javascript - 如何在jQuery中检查'undefined'值(How to check 'undefined' value in jQuery)

Question

Possible Duplicate:(可能重复：)

Detecting an undefined object property in JavaScript(在JavaScript中检测未定义的对象属性)
javascript undefined compare(javascript undefined比较)

How we can add a check for an undefined variable, like:(我们如何为未定义的变量添加检查，例如：)

function A(val) {
  if (val == undefined) 
    // do this
  else
    // do this
}

  ask by Gaurav translate from so

Answer 1

JQuery library was developed specifically to simplify and to unify certain JavaScript functionality.(JQuery库是专门为简化和统一某些JavaScript功能而开发的。)

However if you need to check a variable against undefined value, there is no need to invent any special method, since JavaScript has a typeof operator, which is simple, fast and cross-platform:(但是，如果需要根据undefined值检查变量，则无需创建任何特殊方法，因为JavaScript具有typeof运算符，该运算符简单，快速且跨平台：)

if (typeof value === "undefined") {
    // ...
}

It returns a string indicating the type of the variable or other unevaluated operand.(它返回一个字符串，指示变量的类型或其他未评估的操作数。)

The main advantage of this method, compared to if (value === undefined) { ... } , is that typeof will never raise an exception in case if variable value does not exist.(与if (value === undefined) { ... }相比，此方法的主要优点是，如果变量value不存在， typeof将永远不会引发异常。)