javascript中的变量作用域以及变量提升详细介绍_javascript技巧

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