属性赋值undefined和delete删除属性的不同点 - pod4g/tool GitHub Wiki

两者的不同

两者最大的不同在于：赋值undefined的话，属性还存在，还能被for in遍历到，而delete的属性无法被for in遍历到，相当于这个属性从来没有存在过（不然delete叫delete好意思嘛？）。

var obj = {a: 1};

for(var attr in obj){
  console.log(attr); // a
}

obj.a = undefined; // or null

for(attr in obj){
  console.log(attr); // a
}

delete obj.a;

for(attr in obj){
  console.log(attr); // nothing!
}

IE8下的一个delete的bug

// Known issue: Will throw 'Uncaught ReferenceError: callback_*** is not defined' error if request timeout
function clearFunction(functionName) {
    // IE8 throws an exception when you try to delete a property on window
    // http://stackoverflow.com/a/1824228/751089
    try {
        delete window[functionName];
    } catch (e) {
        window[functionName] = undefined;
    }
}