深入了解JavaScript词法作用域

function foo(a){
	var b = a;
	return a + b;
}
var c = foo(2);
// LHS(3处)：c；a(隐式变量分配)；b;
// RHS(4处)：foo(2)；=a；a；b；

function foo(a){
	console.log(a + b);
	b = a;
}
foo(2);

function foo(str,a){
	eval(str);
	console.log(a, b);			//1 , 3
	console.log(a, window.b);	//1 , 2
}
var b = 2;
foo("var b = 3;", 1);

function foo(str,a){
	"use strict";
	eval(str);
	console.log(a, b);			//1 , 2
	console.log(a, window.b);	//1 , 2
}
var b = 2;
foo("var b = 3;", 1);

function foo(obj){
	with(obj){
		a = 2;
	}
}
var o1 = { a : 3 };
var o2 = { b : 3 };

foo(o1);
console.log( o1.a );	// 2

foo(o2);
console.log( o2.a );	// undefined
console.log( a );		// 2，表明a泄漏到全局作用域上了！

/* 匿名（引用自身只能用过期的arguments.callee引用） */
setTimeout(function(){
	console.log("i wait 1 second!")
},1000);
/* 具名（可读性好） */
setTimeout(function timeoutHandler(){
	console.log("i wait 1 seco nd!")
},1000);

/* IIFE模式 */
var a = 2;
(function IIFE(global){
	var a = 3;
	console.log(a);			//3
	console.log(global.a);	//2
})(window);
/* UMD模式 */
var b = 2;
(function UMD(def){
	def(window);
})(function tmpF(global){
	var b = 3;
	console.log(b);			//3
	console.log(global.b);	//2
});

try{
	undefined();
}catch(err){
	console.log(err);	//可以正常使用
}
console.log(err);	//ReferenceError: err is not defined

/* 坑1 */
for(var i=0;i<10;i++){}
console.log(i);		//10
/* 坑2 */
{
	console.log(bar);	//undefined 不会报错！！
	var bar = 2;
}

/* 填坑1 */
for(let i=0;i<10;i++){}
console.log(i);		//SyntaxError: Block-scoped declarations (let, const, function, class) not yet supported outside strict mode
/* 填坑2 */
{
	console.log(bar);	//SyntaxError 报错！！
	let bar = 2;
}