javascript - 如何检查是否在 JavaScript 中加载了变量？

Question

如何查看某个对象是否已加载，如果没有，如何加载，如下所示？

if (!isObjectLoaded(someVar)) {
    someVar= loadObject();
}

Answer 1

如果它是一个对象，那么您应该能够检查它是否为空或未定义，然后如果是则加载它。

if (myObject === null || myObject === undefined) {
   myObject = loadObject();
}

使用typeof函数也是一种选择，因为它返回所提供对象的类型。但是，如果对象尚未加载，它将返回null 或 undefined，因此在可读性方面可能会归结为个人偏好。

Answer 2

if(typeof(o) != 'object') o = loadObject();

Answer 3

myObject = myObject || loadObject();

Answer 4

我不确定您所说的“已加载”是什么意思...变量是否object存在并且根本没有您想要的类型？在这种情况下，你会想要这样的东西：

function isObjectType(obj, type) {
    return !!(obj && type && type.prototype && obj.constructor == type.prototype.constructor);
}

然后使用if (isObjectType(object, MyType)) { object = loadObject(); }.

如果object在您的测试之前没有填充任何内容（即 - typeof object === 'undefined'），那么您只需要：

if ('undefined' === typeof object) { object = loadObject(); }

Answer 5

您可能想查看是否定义了给定对象

特别是如果它在一个异步线程中完成，并带有 setTimeout 来检查它何时出现。

  var generate = function()
  { 
      window.foo = {}; 
  }; 
  var i = 0;
  var detect = function()
  {
     if( typeof window.foo == "undefined" ) 
     {
           alert( "Created!"); 
           clearInterval( i );
     }
   };
   setTimeout( generate, 15000 ); 
   i = setInterval( detect, 100 ); 

理论上应该检测 window.foo 何时存在。

Answer 6

If by loaded you mean defined, you can check the type of the variable with the typeof function. HOWEVER typeof has a few quirks, and will identify an Object, an Array, and a null as an object

alert(typeof(null));

Identifying a null as a defined object would probably cause your program to fail, so check with something like

if(null !== x && 'object' == typeof(x)){
    alert("Hey, It's an object or an array; good enough!");
}

Answer 7

如果要检测自定义对象：

// craete a custom object
function MyObject(){

}

// check if it's the right kind of object
if(!(object instanceof MyObject)){
   object = new MyObject();
}

Answer 8

您也可以只使用快捷方式if(obj)

Answer 9

typeof(obj)将在其他可能的值中返回类对象的“对象”。

Answer 10

if (!("someVar" in window)) {
  someVar = loadObject();
}

will tell you whether any JS has previously assigned to the global someVar or declared a top-level var someVar.

That will work even if the loaded value is undefined.