Functional programming with Javascript
Functional programming (often abbreviated FP) is the process of building software by composing pure functions, avoiding shared state, mutable data, and side-effects. Functional programming is declarative rather than imperative, and application state flows through pure functions.
WHY函数式编程这两年又火了
根本的原因是摩尔定律不适用。cpu的性能提升将体现在核数增加，这样并行的程序运行速度会越来越快。并行的程序的写法就是找出不能并行的地方，其他地方都尽量并行。如果要这样写，最需要避免的事情就是赋值。函数式编程的本质就是，规避掉“赋值”。
Important concepts
A pure function is a function which:
- Given the same inputs, always returns the same output, and
- Has no side-effects
Function composition is the process of combining two or more functions in order to produce a new function or perform some computation.
Shared state is any variable, object, or memory space that exists in a shared scope, or as the property of an object being passed between scopes. A shared scope can include global scope or closure scopes.
The problem with shared state is that in order to understand the effects of a function, you have to know the entire history of every shared variable that the function uses or affects.
An immutable object is an object that can’t be modified after it’s created. Conversely, a mutable object is any object which can be modified after it’s created.
A side effect is any application state change that is observable outside the called function other than its return value.
A higher order function is any function which takes a function as an argument, returns a function, or both.