javascript 定义一个函数3种方式比较

3种定义方式：

    function a(x){  //方式1
        return xx;
    }

    var b=function(x){   //方式2
        return xx;
    }

    var c=new Function('x','return xx;');  //方式3

要比较不同点当然要首先来看一下他们的构造函数有什么区别

//都是function类型
console.log(typeof a); //function
console.log(typeof b); //function
console.log(typeof c); //function

//都是Function对象的一个实例
console.log(a instanceof Function);//true
console.log(b instanceof Function);//true
console.log(c instanceof Function);//true

//也都是Object对象的一个实例
//(因为在他们的构造函数中他们的__proto__都指向的是Object)
console.log(a instanceof Object);//true
console.log(b instanceof Object);//true
console.log(c instanceof Object);//true

//相同的构造函数
console.log(a.constructor);//function Function() { [native code] }
console.log(b.constructor);//function Function() { [native code] }
console.log(c.constructor);//function Function() { [native code] }
console.log(a.constructor==b.constructor);//true
console.log(a.constructor==c.constructor);//true
console.log(b.constructor==c.constructor);//true

//不同的prototype,相同的__proto__
console.log(a.prototype);//a
console.log(a.__proto__);//function Empty() {}
console.log(b.prototype);//Object
console.log(b.__proto__);//function Empty() {}
console.log(c.prototype);//anonymous
console.log(c.__proto__);//function Empty() {}

console.log(a.prototype==b.prototype);//false
console.log(a.prototype==c.prototype);//false
console.log(b.prototype==c.prototype);//false

console.log(a.__proto__==b.__proto__);//true
console.log(a.__proto__==c.__proto__);//true
console.log(b.__proto__==c.__proto__);//true

console.log(a.prototype.__proto__==b.prototype.__proto__);//true
console.log(a.prototype.__proto__==c.prototype.__proto__);//true
console.log(b.prototype.__proto__==c.prototype.__proto__);//true

//他们的__proto__都是指向Function的原形
console.log(a.__proto__===Function.prototype);//true
console.log(b.__proto__===Function.prototype);//true
console.log(c.__proto__===Function.prototype);//true

//他们的原形的__proto__都是指向Object的原形
console.log(a.prototype.__proto__===Object.prototype);//true
console.log(b.prototype.__proto__===Object.prototype);//true
console.log(c.prototype.__proto__===Object.prototype);//true


console.log(b.prototype==Object);//false  b的原形并不等于Object(虽然构造函数中的写的是Object,我想可能是由于匿名函数没有名字，所以就用了Object代替
console.log(typeof b.prototype);//object

console.log(Function.prototype.__proto__===Object.prototype);//true 说明 Function 是从 Object 继承过来的