JavaScript not resetting a variable in a loop

Question

would like to ask for reason, why JavaScript doesn't reset a variable after every iteration of the loop, in situation when I try to set it equal to a function argument(#1). It does the reset, if the variable is equaled to a specific array(#2).

#1
    function almostIncreasingSequence(sequence) {
            for (var i = 0; i < sequence.length; i++) {
                var testArray=sequence;
                testArray.splice(i, 1);
                console.log(testArray);
            }
        }
        almostIncreasingSequence([1, 3, 2, 1]);
#2
    function almostIncreasingSequence(sequence) {
            for (var i = 0; i < sequence.length; i++) {
                var testArray=[1, 3, 2, 1];
                testArray.splice(i, 1);
                console.log(testArray);
            }
        }
        almostIncreasingSequence([1, 3, 2, 1]);

Will be grateful for every answer. Thank you.

Your var testArray=sequence isn't creating a new array but just referencing sequence. So when you splice testArray you are also splicing sequence. If you want it to reset then you need var testArray = sequence.slice() — Matt
– Matt, Commented Apr 9, 2018 at 16:56
Possible duplicate of Looping through array and removing items, without breaking for loop — Mohammad Usman
– Mohammad Usman, Commented Apr 9, 2018 at 16:57
@MohammadUsman His issue isn't understanding that splicing removes the index while the loop proceeds causing the occasional skipped index. His issue is not understanding variable assignment(s) and what passes a reference and what passes a value. Please remove or update your comment. — Matt
– Matt, Commented Apr 9, 2018 at 17:08

Matt · Accepted Answer · 2018-04-09 17:05:55Z

5

As stated in the comment above is that you have confusion with your variable assignment.

In #1 you are under the impression that var testArray = sequence; is the same as saying var testArray = [1, 3, 2, 1]. This isn't the case. var testArray = sequence is simply a reference to sequence. Whatever you modify in testArray calls back to sequence and modifies it there as well.

To fix #1 to perform as #2 you will have to do var testArray = sequence.slice(). This performs a shallow copy of sequence so that modifying testArray has no impact on sequence.

function almostIncreasingSequence(sequence) {
  for (var i = 0; i < sequence.length; i++) {
      var testArray=sequence.slice();
      testArray.splice(i, 1);
      console.log(testArray);
  }
}

almostIncreasingSequence([1, 3, 2, 1]);

answered Apr 9, 2018 at 17:05

Matt

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