• 不能删除全局变量和局部变量

var globalVal = 1;
delete globalVal; // false

(function(){
    var localVal = 1;
    return delete localVal;
}()); // false

• 不能删除函数

function fd(){}
delete fd; // false

(function(){
    function fd(){};
    return delete fd;
}());//false

• 可以删除隐式全局变量

ohNo = 1;
console.log(window.ohNo); // 1
delete ohNo; //true