Javascript one line If...else...else if statement

In simple words:

var x = (day == "yes") ? "Good Day!" : (day == "no") ? "Good Night!" : "";

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)

Sure, you can do nested ternary operators but they are hard to read.

var variable = (condition) ? (true block) : ((condition2) ? (true block2) : (else block2))