zoukankan      html  css  js  c++  java
  • Function in loop and closure

    This article describe the famious issue “function in loop and closure” in JavaScript.

    The root cause is loop statements (such as for, while) don’t have their own scope.

    Let’s see an example first:

        <ul>
            <li>Item1</li>
            <li>Item2</li>
            <li>Item3</li>
        </ul>
        var liNodes = document.getElementsByTagName("li");
        for (var i = 0; i < liNodes.length; i++) {
            liNodes[i].onclick = function() {
                alert("You click item " + i);
            };
        }

    Now, if you click each of the list, all will produce a “You click item 3″ alert
    box.

    The number 3 comes out of the end execution of the loop (0, 1, 2 and out of the
    loop i === 3).

    Obviously, the result is not expected.

    If you use JSLint to validate this piece of code, you will get the following warning:

    Be careful when making functions within a loop. Consider putting the function in
    a closure.

    According to JSLint’s suggest, we have the first solution:

        // GOOD - 0
        function clickNode(liNode, i) {
            liNode.onclick = function() {
                alert("You click item " + i);
            };
        }  
    
        var liNodes = document.getElementsByTagName("li");
        for (var i = 0; i < liNodes.length; i++) {
            clickNode(liNodes[i], i);
        }

    If you don’t want to create another function, consider using anonymous funtion:

        // GOOD - 1
        var liNodes = document.getElementsByTagName("li");
        for (var i = 0; i < liNodes.length; i++) {
            (function(i) {
                liNodes[i].onclick = function() {
                    // You click item 0
                    // You click item 1
                    // You click item 2
                    alert("You click item " + i);
                };
            })(i);
        }

    Notice: The self-executing function create a context scope which contains a local
    variable i.

    When the click event occurs, the variable i is coming from the closure which is
    just the self-executing function scope.

    There are many ways to solve this problem, following are another three ways:

        // GOOD - 2
        var liNodes = document.getElementsByTagName("li");
        $.each(liNodes, function(i, item) {
            $(item).click(function() {
                // You click item 0
                // You click item 1
                // You click item 2
                alert("You click item " + i);
            });
        });  
    
        // GOOD - 3
        $("li").each(function(i, item) {
            $(item).click(function() {
                // You click item 0
                // You click item 1
                // You click item 2
                alert("You click item " + i);
            });
        });  
    
        // PREFERED - 4
        var liNodes = $("li").click(function(event) {
            var i = liNodes.index(this);
            // You click item 0
            // You click item 1
            // You click item 2
            alert("You click item " + i);
        });
  • 相关阅读:
    Ueditor富文本编辑器
    ashx误删后,未能创建类型
    一个菜鸟的ASP.NET观光路线图
    地位尴尬的WebForm、ASP.NET核心知识(10)
    经典排序算法--归并排序
    经典排序算法--快速排序
    经典排序算法--冒泡排序
    经典排序算法--堆排序
    经典排序算法--简单选择排序
    经典排序算法--希尔排序
  • 原文地址:https://www.cnblogs.com/sanshi/p/1514065.html
Copyright © 2011-2022 走看看