在JavaScript函数中定义全局变量 [英] Define a global variable in a JavaScript function
问题描述
是否可以在JavaScript函数中定义全局变量?
Is it possible to define a global variable in a JavaScript function?
我想在其他功能中使用trailimage变量(在makeObj函数中声明).
I want use the trailimage variable (declared in the makeObj function) in other functions.
<html xmlns="http://www.w3.org/1999/xhtml">
<head id="Head1" runat="server">
<title></title>
<script type="text/javascript">
var offsetfrommouse = [10, -20];
var displayduration = 0;
var obj_selected = 0;
function makeObj(address) {
**var trailimage = [address, 50, 50];**
document.write('<img id="trailimageid" src="' + trailimage[0] + '" border="0" style=" position: absolute; visibility:visible; left: 0px; top: 0px; width: ' + trailimage[1] + 'px; height: ' + trailimage[2] + 'px">');
obj_selected = 1;
}
function truebody() {
return (!window.opera && document.compatMode && document.compatMode != "BackCompat") ? document.documentElement : document.body;
}
function hidetrail() {
var x = document.getElementById("trailimageid").style;
x.visibility = "hidden";
document.onmousemove = "";
}
function followmouse(e) {
var xcoord = offsetfrommouse[0];
var ycoord = offsetfrommouse[1];
var x = document.getElementById("trailimageid").style;
if (typeof e != "undefined") {
xcoord += e.pageX;
ycoord += e.pageY;
}
else if (typeof window.event != "undefined") {
xcoord += truebody().scrollLeft + event.clientX;
ycoord += truebody().scrollTop + event.clientY;
}
var docwidth = 1395;
var docheight = 676;
if (xcoord + trailimage[1] + 3 > docwidth || ycoord + trailimage[2] > docheight) {
x.display = "none";
alert("inja");
}
else
x.display = "";
x.left = xcoord + "px";
x.top = ycoord + "px";
}
if (obj_selected = 1) {
alert("obj_selected = true");
document.onmousemove = followmouse;
if (displayduration > 0)
setTimeout("hidetrail()", displayduration * 1000);
}
</script>
</head>
<body>
<form id="form1" runat="server">
<img alt="" id="house" src="Pictures/sides/right.gif" style="z-index: 1; left: 372px;
top: 219px; position: absolute; height: 138px; width: 120px" onclick="javascript:makeObj('Pictures/sides/sides-not-clicked.gif');" />
</form>
</body>
</html>
推荐答案
正如其他人所说，您可以在全局范围内(在所有函数和模块之外)使用var来声明全局变量:
As the others have said, you can use var at global scope (outside of all functions and modules) to declare a global variable:
<script>
var yourGlobalVariable;
function foo() {
// ...
}
</script>
(请注意，这仅在 global 范围内是正确的.如果该代码在模块-<script type="module">...</script>中-它将不在全局范围内，因此不会创建全局范围. )
(Note that that's only true at global scope. If that code were in a module — <script type="module">...</script> — it wouldn't be at global scope, so that wouldn't create a global.)
或者:
In modern environments, you can assign to a property on the object that globalThis refers to (globalThis was added in ES2020):
<script>
function foo() {
globalThis.yourGlobalVariable = ...;
}
</script>
在浏览器上，您可以使用称为window的全局变量来做同样的事情:
On browsers, you can do the same thing with the global called window:
<script>
function foo() {
window.yourGlobalVariable = ...;
}
</script>
...由于在浏览器中，所有用全局变量是var声明的全局变量window对象的属性. (在最新规范ECMAScript 2015中，在全局范围内的新let，const和class语句创建的不是全局对象属性的全局变量；这是ES2015中的新概念.)
...because in browsers, all global variables global variables declared with var are properties of the window object. (In the latest specification, ECMAScript 2015, the new let, const, and class statements at global scope create globals that aren't properties of the global object; a new concept in ES2015.)
(还有隐式全局变量的恐怖 ，但不要故意这样做，并尽最大努力避免意外地这样做，也许可以使用ES5的"use strict".
(There's also the horror of implicit globals, but don't do it on purpose and do your best to avoid doing it by accident, perhaps by using ES5's "use strict".)
所有说明:如果可能(我几乎可以肯定)，我会避免使用全局变量.正如我提到的，它们最终是window的属性，而window已经是 plenty 足够拥挤 ，其中所有带有id(很多都带有name)元素都被转储到其中(无论即将发布的规范如何，IE都将转储带有name的几乎所有内容)在那儿).
All that said: I'd avoid global variables if you possibly can (and you almost certainly can). As I mentioned, they end up being properties of window, and window is already plenty crowded enough what with all elements with an id (and many with just a name) being dumped in it (and regardless that upcoming specification, IE dumps just about anything with a name on there).
相反，在现代环境中，请使用模块:
Instead, in modern environments, use modules:
<script type="module">
let yourVariable = 42;
// ...
</script>
模块中的顶级代码位于模块范围内，而不是全局范围内，因此会创建一个变量，该模块中的所有代码都可以看到，但不是全局的.
The top level code in a module is at module scope, not global scope, so that creates a variable that all of the code in that module can see, but that isn't global.
在没有模块支持的过时环境中，将代码包装在作用域函数中，并使用该作用域局部变量，并在其中封闭其他函数:
In obsolete environments without module support, wrap your code in a scoping function and use variables local to that scoping function, and make your other functions closures within it:
<script>
(function() { // Begin scoping function
var yourGlobalVariable; // Global to your code, invisible outside the scoping function
function foo() {
// ...
}
})(); // End scoping function
</script>
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