Lykn

Truthiness

JavaScript’s if doesn’t require a boolean. It accepts any value and coerces it to a boolean. The values that become false are called falsy. Everything else is truthy.

The Falsy Values

There are exactly eight:

Everything else is truthy. [] is truthy. {} is truthy. "false" is truthy. "0" is truthy. new Boolean(false) is truthy — a wrapped false that evaluates as true, which is the sort of thing that makes you question whether the language is being philosophical on purpose.

The Danger

Truthiness checks conflate several very different conditions:

if (user) { ... }       // false if user is: null, undefined, 0, "", false, NaN

If user is 0 — perhaps a user ID — the truthiness check fails. If user is "" — perhaps a username that hasn’t been validated yet — the truthiness check fails. The check is convenient but imprecise: it can’t distinguish “this value doesn’t exist” from “this value is zero or empty.”

Lykn’s Approach

In Lykn surface code, truthiness works the same way — if evaluates its test using JavaScript’s truthiness rules, because the compiled output is JavaScript. But the surface language encourages a more precise pattern: Option types with if-let:

;; Truthiness check (JS-style — works but imprecise)
(if user
  (greet user)
  (redirect-to-login))

;; Option check (Lykn-style — explicit and precise)
(if-let ((Some user) (find-user id))
  (greet user)
  (redirect-to-login))

The Option version distinguishes “user not found” (returns None) from “user exists but has a falsy value” (returns (Some 0) or (Some "")). This is a preview of Chapter 10, where Option, Result, and exhaustive pattern matching get their full treatment. For now, the point is: Lykn gives you truthiness because JavaScript gives you truthiness, but it also gives you better alternatives.