Javascript_备忘录3

今天备忘的是Variable Declaration and Variable Scope。今天直接上原汁原味的代码例子反而觉得更清楚：

var scope = "global"; // Declare a global variable
function checkscope() {
var scope = "local"; // Declare a local variable with the same name
return scope; // Return the local value, not the global one
}
checkscope() // => "local"

scope = "global"; // Declare a global variable, even without var.
function checkscope2() {
scope = "local"; // Oops! We just changed the global variable.          最重要的是这里，在函数体内没使用var，则修改的是全局变量。
myscope = "local"; // This implicitly declares a new global variable.
return [scope, myscope]; // Return two values.
}
checkscope2() // => ["local", "local"]: has side effects!
scope // => "local": global variable has changed.
myscope // => "local": global namespace cluttered up.

//这里主要讲函数可以嵌套
var scope = "global scope"; // A global variable
function checkscope() {
var scope = "local scope"; // A local variable
function nested() {
var scope = "nested scope"; // A nested scope of local variables
return scope; // Return the value in scope here
}
return nested();
}
checkscope() // => "nested scope"

//这个例子很重要，说明了变量声明范围遍布了整个函数体，所以局部变量屏蔽了全局变量，但是注意这里的变量初始化，他发生在声明代码之后的范围，之前的范围是没初始化的。
var scope = "global";
function f() {
console.log(scope); // Prints "undefined", not "global"
var scope = "local"; // Variable initialized here, but defined everywhere
console.log(scope); // Prints "local"
}