Javascript 检查全局变量是否存在的正确方法是什么？
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What is the correct way to check if a global variable exists?
提问by Ebrahim Byagowi
JSLint is not passing this as a valid code:
JSLint 没有将此作为有效代码传递：
/* global someVar: false */
if (typeof someVar === "undefined") {
var someVar = "hi!";
}
What is the correct way?
什么是正确的方法？
回答by bigoldbrute
/*global window */
if (window.someVar === undefined) {
window.someVar = 123456;
}
if (!window.hasOwnProperty('someVar')) {
window.someVar = 123456;
}
回答by gvlasov
/**
* @param {string} nameOfVariable
*/
function globalExists(nameOfVariable) {
return nameOfVariable in window
}
It doesn't matter whether you created a global variable with var foo or window.foo — variables created with var in global context are written into window.
无论你是用 var foo 还是 window.foo 创建了一个全局变量都没有关系——在全局上下文中用 var 创建的变量被写入到 window 中。
回答by mrak
If you are wanting to assign a global variable only if it doesn't already exist, try:
如果您只想分配一个不存在的全局变量，请尝试：
window.someVar = window.someVar || 'hi';
or
或者
window['someVar'] = window['someVar'] || 'hi';
回答by gion_13
try
尝试
variableName in window
or
或者
typeof window[variableName] != 'undefined'
or
或者
window[variableName] !== undefined
or
或者
window.hasOwnProperty(variableName)
回答by bfavaretto
I think this is actually a problem with JSLint. It will issue the following error:
我认为这实际上是 JSLint 的一个问题。它将发出以下错误：
Unexpected 'typeof'. Compare directly with 'undefined'.
意外的“typeof”。直接与“未定义”进行比较。
I believe this is bad advice. In JavaScript, undefinedis a global variable that is, usually, undefined. But some browsers allow scripts to modify it, like this: window.undefined = 'defined'. If this is the case, comparing directly with undefinedcan lead to unexpected results. Fortunately, current ECMA 5 compliant browsers do not allow assignments to undefined(and will throw an exception in strict mode).
我相信这是一个糟糕的建议。在 JavaScript 中，undefined是一个全局变量，通常是未定义的。但是，一些浏览器允许脚本来修改它，就像这样：window.undefined = 'defined'。如果是这种情况，直接比较undefined可能会导致意想不到的结果。幸运的是，当前的 ECMA 5 兼容浏览器不允许赋值undefined（并且会在严格模式下抛出异常）。
I prefer typeof someVar === "undefined", as you posted, or someVar in windowas Susei suggested.
我更喜欢typeof someVar === "undefined"，正如您发布的那样，或者someVar in window像 Susei 建议的那样。
回答by Vatev
if (typeof someVar === "undefined") { var someVar = "hi!"; }
if (typeof someVar === "undefined") { var someVar = "hi!"; }
will check if someVar(local or global) is undefined.
将检查someVar（本地或全局）是否未定义。
If you want to check for a global variable you can use
如果要检查全局变量，可以使用
if(window['someVar'] === undefined) {
...
}
assuming this is in a browser :)
假设这是在浏览器中:)
回答by Michael
bfavaretto is incorrect.
bfavaretto 是不正确的。
Setting the global undefined to a value will not alter tests of objects against undefined. Try this in your favorite browsers JavaScript console:
将全局 undefined 设置为一个值不会改变对象对 undefined 的测试。在您最喜欢的浏览器 JavaScript 控制台中试试这个：
var udef; var idef = 42;
alert(udef === undefined); // Alerts "true".
alert(idef === undefined); // Alerts "false".
window.undefined = 'defined';
alert(udef === undefined); // Alerts "true".
alert(idef === undefined); // Alerts "false".
This is simply due to JavaScript ignoring all and any values attempted to be set on the undefined variable.
这仅仅是因为 JavaScript 忽略了所有试图在未定义变量上设置的值。
window.undefined = 'defined';
alert(window.undefined); // Alerts "undefined".
回答by Prem Anand
This would be a simple way to perform the check .
这将是执行检查的简单方法。
But this check would fail if variableNameis declared and is assigned with the boolean value: false
但是如果variableName声明并分配了boolean value: false
if(window.variableName){
}
