 at 16:01
• Professional developer that uses spread syntax but not includes or Set? .oO – Bergi Commented Apr 27, 2020 at 16:35

Add a ment  | 

2 Answers 2

Sorted by: Reset to default 6

It is just a fancy way to concat two arrays together. This is how it looks if you did not use the spread operator

function diffArray(arr1, arr2) {
  function diff(a, b) {
    return a.filter(item => b.indexOf(item) === -1);
  }

  var diff1 = diff(arr1, arr2)   // [0, 1]
  var diff2 = diff(arr2, arr1)   // [5, 6]
  return [].concat(diff1, diff2) // [0, 1, 5, 6]

}

var res = diffArray([0,1,2,3,4], [2,3,4,5,6])
console.log(res)

The spread operator used on an array takes the elements out of the array and puts them directly into the data structure that contains them. So if A = [1,2,3] and B = [4,5,6] then [...A, ...B] === [1,2,3,4,5,6]. Without the spread operator, they would be [[1,2,3], [4,5,6]].

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