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

    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);
        });

    转自:http://www.cnblogs.com/sanshi/archive/2009/06/30/1514065.html

  • 相关阅读:
    PHP中使用Redis
    找回Xcode7的代码折叠功能
    使用 Kingfisher 处理网络图片的读取与缓存
    Swift的两个小窍门
    斯坦福公开课:Developing IOS 8 App with Swift(1-3)心得体会
    What is the relationship between Xcode, Swift and Cocoa?
    C++中static用法总结
    C++中const用法总结
    数据库概念要点
    虚拟内存地址和物理内存地址
  • 原文地址:https://www.cnblogs.com/johnwonder/p/1688909.html
Copyright © 2011-2022 走看看