作用域
編程語言中作用于域的定義
Scope refers to the visibility of variables. In other words, which parts of your program can see or use it.
譯為：作用域指的是變量的可見性鹉动， 換句話說僻族， 就是程序可以看見和使用的那部分變量。
JS中的作用域
JS采用的是詞法作用域（Lexical scope, static scope）
Lexical scoping (sometimes known as static scoping ) is a convention used with many programming languages that sets the scope (range of functionality) of a variable so that it may only be called (referenced) from within the block of code in which it is defined. The scope is determined when the code is compiled.
譯為： 詞法作用域（有時(shí)也稱為靜態(tài)作用域）是許多編程語言使用的一種約定命爬，它設(shè)置變量的范圍笔宿，以便于只能從定義它的代碼塊中調(diào)用它律姨。作用域在代碼編譯階段決定米苹。
當(dāng)然有靜態(tài)作用域就會(huì)有動(dòng)態(tài)作用域，這里不多做討論忍啸。
舉個(gè)例子
let a = 'global'
function showA() {
console.log(a)
}
function foo() {
let a = 'foo'
showA()
}
foo() // 'global'
例子中仰坦， 在foo方法中調(diào)用了showA方法，最終打印出來的結(jié)果居然是'global'! 為什么计雌？ showA方法在編譯階段就將a指向了全局中的a悄晃。牢記那句話作用域在代碼編譯階段決定!
