482 私有栈内存处理

闭包：函数执行形成的私有栈内存，会把内存中的所有私有变量都保护起来，和外面没有任何关系，函数的这种保护机制就是闭包

console.log(a, b);
var a = 12,
	b = 12;

function fn() {
	console.log(a, b);
	var a = b = 13;
	console.log(a, b);
}
fn();
console.log(a, b);

---

console.log(a, b, c);
var a = 12,
	b = 13,
	c = 14;

function fn(a) {
	console.log(a, b, c);
	a = 100;
	c = 200;
	console.log(a, b, c);
}
b = fn(10);
console.log(a, b, c);

---

function sum(a) {
	console.log(a);
	let a = 100; // => Uncaught SyntaxError: Identifier 'a' has already been declared
	console.log(a);
}
sum(200);

---

var ary = [12, 23];

function fn(ary) {
	console.log(ary);
	ary[0] = 100;
	ary = [100];
	ary[0] = 0;
	console.log(ary);
}
// 注意，这里传递的是全局变量arr保存的地址值，不是传递[12, 23]
fn(ary);
console.log(ary);