参考资料：
1.函数图解（SICP-1.2.6 ）
2.https://medium.com/javascript-scene/master-the-javascript-interview-what-is-a-pure-function-d1c076bec976
3.https://medium.com/@jamesjefferyuk/javascript-what-are-pure-functions-4d4d5392d49c

纯函数（Pure functions）：

Functions have some input (their arguments) and return some output (the result of applying them).
具有一些输入（参数）以及返回一些输出（调用结果）的函数。

  • Given the same input, will always return the same output.
  • Produces no side effects.
1
Math.abs(-2)

可以描述为接受输入并产生输出的小型机器

使用纯函数的好处：

One of the major benefits of using pure functions is they are immediately testable. They will always produce the same result if you pass in the same arguments.

非纯函数（Non-pure functions）：
In addition to returning a value, applying a non-pure function can generate side effects, which make some change to the state of the interpreter or computer. A common side effect is to generate additional output beyond the return value, using the print function.

除了返回一个值之外，调用非纯函数会产生副作用，这会改变解释器或计算机的 一些状态。一个普遍的副作用就是在返回值之外生成额外的输出

Side effects include, but are not limited to:

  • Making a HTTP request
  • Mutating data
  • Printing to a screen or console
  • DOM Query/Manipulation
  • Math.random()
  • Getting the current time
1
2
3
Math.random()
console.log(1)
console.log(console.log(1))

调用console.log的嵌套表达式，会凸显出它的非纯特性

签名（Signatures）：
A description of the arguments that a function can take is called the function’s signature.
函数的可接受参数的描述叫做函数的签名。

Math.abs(number)
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/abs
console.log(...)
https://developer.mozilla.org/zh-CN/docs/Web/API/Console/log

借助抽象，不关系底层的具体计算过程，在根高层次shanghai思考问题； 函数是最基本的一种代码抽象方式