var name = "Richard"; function showName () {     var name = "Jack"; // local variable; only accessible in this showName function     console.log (name); // Jack } console.log (name); // Richard: the global variable

var name = "Richard"; // the blocks in this if statement do not create a local context for the name variable if (name) {     name = "Jack"; // this name is the global name variable and it is being changed to "Jack" here     console.log (name); // Jack: still the global variable } // Here, the name variable is the same global name variable, but it was changed in the if statement console.log (name); // Jack //    不要忘记使用var关键字 //    如果声明一个变量的时候没有使用var关键字，那么这个变量将是一个全局变量！ // If you don't declare your local variables with the var keyword, they are part of the global scope var name = "Michael Jackson"; function showCelebrityName () {     console.log (name); } function showOrdinaryPersonName () {        name = "Johnny Evers";     console.log (name); } showCelebrityName (); // Michael Jackson // name is not a local variable, it simply changes the global name variable showOrdinaryPersonName (); // Johnny Evers // The global variable is now Johnny Evers, not the celebrity name anymore showCelebrityName (); // Johnny Evers // The solution is to declare your local variable with the var keyword function showOrdinaryPersonName () {        var name = "Johnny Evers"; // Now name is always a local variable and it will not overwrite the global variable     console.log (name); } //    局部变量优先级大于全局变量 //如果在全局作用域中什么的变量在局部作用域中再次声明，那么在局部作用域中调用这个变量时，优先调用局部作用域中声明的变量： var name = "Paul"; function users () {     // Here, the name variable is local and it takes precedence over the same name variable in the global scope var name = "Jack"; // The search for name starts right here inside the function before it attempts to look outside the function in the global scope console.log (name); } users (); // Jack

// name and sex is not in any function var myName = "zhou"; var sex = "male"; //他们都处在window对象中 console.log(window.myName); //paul console.log('sex' in window); //true

function showAge(){   //age初始化时没有使用var关键字，所以它是一个全局变量   age = 20;   console.log(age); } showAge();  //20 console.log(age); //因为age是全局变量，所以这里输出的也是20

var Value1 = 200; var Value2 = 20; var myObj = {   Value1 : 10,   Value2 : 1,   caleculatedIt: function(){     setTimeout(function(){       console.log(this.Value1 * this.Value2);     }, 1000);   } } myObj.caleculatedIt(); //4000

function showName () { console.log ("First Name: " + name); var name = "Ford"; console.log ("Last Name: " + name); } showName (); // First Name: undefined // Last Name: Ford // The reason undefined prints first is because the local variable name was hoisted to the top of the function // Which means it is this local variable that get calls the first time. // This is how the code is actually processed by the JavaScript engine: function showName () {     var name; // name is hoisted (note that is undefined at this point, since the assignment happens below) console.log ("First Name: " + name); // First Name: undefined name = "Ford"; // name is assigned a value // now name is Ford console.log ("Last Name: " + name); // Last Name: Ford }

// Both the variable and the function are named myName var myName;? function myName () { console.log ("Rich"); } // The function declaration overrides the variable name console.log(typeof myName); // function

// But in this example, the variable assignment overrides the function declaration var myName = "Richard"; // This is the variable assignment (initialization) that overrides the function declaration. function myName () { console.log ("Rich"); } console.log(typeof myName); // string