我想这是因为 person 是可见的，所以它暴露了它的所有属性。
不完全是。首先，person 是一个正则函数。它可以在没有 new-keyword 的情况下完美地调用，但结果会炸毁你的整个应用程序。
要了解原因，您应该首先了解 new-keyword 在幕后的作用。这将是一个 js 实现：
function fakeNew(constructor, ...args){
if(typeof constructor !== "function"){
throw new TypeError(constructor + " is not a constructor");
}
//create a new Instance of the constructors prototype-property
var instance = Object.create(constructor.prototype);
//invoke the constructor with the scope set to the instance and the provided arguments
var result = constructor.apply(instance, args);
//check wether the returned value is an Object (and functions are considered as Objects)
if(result === Object(result)){
//then return the result-value in favor to the instance
return result;
}
//otherwise return the instance
return instance;
}
另一方面，任何函数也可以是构造函数；没有特殊需求，一切由你决定。
所以回到杰克
var jack = person("Jack", 30); //would result in the following desaster:
console.log(jack); //undefined, since person doesn't return anthing
console.log(jack.getName());
//will throw, since jack is still undefined, and therefore doesn't have any properties
//BUT:
console.log(window.getName()) //will return "Jack" now
console.log(window.getAge); //undefined, but this is fine
//global scope has not been polluted with this one, cause getAge was a local variable inside the function-call
console.log(window.giveAge()) //can still call the enclosed (private) function getAge()
然后
var jill = person("Jill", 28);
//will overwrite the global functions and expose new values now
console.log(window.getName(), window.giveAge()) //"Jill", 28
//and Jack is kind of gone, well the variable is left but the variable contained undefined, so...
接下来是范围界定。让我们假设你做对了
//first let's add a function that executes on the scope
//inside the constructor
this.getNameAndAge = function(){
return this.getName() + ": " + getAge();
}
.
var andy = new person("Andy", 45);
var joe = new person("Joe", 32);
//let's make Andy a little younger
andy.getNameAndAge = joe.getNameAndAge;
console.log(andy.getNameAndAge(), andy.getName() + ": " + andy.giveAge());
//will result in "Andy: 32", "Andy": 45
哇？
好吧，您已经覆盖了公共方法 getNameAndAge。
通过在当前 Object上调用（也是公共的）方法 getName() 来访问名称。
但是 giveAge() 仍然是声明这个特定“getNameAndAge 函数的实例”的范围内的封闭变量，因此它来自 Joe 的函数调用。
为了理解这一点的影响，让我们让范围更奇怪
funciton Animal(species, _name){
//species is likewise a local variable and can be enclosed, modified, or whatever
//we don't need to write it to some different variable
//but we want to expose the name of this animal, since it should be possible to change it later
//without the need to create a getter and a setter just to change the property of _name
this.name = _name;
this.whoAreYou = function(){
//so we concat the enclosed value from species with the name-argument on this object
//in the hope that everything will be alright.
return species + " " + this.name;
}
}
var jack = new Animal("dog", "Jack");
var jill = new Animal("cat", "Jill");
var joe = new Animal("fish", "Joe");
console.log(jack.whoAreYou()); //"dog Jack"
console.log(jill.whoAreYou()); //"cat Jill"
console.log(joe.whoAreYou()); //"fish Joe"
//as far so good; till now ...
//since these properties are still writable someone will start and move them around
//maybe like a callback
function someFunction(someArg, callback){
console.log(someArg, callback());
}
someFunction("Who are you?", jack.whoAreYou);
//or sth. like this:
//you may not believe that someone would ever do that, but it will happen!
jack.whoAreYou = jill.whoAreYou;
console.log(jack.whoAreYou());
//and now the poor dog has an Identity-crisis.
//the first one will result in:
"Who are you?", "dog undefined"
//the latter will log "cat Jack"
or even more fummy if sth. like this happens:
var fn = joe.whoAreYou;
console.log(fn.call(jack), fn.call(jill), fn.call(joe), fn.call(Animal));
//cause now they are all fishes, even the Animal-constuctor
我不想说这很糟糕，或者你应该避免它，但它有其工作方式，应该考虑这一点。
因为这种方式为我们提供了原型继承，以及一种编写 mixin 的好方法，而无需一直编写包装方法。
您可以将其视为“我需要保护我的私人状态”，或“我在您提供给我的任何环境中工作”
还有一个问题，为什么 console.log(jack.getAge()); 抛出错误？
因为 jack.getAge 是未定义的并且未定义是没有功能的
var getAge = function() {
return age;
}
对此行的另一条评论
在 JS 中，函数和变量声明被提升，因此可以从函数的开头使用。表达式不是。
var person = function(){
//...
foo();
bar();
baz();
function foo(){ console.log("this will work"); }
var bar = function(){ console.log("this will fail"); }
//because to this point, bar was delared and assigned with undefined,
//and we remember? undefined can't be invoked
return whatever;
function baz(){ console.log("event this would work"); }
//unless some preprocessor decided (falsely), that this function can be removed
//since it is after the return-statement, and is therefore unreachable
}