来看看下面的代码：
const name = "Lydia"
const age = 21
const city = "San Francisco"
function getPersonInfo() {
const name = "Sarah"
const age = 22
return `${name} is ${age} and lives in ${city}`
}
console.log(getPersonInfo())
We're invoking the getPersonInfo function, which returns a string containing the values of the name, age and city variables:
我们正在调用 getPersonInfo 函数，该函数将返回一个包含姓名、年龄和城市变量值的字符串：Sarah is 22 and lives in San Francisco.
But, the getPersonInfo function doesn't contain a variable named city 🤨? How did it know the value of city?
First, memory space is set up for the different contexts.
首先，为不同的上下文设置内存空间。
We have the default global context (window in a browser, global in Node),
我们有默认的全局上下文（浏览器中的window，Node 中的 global上下文），
and a local context for the getPersonInfo function which has been invoked.
以及用于调用 getPersonInfo 函数的本地上下文。
Each context also has a scope chain.
每个上下文还有一个作用域链。
For the getPersonInfo function, the scope chain looks something like this :
对于 getPersonInfo 函数，作用域链看起来是这样的：
local execution context -> local scope chain 本地执行上下文 -> 本地作用域链
The scope chain is basically a "chain of references" to objects
作用域链基本上是对象的 "引用链"，
that contain references to values (and other scopes) that are referencable in that execution context.
其中包含对该执行上下文中可引用的值（和其他作用域）的引用。
In the following examples,
在下面的示例中，
the key/value pairs in the execution contexts represent the references that the scope chain has to the variables.
执行上下文中的键/值对表示作用域链对变量的引用。
The scope chain of the global execution context has a reference to 3 variables: name with the value Lydia, age with the value 21, and city with the value San Francisco.
全局执行上下文的作用域链引用了 3 个变量：姓名（值为 Lydia）、年龄（值为 21）和城市（值为旧金山）。
In the local context, we have a reference to 2 variables: name with the value Sarah, and age with the value 22.
在本地上下文中，我们引用了 2 个变量：姓名（值为 Sarah）和年龄（值为 22）。
When we try to access the variables in the getPersonInfo function, the engine first checks the local scope chain.
当我们尝试访问 getPersonInfo 函数中的变量时，引擎会首先检查本地作用域链。
In the global context,
在全局上下文中，
we declared the variable city with the value of San Francisco,
我们声明了值为 San Francisco 的变量 city，
thus has a reference to the variable city.
因此有了对变量 city 的引用。
Now that we have a value for the variable,
现在我们有了变量的值，
the function getPersonInfo can return the string Sarah is 22 and lives in San Francisco 🎉
函数 getPersonInfo 可以返回字符串 Sarah is 22 and lives in San Francisco 🎉
We can go down the scope chain, but we can't go up the scope chain.
我们可以向下移动作用域链，但不能向上移动作用域链。