if (true) {
var x = 5;
}
console.log(x); // 5 

i f (true) {
let y = 5;
}
console.log(y); // ReferenceError: y is not defined1234 

 /**
* 全局变量上浮
*/
console.log(x === undefined); // logs "true"
var x = 3;

/**
* 方法变量上浮
*/
var myvar = "my value";
// 打印变量myvar结果为：undefined
(function() {
console.log(myvar); // undefined
var myvar = "local value";
})(); 
上面代码和下面代码是等价的：
 /**
* 全局变量上浮
*/
var x;
console.log(x === undefined); // logs "true"
x = 3;

/**
* 方法变量上浮
*/
var myvar = "my value";
(function() {
var myvar;
console.log(myvar); // undefined
myvar = "local value";
})(); 

 version = "1.0.0";
console.log(window.version); //输出1.0.0 

parseInt("0xF", 16);
parseInt("F", 16);
parseInt("17", 8);
parseInt(021, 8);
parseInt("015", 10);
parseInt(15.99, 10);
arseInt("15,123", 10);
parseInt("FXX123", 16);
parseInt("1111", 2);
parseInt("15*3", 10);
parseInt("15e2", 10);
parseInt("15px", 10); 

parseFloat("3.14"); //返回数字
parseFloat("314e-2"); //返回数字
parseFloat("more non-digit characters"); //返回NaN 

 0, 117 and -345 //十进制
015, 0001 and -0o77 //八进制
0x1123, 0x00111 and -0xF1A7 //十六进制
0b11, 0b0011 and -0b11 1234 //二进制 
浮点数：[(+|-)][digits][.digits][(E|e)[(+|-)]digits]。例如：
 3.1415926，-.123456789，-3.1E+12（3100000000000），.1e-23（1e-24） 

 var car = { manyCars: {a: "Saab", "b": "Jeep"}, 7: "Mazda" };
console.log(car.manyCars.b); // Jeep
console.log(car[7]); // Mazda 

 var unusualPropertyNames = {
"": "An empty string",
"!": "Bang!"
}
console.log(unusualPropertyNames.""); // SyntaxError: Unexpected string
console.log(unusualPropertyNames[""]); // An empty string
console.log(unusualPropertyNames.!); // SyntaxError: Unexpected token !
console.log(unusualPropertyNames["!"]); // Bang! 

 var quote = "He read \"The Cremation of Sam McGee\" by R.W. Service.";
console.log(quote);
//输出：He read "The Cremation of Sam McGee" by R.W. Service.1。 

 var str = "this string \
is broken \
across multiple\
lines."
console.log(str); // this string is broken across multiplelines. 

 while (x < 10) {
x++;
} 

 var x = 1;
{
var x = 2;
}
console.log(x); // outputs 2 

 var b = new Boolean(false);
if (b) // 返回true
if (b == true) // 返回false 

 throw "Error2"; // 字符串类型
throw 42; // 数字类型
throw true; // 布尔类型
throw {toString: function() { return "I'm an object!"; } }; //对象类型 

 // 创建一个对象类型UserException
function UserException(message) {
this.message = message;
this.name = "UserException";
}

//重写toString方法，在抛出异常时能直接获取有用信息
UserException.prototype.toString = function() {
return this.name + ': "' + this.message + '"';
}

// 创建一个对象实体并抛出它
throw new UserException("Value too high"); 

 function f() {
  try {
    console.log(0);
    throw "bogus";
  } catch(e) {
    console.log(1);
    return true; // 返回语句被暂停，直到finally执行完成
    console.log(2); // 不会执行的代码
  } finally {
    console.log(3);
    return false; //覆盖try.catch的返回
    console.log(4); //不会执行的代码
  }
  // "return false" is executed now 
  console.log(5); // not reachable
}
f(); // 输出 0, 1, 3; 返回 false 

 function f() {
  try {
    throw "bogus";
  } catch(e) {
    console.log('caught inner "bogus"');
    throw e; // throw语句被暂停，直到finally执行完成
  } finally {
    return false; // 覆盖try.catch中的throw语句
  }
  // 已经执行了"return false"
}

try {
  f();
} catch(e) {
  //这里不会被执行，因为catch中的throw已经被finally中的return语句覆盖了
  console.log('caught outer "bogus"');
}
// 输出
// caught inner "bogus"