Vincent Gable’s Blog

One thing about JavaScript I really like is that its ||, the Logical Or operator, is really a more general ‘Eval Until True‘ operation. (If you have a better name for this operation, please leave a comment!) It’s the same kind of or operator used in Lisp. And I believe it’s the best choice for a language to use.

In C/C++, a || b is equivalent to,

  if a evaluates to a non-zero value:
    return true;
  if b evaluates to a non-zero value:
    return true;
  otherwise:
    return false;

Note that if a can be converted to true, then b is not evaluated. Importantly, in C/C++ || always returns a bool.

But the JavaScript || returns the value of the first variable that can be converted to true, or the last variable if both variables can’t be interpreted as true,

  if a evaluates to a non-zero value:
    return a;
  otherwise:
    return b;

Concise

JavaScript’s || is some sweet syntactic sugar.

We can write,

return playerName || "Player 1";

instead of,

return playerName ? playerName : "Player 1";

And simplify assert-like code in a perl-esq way,

x || throw "x was unexpectedly null!";

It’s interesting that a more concise definition of || allows more concise code, even though intuitively we’d expect a more complex || to “do more work for us”.

General

Defining || to return values, not true/false, is much more useful for functional programming.

The short-circuit-evaluation is powerful enough to replace if-statements. For example, the familiar factorial function,

function factorial(n){
	if(n == 0) return 1;
	return n*factorial(n-1);
}

can be written in JavaScript using && and || expressions,

function factorial2(n){ return n * (n && factorial2(n-1)) || 1;}

Yes, I know this isn’t the clearest way to write a factorial, and it would still be an expression if it used ?:, but hopefully this gives you a sense of what short-circuiting operations can do.

Unlike ?:, the two-argument || intuitively generalizes to n arguments, equivalent to a1 || a2 || ... || an. This makes it even more useful for dealing with abstractions.

Logical operators that return values, instead of simply booleans, are more expressive and powerful, although at first they may not seem useful — especially coming from a language without them.