Discussion on: Rethinking JavaScript: The if statement

[edA‑qa mort‑ora‑y]

It's this pattern I disagree with:

const doSomething = thing =>
  checkTheThing(thing)
    ? doOneThing(thing)
    : doOtherThing(thing)

In particular I disagree with the do... functions. Ternary is meant for evaluation, such as:

const result = thing =>
  checkTheThing(thing)
    ? evalOneThing(thing)
    : evalOtherThing(thing)

There is a subtle, but important difference. This clearly results in a value that we want to use, whereas the do... form is branching to perform some action, it's unclear what result is desired.

Consider if the functions don't return anything. Then you'd have:

var voidResult = check ? voidFoo() : voidBar()

This won't even be allowed in many statically type languages, as they can't evaluate void. It also means the result cannot be used, implying that the expression itself isn't being used. Logically if a value isn't being used you'd expect you can skip the evaluation of it. But instead you have code where despite not using the result, you require the code to be evaluated nonetheless.

Moreso, you're implicitly relying on a short-circuiting of this expression, which may not actually be case in all languages. Consider that operators are short-forms for functions, in this case you have a function like this:

ternaryOp( cond, eval1, eval2 )

But you can't translate your code to this:

var result = ternaryOp( checkTheThing(thing), evalOneThing(thing), evalOtherThing(thing) );

This always evaluates both of the arguments, which is not correct for your code. It's failed a logical substituion test.

The form that I would allow is this:

var result = (checkTheThing(thing) ? evalOneThing : evalOtherThing)(thing);

This is okay because the ternary is a proper evaluation. It doesn't require compiler short-circuiting, it isn't hiding any code branch. It works with void return as well (dropping the assignment).

[JavaScript Joel]

var voidResult = check ? voidFoo() : voidBar()
This won't even be allowed in many statically type languages, as they can't evaluate void

That is absolutely correct. Though this article is exclusive to JavaScript. There is quite a bit of JavaScript that doesn't translate over to other languages.

You can't even do this in C#

// ERROR
var result = voidFoo();

You still always need to follow order of operations. For example:

// this function...
const result = ternaryOp( checkTheThing(thing), evalOneThing(thing), evalOtherThing(thing) );

// ...is equivalent to
const val1 = checkTheThing(thing)
const val2 = evalOneThing(thing)
const val3 = evalOtherThing(thing)
const result = ternaryOp(val1, val2, val3)

...when compared to...

// this thing...
const result =
  checkTheThing(thing)
    ? doOneThing(thing)
    : doOtherThing(thing)

// ...is equivalent to
const val1 = checkTheThing(thing)
let result
if (val1) {
  result = doOneThing(thing)
} else {
  result = doOtherThing(thing)
}

Moreso, you're implicitly relying on a short-circuiting of this expression

For sure. Though again, this article is exclusively about JavaScript. I'm still not completely sure this is short-circuiting, I have only heard that in reference to the && and || operators.

Another example:

const result = obj ? obj.name : 'unknown'

The reason why this works is because both branches are not evaluated. This is not exclusive to my examples, but fundamental to how the ternary operator works in JavaScript.

The ternary works differently than VB.net's IIf operator, which evaluates both sides (this is close to your example). VB.net later added the ability to do Dim foo as String = If(bar = buz, cat, dog), which will only evaluate one side.

I would expect an understanding to how any operator works in your language prior to using it to be mandatory. JavaScript's ternary operator works in a fairly common way, for example, it works the same way in C and Java.

I will consider updating the article to clarify this is JavaScript as well as more examples on how the fundamentals of the ternary work.

All very good feedback.

Cheers!