如何检查是否在Javascript中设置了变量？

Question

I have this object/array thing:

var states = {};

states["CA"] = new State("CA", "California");
states["AR"] = new State("AR", "Arizona");
....

How can I check if states["AL"] has been set or not? Will the following work (in all browsers)?

if (states["AL"] == undefined)
   alert("Invalid state");

Answer 1

Recommended:

if (typeof states["AL"] == 'undefined')
    alert("Invalid state");

Litteral undefined is possible and will work most of the time, but won't in some browser (IE mac, maybe others).

Answer 2

You don't have to do a literal comparison at all; the following will work just fine...

if(states["AL"]) 
  alert("State exists"); 
else
  alert("State does not exist"); 

Answer 3

或者，您可以说if ("AL" in states) { ... } 。

Answer 4

function State(ab, name){
    if(states && states[ab])return states[ab];

    this.ab=ab;
    this.name=name;
    //whatever else, no return;
}