我们都知道 global variables绝非最佳实践。但是有几个例子，没有它们就很难编码。您使用什么技术来避免使用全局变量？
例如，给定以下场景，您将如何不使用全局变量？
JavaScript 代码:
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();
}
相关标记:
<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 <input type="file"> 的 Web 表单.它一次上传一个文件以防止大量请求。它通过 POST 做到这一点进入 iframe，等待触发 iframe onload 的响应，然后触发下一次提交。
这个例子大家可以不用具体回答，我只是提供给自己想不出避免全局变量的方法的情况下引用。
最佳答案
最简单的方法是将您的代码包装在闭包中，并手动仅将全局需要的那些变量公开到全局范围:
(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，开发人员需要更改问题中假设的一些事情。它看起来更像这样:
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>
您不需要 <iframe> 上的内联事件处理程序，它仍然会在使用此代码的每次加载时触发。
关于加载事件
这是一个测试用例，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)，否则您应该能够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>
每当我在 Safari、Firefox、IE 6、7 和 8 中单击提交按钮时都会触发警报。
关于javascript - 如何避免 JavaScript 中的全局变量？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/1841916/