Bug 142792

There is a timing bug in iOS8 that causes mobile Safari to incorrectly report a 'length' on objects that don't have one. To the best of my knowledge, this happens on iOS8+, possibly only on 64-bit systems. The bug is triggered for objects that have only numeric properties. For example: foo = { 1: 'a', 2: 'b', 3: 'c' } In this case, if you query foo.length then mobile Safari will sometimes return 4 (the highest property + 1). This causes strange failures in functions like jQuery's $.each() or Underscore's _.each() since the appearance of the '.length' property makes them believe the object is an array. When the bug manifests itself, the following be true: a = foo.length; b = typeof a; c = foo.hasOwnProperty('length') --> a == 4 // should have been undefined b == 'number' // should have been 'undefined' c == false // correct Both jQuery and Underscore to use the 'typeof' test on the length property, and I have advised them to add an 'hasOwnProperty' check as well. You can see more background and some repro steps at the following Stack Overflow discussion: http://stackoverflow.com/questions/28155841/misterious-failure-of-jquery-each-and-underscore-each-on-ios The Underscore team have apparently managed to create an automated test for this: https://github.com/jashkenas/underscore/issues/2081 The jQuery issue is here: https://github.com/jquery/jquery/issues/2145