mutation

One pattern many people follow is to be liberal with mutation when constructing data, but conservative with mutation when consuming data.

let

let permits us to rebind variables.

let age = 52;
age = 53;
age
//=> 53

Declaring a variable const does not prevent us from mutating its value, only prevent us from rebinding its name. This is an important distinction.

const ls = []
ls = [1] // not allowed
ls[0] = 1 // allowed

js没有immutable变量，F#有，它不仅不允许rebinding, 也不允许修改它的值。

js模块跨文件的exprot和import都应该是immutable变量。但它不是，我们假设它是的，从来不会修改它，只是会读取它，执行它。