Javascript: array difference but occurrencies between parentheses

Say we have 2 arrays

a = ['1', '2', '3', '4', '5']
b = ['1', '3', '5']

The difference between these 2 arrays

diff = a.filter(x => !b.includes(x));

so diff is equal to ['2', '4']

My problem is that I could have

a = ['1', '2', '3(4)', '4', '5'] // (I have 4 occurrencies of "3")
b = ['1', '3', '5'] 
// or 
b = ['1', '3(3)', '5']

and I want to get this array

diff = ['2', '3(3)', '4'] 
// or 
diff = ['2', '3', '4']

Answer

Your data format is awkward. If you can take decisions on that, I advise you to do instead (for an array like ['1', '2', '3(4)', '4', '5']):

• use a flat array with as many occurrences as it gets for each element, like:

   ['1', '2', '3', '3', '3', '3', '4', '5']
  

• use an array of tuples, whose second value in each element is its count, like:

   [['1',1], ['2',1], ['3',4], ['4',1], ['5',1]]
  

• use a map-like object, whose value is the count, like:

   {'1':1, '2':1, '3':4, '4':1, '5':1}
  

• use an array of only the counts, where the index is implicitly the key:

   [0, 1, 1, 4, 1, 1]
  

Any of those would make your like easier.

---

If none of that is a valid option for you, I will point you in the right direction of implementing a function to compare 2 elements are yield the merged difference value according to the rules you have:

function diff(s1,s2) {
  const re = /([0-9]+)(?:(([0-9]+)))?/
  const [,n1,q1] = re.exec(s1)
  const [,n2,q2] = re.exec(s2)
  if (n1!=n2) return  
  const q = Number(q1||1) - Number(q2||1)
  if (q <= 0) return 
  if (q == 1) return n1
  return n1 + `(${q})`
}

console.log(diff("3(4)", "3"   )) // "3(3)"
console.log(diff("3(4)", "3(2)")) // "3(2)"
console.log(diff("3(4)", "3(3)")) // "3"
console.log(diff("3(4)", "3(4)")) // undefined
console.log(diff("3(4)", "3(5)")) // undefined
console.log(diff("3"   , "3"   )) // undefined
console.log(diff("3"   , "3(2)")) // undefined
console.log(diff("3"   , "5"   )) // undefined

You can start from here to iterate through your arrays and generate the differences with the merged values.