Array state will be cached in iOS 12 Safari. Is it a bug or feature?

It's definitely a BUG! And it's a very serious bug.

The bug is due to the optimization of array initializers in which all values are primitive literals. For example, given the function:

function buildArray() {
    return [1, null, 'x'];
}

All returned array references from calls to buildArray() will link to the same memory, and some methods such as toString() will have their results cached. Normally, to preserve consistency, any mutable operation on such optimized arrays will copy the data to a separate memory space and link to it; this pattern is called copy-on-write, or CoW for short.

The reverse() method mutates the array, so it should trigger a copy-on-write. But it doesn't, because the original implementor (Keith Miller of Apple) missed the reverse() case, even though he had written many testcases.

This bug was reported to Apple on August 21. The fix landed in the WebKit repository on August 27 and shipped in Safari 12.0.1 and iOS 12.1 on October 30, 2018.

I wrote a lib to fix the bug. https://www.npmjs.com/package/array-reverse-polyfill

This is the code:

(function() {
  function buggy() {
    var a = [1, 2];
    return String(a) === String(a.reverse());
  }
  if(!buggy()) return;
  var r = Array.prototype.reverse;
  Array.prototype.reverse = function reverse() {
    if (Array.isArray(this)) this.length = this.length;
    return r.call(this);
  }
})();

This is a bug in webkit. Though this has been solved at their end but not yet shipped with iOS GM release. One of the solutions to this problem:

(function() {
  function getReverseStr() {
    return [1, 2].reverse();
  }

  var n1 = getReverseStr()[0];
  var n2 = getReverseStr()[0];
  // check if there is an issue
  if(n1 != n2) {
    var origReverseFunction = Array.prototype.reverse;
    Array.prototype.reverse = function() {
      var newArr = this.slice();
      // use original reverse function so that edge cases are taken care of
      origReverseFunction.apply(newArr, arguments);
      var that = this;
      // copy reversed array
      newArr.forEach(function(value, index) {
        that[index] = value;
      });
      return this;
    }
  }
})();

It seems not to be cached if the number of elements changes.
I was able to avoid this like this.

<!DOCTYPE html>
<html>
<head>
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0, maximum-scale=1.0, user-scalable=0">
    <title>iOS 12 Safari bugs</title>
    <script type="text/javascript">
    window.addEventListener("load", function ()
    {
        let arr = [1, 2, 3, 4, 5];
        arr.push('');
        arr.pop();
        alert(arr.join());

        document.querySelector("button").addEventListener("click", function ()
        {
            arr.reverse();
        });
    });
    </script>
</head>
<body>
    <button>Array.reverse()</button>
    <p style="color:red;">test: click button and refresh page, code:</p>
</body>
</html>

Array state will be cached in iOS 12 Safari. Is it a bug or feature?

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