JavaScript中变量声明有var和没var的区别

(function(){ 
// ... 
})();

var fff = 2; 
window.ffa = 3; 
ffb = 4; 
this.ffc = 4; 
var ffftx = Object.getOwnPropertyDescriptor(window, 'fff'); //configurable:false,enumerable:true,value:2,writable:true 
var ffatx = Object.getOwnPropertyDescriptor(window, 'ffa'); //configurable:true,enumerable:true,value:2,writable:true 
var ffbtx = Object.getOwnPropertyDescriptor(window, 'ffb'); //configurable:true,enumerable:true,value:2,writable:true 
var ffctx = Object.getOwnPropertyDescriptor(window, 'ffc'); //configurable:true,enumerable:true,value:2,writable:true

delete fff; // 无法删除 
delete ffa; // 可删除 
delete ffb; // 可删除 
delete ffc; // 可删除

// 函数定义表达式 
var fns = function (){ 
// ... 
}； 
// 函数声明语句 
function fns(){ 
// ... 
}

(function() { 
testa(); // 打印出testa 
testb(); // 报错：提示undefined is not a function 
console.log(testc); //undefined,如果移到上面就可以了 
function testa() { 
console.log("testa"); 
} 
var testb = function() { 
console.log("tesb"); 
} 
var testc = "testc"; 
})();