如何在没有变异的情况下分配条件变量？ [英] How do I assign a conditional variable without mutation?

问题描述

我坚持严格的函数式编程原则，没有变异。

I am adhering to strict functional programming principles with no mutation.

我怎样才能以不改变<$的方式编写类似下面代码的内容c $ c>问候变量，如果阻止，则不在每个内返回？

How can I write something like the below code in a way that doesn't mutate the greeting variable, and without returning it within each if block?

const greet = (name, time) => { 
  let greeting = 'Morning';

  if(time >= 12) {
    greeting = 'Afternoon';
  }
  if(time >= 17) {
    greeting = 'Evening';
  }

  return `Good ${greeting} ${name}!`;
};

如果只是两个条件我会做以下事情，但是当它有时，它将不起作用3个条件：

If it was just two conditions I would do the following, but it won't work when there are 3 conditions:

const greeting = time > 12 ? 'Afternoon' : 'Morning'

推荐答案

三元表达式可以组成其他三元表达式 - 允许我们排序逻辑选择

Ternary expressions can be made up other ternary expressions – allowing us to sequence logical choices

const greeting = time > 12 ? (time > 17 ? 'Evening' : 'Afternoon') : 'Morning'

但是，我认为变量使变量成为变量 ...

However, I think it's the variable that makes the variable a variable...

您有两个担忧，并且它将使您分开它们将受益

You have two concerns though, and it it will benefit you to separate them

1. 确定从小时开始的日期


2. 组装问候语字符串

按这样做，你可以避免

1. 变异（本地重新分配问候语）


2. 单分支如果语句


3. 命令式风格的陈述（即让， if ， return ， x = ... ）

1. mutation (local reassignment of greeting)
2. single-branch if statements
3. imperative-style statements altogether (ie let, if, return, x = ...)

结果是两个纯粹的（ referentially transparent ）使用表达式编写的函数 - 有没有任务（或重新分配），也没有副作用。

The result is two pure (referentially transparent) functions written using expressions – there is no assignment (or reassignment), and there are no side-effects.

const timeToPeriod = time =>
  time >= 17
    ? 'Evening'
    : time >= 12
      ? 'Afternoon'
      : 'Morning'

const greet = (name, time) =>
  `Good ${timeToPeriod(time)} ${name} !`
  
console.log(greet('Jonas', 9))  // Good Morning Jonas !
console.log(greet('Jonas', 13)) // Good Afternoon Jonas !
console.log(greet('Jonas', 22)) // Good Evening Jonas !

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