How do I assign a conditional variable without mutation?

Question

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

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}!`;
};

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'

Answer 1

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

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

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

---

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

1. determining the day period from the hour
2. assembling the greeting string

By doing this, you avoid

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

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 !

Answer 2

const greeting = [
  'Morning', 'Morning', 'Morning', 'Morning', 'Morning', 'Morning', 'Morning', 
  'Morning', 'Morning', 'Morning', 'Morning', 'Morning',
  'Afternoon', 'Afternoon', 'Afternoon', 'Afternoon', 'Afternoon',
  'Evening', 'Evening', 'Evening', 'Evening', 'Evening', 'Evening', 'Evening'
]

return `Good ${greeting[time]} ${name}!`

This technically gives you the flexibility to add more times of day in the future, such as adding 'Noon' on the 12th hour. Additionally, it makes localization easier for similar reasons; Some locales may have a Noon others may not.

This was originally a joke ;)