Using the Switch Statement with Logical Operators

JavaScript's switch statement is a pretty powerful tool, but one I've generally avoided because it's not as predictable as if statements or the ternary operator. But while working through Codesmith's CSX challenges, I set my mind to using switch for one of the problems and learned something interesting in the process.

This is the challenge:

Create a function gradeCalculator which takes a grade (number) and returns its 
letter grade.

grades 90 and above should return "A"
grades 80 to 89 should return "B"
grades 70 to 79 should return "C"
grades 60 to 69 should return "D"
59 and below should return "F"

Below is my initial solution:

function gradeCalculator(grade) {
  switch (grade) {
    case (grade >= 90):
      return "A"
    case grade >= 80:
      return "B"
    case grade >= 70:
      return "C"
    case grade >= 60:
      return "D"
    case grade <= 59:
      return "F"
  }
}

Can you spot the mistake? At first, I couldn't understand why the terminal returned

undefined
undefined
undefined
undefined
undefined

But a quick Google search brought me to a StackOverflow discussion that addressed the issue.

The Answer

Basically, JavaScript is trying to compare the expression in the parentheses to the values of the cases.

If grade = 92, grade >= 90: would return true, but I had my switch statement to comparing true to grade (or 92). True === 92 returns undefined

The proper way to formulate my switch statement is:

function gradeCalculator(grade) {
  switch (true) {
    case (grade >= 90):
      return "A"
    case grade >= 80:
      return "B"
    case grade >= 70:
      return "C"
    case grade >= 60:
      return "D"
    case grade <= 59:
      return "F"
  }
}

Check out the StackOverflow discussion here.

Comments

[Vesa Piittinen]

You don't need a break if you return :)

[Jasterix]

good point. I'll update the code to reflect this

[Rémy 🤖]

What is the advantage of this syntax over if/elseif?

[Jasterix]

I decided to use it here to get more practice with switch statements, but in this scenario, if/else would be more appropriate

this article explores when to use if/else vs a switch statement

[Robert P.]

Adding to this, MDN's article linked from The Odin Project says that one of the cases switch should be used over if...else blocks is when you have to deal with many choices.

Thank you for the article! I didn't understand why logical operators doesn't work for switch.

[Roshand79]

Thanks for this post!! I was stumped at why the switch statement I coded did not work. Chat GPT couldn't even answer the question and started waffling for about 20 minutes. I must have went back and for with GPT with no results. Once I followed suit from this post and put a Boolean in the switch expression it worked so I avoided punching my screen in the face lol.