JavaScript函数及其prototype详解

function a(){
console.log('bbb');
}

var a = function(){
console.log('aaa');
}

var a = function a(){
console.log('bbb');
}

var a = function(){
console.log('aaa');
}
function a(){
console.log('bbb');
}
a(); //结果：aaa

(function() {
console.log(noSuchVariable);
})();
//报错，因为noSuchVariable变量根本没定义

</pre><pre name="code" class="javascript">(function() {
console.log(declaredLater); //"undefined"
var declaredLater = "Now it's defined!";
console.log(declaredLater); //"Now it's defined!"
})();
//上面这段代码是正确的，没有任何问题。

var a; // 声明一个变量，标识符为a
function a() {} // 声明一个函数，标示符也为a
alert(typeof a); // function

function a() {}
var a;
alert(typeof a); //function

function a() {}
var a = 1;
alert(typeof a); //number

var a = 1;
function a() {}
alert(typeof a); //number

typeof 2; //'number'
typeof '2'; //'string'
typeof true; //'boolean'
typeof []; //'object'
typeof null; //'object'
typeof undefined; //'undefined'
var a = function(){}; typeof a; //'function'

typeof new Number(2); //'object'
typeof new String('2'); //'object'
typeof new Boolean(true); //'object'

[] instanceof Object; //true
2 instanceof Number; //false
new Number(2) instanceof Number; //true

var a = function(){};
var b = function(){};
a.prototype = new b();
new a() instanceof b; //true
new a() instanceof Object; //true

var b = {};
b.prototype.bbb='bbb'; //报错b.prototype为undefined

function b(){}
b.prototype.bbb='bbb';
console.log(b); //结果：function b(){}
console.log(b.prototype); //结果：b {bbb: "bbb"}
console.log(b.bbb); //结果：undefined
console.log(new b().bbb); //结果：bbb

function test(){}
test.prototype; //结果：test {}
new test().prototype; //结果：undefined，对象没有prototype
new test().__proto__; //结果：test {}

function a(){
this.v='vvv';
}
a.prototype; //a {}
new a.__proto__; //a {}
new a().__proto__ === a.prototype; //true 实例对象的__proto__，指向函数的原型

function a(){
this.v='vvv';
}
a.prototype.constructor === a; //true[物理地址一样]
a.prototype.constructor; //constructor属性指代这个类
a.prototype.constructor.__proto__ === a.__proto__ === Function.prototype; //true
Object.prototype.constructor.__proto__ === Object.__proto__ === Function.prototype; //true

function a(){}
function b(){}
a.prototype = {aaa:'aaa'};
b.prototype = a.prototype;
var aa1 = new a();
var bb1 = new b();
//aa1.prototype.aaa='xxx'; //错误，对象没有原型
a.prototype.aaa = 'xxx';
console.log( aa1.aaa ); //xxx
console.log( bb1.aaa ); //xxx

function a(){}
function b(){ this.bbb='bbb'}
a.prototype = new b(); //相当于a.prototype = {bbb: "bbb"};
console.log(new a().bbb ); //结果：bbb

function a(){}
a.prototype.count = 0;
var a1 = new a();
a1.count++;
var a2 = new a();
a2.count++;
a.prototype.count; //0

function a(){}
a.count = 0;
new a().count(); //undefined，只能使用a.count调用

var scope = "global scope";
function checkScope(){
var scope = "local scope";
function f(){
return scope;
}
return f();
}
checkScope(); //local scope

function counter(){
var n=0;
return{
count:function(){
return ++n;
},
reset:function(){
n=0;
}
};
}
var c1 = new counter();
c1.count(); //1
var c2 = new counter();
c2.count(); //1
c1.reset(); //c1置为0，c2不受影响
c2.count(); //2

function a(){
console.log(this.x+this.y);
}
a.call({x:100,y:100}); //200

function C(){
this.m = function(){
console.log(this.x+this.y);
}
}
var c = new C();
c.m.call({x:100,y:100}); //200

function a(a,b,c,d){

  //将伪数组转变为真数组

  var args = [].slice.call(arguments);

}

function a(){
console.log(this); //a {}
function b(){
console.log(this); //window
}
b();
}
new a();

function a(callback){

callback();

}

a(function(){

console.log('aaa');

});