Go vs JavaScript: Switch Statements

Both Go and JavaScript feature a switch statement, but there are a few distinct differences in behavior between the languages. Let’s take some time to explore those differences.

Go switch statements do not have fall-through by default

Fall through happens when a break statement is omitted and execution continues to the next case clause all the way down to the default clause regardless of whether there is a value match. In Go, only the matched case is run, and any subsequent cases will be ignored. Let’s take a look at an example:

JavaScript

const weekday = new Date().getDay()

switch (weekday) {
    case 0:
        console.log('Sunday')
    case 1:
        console.log('Monday')
    case 2:
        console.log('Tuesday')
    case 3:
        console.log('Wednesday')
    case 4:
        console.log('Thursday')
    case 5:
        console.log('Friday')
    case 6:
        console.log('Saturday')
    default:
        console.log('Unknown day')
}

Assuming today is Monday, what do you think the JavaScript program will output?

Monday
Tuesday
Wednesday
Thursday
Friday
Saturday
Unknown day

That doesn’t seem like something we want! You’ll observe that once the matching case was hit, JavaScript continued to execute all the following cases since we omitted a break statement.

To fix our JavaScript program, we’d need to add break statements for each case:

const weekday = new Date().getDay()

switch (weekday) {
    case 0:
        console.log('Sunday')
        break
    case 1:
        console.log('Monday')
        break
    case 2:
        console.log('Tuesday')
        break
    case 3:
        console.log('Wednesday')
        break
    case 4:
        console.log('Thursday')
        break
    case 5:
        console.log('Friday')
        break
    case 6:
        console.log('Saturday')
        break
    default:
        console.log('Unknown day')
}

Monday

Now that seems to match our desired behavior!

Go

And now let’s compare that to the Go program:

package main

import (
	"fmt"
	"time"
)

func main() {
	var weekday = time.Now().Weekday()

	switch weekday {
	case time.Sunday:
		fmt.Println("Sunday")
	case time.Monday:
		fmt.Println("Monday")
	case time.Tuesday:
		fmt.Println("Tuesday")
	case time.Wednesday:
		fmt.Println("Wednesday")
	case time.Thursday:
		fmt.Println("Thursday")
	case time.Friday:
		fmt.Println("Friday")
	case time.Saturday:
		fmt.Println("Saturday")
	default:
		fmt.Println("Unknown day")
	}
}

Monday

Much better!

For this difference, I prefer Go’s approach as it can be easy to miss a break when writing a switch statement which in turn can cause some unwanted bugs! There is an eslint rule to warn you on unintentional fall throughs.

Go switch statements allow multiple expressions in the same case statement

In Go, you can have multiple expressions in the same case statement so that if one of those expressions matches, the case is executed. Although JavaScript doesn’t allow this, a similar result can be achieved by using fall through. Let’s take a look at an example:

JavaScript

const weekday = new Date().getDay()

switch (weekday) {
    case 0:
    case 6:
        console.log('The weekend!')
        break
    default:
        console.log('Weekday')
}

You’ll notice that when either case: 0 or case: 1 is met, we’ll see the output “The weekend!”. Assuming today is Monday, here is the output:

Weekday

Go

package main

import (
	"fmt"
	"time"
)

func main() {
	var weekday = time.Now().Weekday()

	switch weekday {
	case time.Sunday, time.Saturday:
		fmt.Println("The weekend!")
	default:
		fmt.Println("Weekday")
	}
}

In Go, the criteria to meet the single case can either be Sunday or Saturday. Assuming today is Monday, here is the output:

Weekday

Go can omit the switch expression

In Go, you can omit the switch expression to create an alternate else/if logic. Although this also isn’t supported in JavaScript, you can do something similar by using switch (true):

JavaScript

In the following example, the case is looking for the statement to be true, so it will short circuit on the first case that meets this condition, which is similar to a traditional if/else block.

const weekday = new Date().getDay()

switch (true) {
    case weekday === 1:
        console.log('Monday')
        break
    default:
        console.log('Not Monday')
}

Monday

Go

Similar to the above example, the first case that is true will be executed.

package main

import (
	"fmt"
	"time"
)

func main() {
	var weekday = time.Now().Weekday()

	switch {
	case weekday == time.Monday:
		fmt.Println("Monday")
	default:
		fmt.Println("Not Monday")
	}
}

Conclusion

And there you have it! There are the differences between Go and JavaScript switch statements. I’m currently in the process of learning Go and I found these differences to be quite interesting. I look forward to sharing any other interesting pieces of information I find on my learning journey!