Just add a space:

if (...) {

} else if (...) {

} else {

}

tl;dr

Yes, you can... If a then a, else if b then if c then c(b), else b, else null

a ? a : (b ? (c ? c(b) : b) : null)

a
  ? a
  : b
      ? c
        ? c(b)
        : b
      : null

longer version

Ternary operator ?: used as inline if-else is right associative. In short this means that the rightmost ? gets fed first and it takes exactly one closest operand on the left and two, with a :, on the right.

Practically speaking, consider the following statement (same as above):

a ? a : b ? c ? c(b) : b : null

The rightmost ? gets fed first, so find it and its surrounding three arguments and consecutively expand to the left to another ?.

   a ? a : b ? c ? c(b) : b : null
                 ^                  <---- RTL
1.            |1-?-2----:-3|
             ^ <-
2.        |1-?|--2---------|:-3---|
     ^ <-
3.|1-?-2-:|--3--------------------|

result: a ? a : (b ? (c ? c(b) : b) : null)

This is how computers read it:

1. Term a is read.
   Node: a
2. Nonterminal ? is read.
   Node: a ?
3. Term a is read.
   Node: a ? a
4. Nonterminal : is read.
   Node: a ? a :
5. Term b is read.
   Node: a ? a : b
6. Nonterminal ? is read, triggering the right-associativity rule. Associativity decides:
   node: a ? a : (b ?
7. Term c is read.
   Node: a ? a : (b ? c
8. Nonterminal ? is read, re-applying the right-associativity rule.
   Node: a ? a : (b ? (c ?
9. Term c(b) is read.
   Node: a ? a : (b ? (c ? c(b)
10. Nonterminal : is read.
    Node: a ? a : (b ? (c ? c(b) :
11. Term b is read.
    Node: a ? a : (b ? (c ? c(b) : b
12. Nonterminal : is read. The ternary operator ?: from previous scope is satisfied and the scope is closed.
    Node: a ? a : (b ? (c ? c(b) : b) :
13. Term null is read.
    Node: a ? a : (b ? (c ? c(b) : b) : null
14. No tokens to read. Close remaining open parenthesis.
    #Result is: a ? a : (b ? (c ? c(b) : b) : null)

Better readability

The ugly oneliner from above could (and should) be rewritten for readability as:
(Note that the indentation does not implicitly define correct closures as brackets () do.)

a
  ? a
  : b
      ? c
        ? c(b)
        : b
      : null

for example

return a + some_lengthy_variable_name > another_variable
        ? "yep"
        : "nop"

More reading

Mozilla: JavaScript Conditional Operator
Wiki: Operator Associativity

Bonus: Logical operators

var a = 0 // 1
var b = 20
var c = null // x=> {console.log('b is', x); return true} // return true here!

a
  && a
  || b
      && c
        && c(b) // if this returns false, || b is processed
        || b
      || null

Using logical operators as in this example is ugly and wrong, but this is where they shine...

"Null coalescence"

This approach comes with subtle limitations as explained in the link below. For proper solution, see Nullish coalescing in Bonus2.

function f(mayBeNullOrFalsy) {
  var cantBeNull = mayBeNullOrFalsy || 42                    // "default" value
  var alsoCantBe = mayBeNullOrFalsy ? mayBeNullOrFalsy : 42  // ugly...
  ..
}

Short-circuit evaluation

false && (anything) // is short-circuit evaluated to false.
true || (anything)  // is short-circuit evaluated to true.

Logical operators
Null coalescence
Short-circuit evaluation

Bonus2: new in JS

Proper "Nullish coalescing"

developer.mozilla.org~Nullish_coalescing_operator

function f(mayBeNullOrUndefined, another) {
  var cantBeNullOrUndefined = mayBeNullOrUndefined ?? 42
  another ??= 37 // nullish coalescing self-assignment
  another = another ?? 37 // same effect
  ..
}

Optional chaining

Stage 4 finished proposal https://github.com/tc39/proposal-optional-chaining https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Optional_chaining

// before
var street = user.address && user.address.street
// after
var street = user.address?.street

// combined with Nullish coalescing
// before
var street = user.address
  ? user.address.street
  : "N/A"

// after
var street = user.address?.street ?? "N/A"

// arrays
obj.someArray?.[index]

// functions
obj.someMethod?.(args)