Diff Two Arrays Exclamation Mark

Tell us what’s happening:
Hello, in the solution of the problem appears an exclamation mark before the array.
What’s is the meaning of this? I understand the logic without the exclamation mark, i’m very confused.

Thank you.

Your code so far

function diffArray(arr1, arr2) {
  return arr1
      (element) => !arr1.includes(element) || !arr2.includes(element)
  // Same, same; but different.
  return newArr;

diffArray([1, 2, 3, 5], [1, 2, 3, 4, 5]);

Your browser information:

User Agent is: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/75.0.3770.142 Safari/537.36.

Link to the challenge:

Your code has been blurred out to avoid spoiling a full working solution for other campers who may not yet want to see a complete solution. In the future, if you post a full passing solution to a challenge and have questions about it, please surround it with [spoiler] and [/spoiler] tags on the line above and below your solution code.

Thank you.

The exclamation mark is a NOT operator, it flips the Boolean value of whatever it is put in front of

So if includes return false, it becomes true and viceversa