如何避免JavaScript中的全局变量？

Z时代
2024-01-10
分类：问答

我们都知道全局变量只是最佳实践。但是在有些情况下，没有它们很难编写代码。您使用什么技术来避免使用全局变量？

例如，在以下情况下，您将如何不使用全局变量？

var uploadCount = 0;
window.onload = function() {
    var frm = document.forms[0];
    frm.target = "postMe";
    frm.onsubmit = function() {
        startUpload();
        return false;
    }
}
function startUpload() {
    var fil = document.getElementById("FileUpload" + uploadCount);
    if (!fil || fil.value.length == 0) {
        alert("Finished!");
        document.forms[0].reset();
        return;
    }
    disableAllFileInputs();
    fil.disabled = false;
    alert("Uploading file " + uploadCount);
    document.forms[0].submit();
}

<iframe src="test.htm" name="postHere" id="postHere"
  onload="uploadCount++; if(uploadCount > 1) startUpload();"></iframe>
<!-- MUST use inline JavaScript here for onload event
     to fire after each form submission. -->

此代码来自具有多个的网络表单<inputtype="file">。它一次上传一个文件，以防止大量请求。它通过POST到iframe来完成此操作，等待响应触发iframeonload，然后触发下一个提交。

您不必专门回答此示例，我只是将其提供给我无法思考避免全局变量的情况的参考。

回答：

最简单的方法是将代码包装在闭包中，然后仅将全局所需的那些变量手动公开给全局范围：

(function() {
    // Your code here
    // Expose to global
    window['varName'] = varName;
})();

为了解决Crescent Fresh的评论：为了从场景中完全删除全局变量，开发人员将需要更改问题中假设的许多内容。它看起来像这样：

Javascript：

(function() {
    var addEvent = function(element, type, method) {
        if('addEventListener' in element) {
            element.addEventListener(type, method, false);
        } else if('attachEvent' in element) {
            element.attachEvent('on' + type, method);
        // If addEventListener and attachEvent are both unavailable,
        // use inline events. This should never happen.
        } else if('on' + type in element) {
            // If a previous inline event exists, preserve it. This isn't
            // tested, it may eat your baby
            var oldMethod = element['on' + type],
                newMethod = function(e) {
                    oldMethod(e);
                    newMethod(e);
                };
        } else {
            element['on' + type] = method;
        }
    },
        uploadCount = 0,
        startUpload = function() {
            var fil = document.getElementById("FileUpload" + uploadCount);
            if(!fil || fil.value.length == 0) {    
                alert("Finished!");
                document.forms[0].reset();
                return;
            }
            disableAllFileInputs();
            fil.disabled = false;
            alert("Uploading file " + uploadCount);
            document.forms[0].submit();
        };
    addEvent(window, 'load', function() {
        var frm = document.forms[0];
        frm.target = "postMe";
        addEvent(frm, 'submit', function() {
            startUpload();
            return false;
        });
    });
    var iframe = document.getElementById('postHere');
    addEvent(iframe, 'load', function() {
        uploadCount++;
        if(uploadCount > 1) {
            startUpload();
        }
    });
})();

HTML：

<iframe src="test.htm" name="postHere" id="postHere"></iframe>

您在上使用内联事件处理程序<iframe>，它仍会在每次加载此代码时触发。

这是一个测试案例，表明您不需要内联onload事件。这取决于引用同一服务器上的文件（/emptypage.php），否则您应该可以将其粘贴到页面中并运行它。

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
    "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
<head>
    <meta http-equiv="Content-Type" content="text/html; charset=utf-8"/>
    <title>untitled</title>
</head>
<body>
    <script type="text/javascript" charset="utf-8">
        (function() {
            var addEvent = function(element, type, method) {
                if('addEventListener' in element) {
                    element.addEventListener(type, method, false);
                } else if('attachEvent' in element) {
                    element.attachEvent('on' + type, method);
                    // If addEventListener and attachEvent are both unavailable,
                    // use inline events. This should never happen.
                } else if('on' + type in element) {
                    // If a previous inline event exists, preserve it. This isn't
                    // tested, it may eat your baby
                    var oldMethod = element['on' + type],
                    newMethod = function(e) {
                        oldMethod(e);
                        newMethod(e);
                    };
                } else {
                    element['on' + type] = method;
                }
            };
            // Work around IE 6/7 bug where form submission targets
            // a new window instead of the iframe. SO suggestion here:
            // http://stackoverflow.com/q/875650
            var iframe;
            try {
                iframe = document.createElement('<iframe name="postHere">');
            } catch (e) {
                iframe = document.createElement('iframe');
                iframe.name = 'postHere';
            }
            iframe.name = 'postHere';
            iframe.id = 'postHere';
            iframe.src = '/emptypage.php';
            addEvent(iframe, 'load', function() {
                alert('iframe load');
            });
            document.body.appendChild(iframe);
            var form = document.createElement('form');
            form.target = 'postHere';
            form.action = '/emptypage.php';
            var submit = document.createElement('input');
            submit.type = 'submit';
            submit.value = 'Submit';
            form.appendChild(submit);
            document.body.appendChild(form);
        })();
    </script>
</body>
</html>