tagdocs/funcsInLoop.html at master · bleveque/tagdocs · GitHub

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<html>
<head>
    <script src="http://code.jquery.com/jquery-1.10.1.min.js"></script>
    <script src="./jquery.js"></script>
    <script>
        /**
         * Ben LeVeque, 7/4/13
         *
         * Don't create functions like handlers within loops. You might want them to use e.g.
         * array[i], but inner functions have access to the increment var i, so by the time the
         * function is used, i is usually not what you want (this could manifest itself as code
         * only seeming to work for the last element in an array).
         *
         * Instead, create a helper function into which you can send i; this helper won't have
         * access to i, so you get the right behavior.
         *
         * See p. 39 of JS The Good Parts.
         *
         * In the example below, buttonX should produce a pop-up displaying X when clicked.
         */
        window.onload = load;

        function load() {
            var buttons, i, correct;
            buttons = $('button');
            correct = false;
            var myArr = [];
            if (!correct) {
                // doesn't work; inner function has access to variable i, so will always give 1000
                for (i = 0; i < buttons.length; i++) {
                    $(buttons[i]).on('click', function() {
                        alert(i);
                    });
                }
            } else {
                // works; buttonClicker doesn't have access to i
                for (i = 0; i < buttons.length; i++) {
                    $(buttons[i]).on('click', buttonClicker(i, myArr));
                }
            }
            myArr.push(1);
            i = 1000;
        }

        function buttonClicker(i, arr) {
            return function() {
                alert("i = "+i+", arr.length = "+arr.length); //need fix for objects, too, since they are passed by reference
            }
        }
    </script>
</head>
<title>no functions in loop!</title>
<body>
    <button>Button 0</button>
    <button>Button 1</button>
    <br /><br />
    <!-- pre-formatted -->
</body>
</html>