f that operand evaluates to a value likened to logically true (according to a language-dependent convention, in other words, a truthy value)

false is a perfectly non-nil object, and you're excluding it out of convenience!

Check whether a value is defined (non-nullable), meaning it is neither `null` or `undefined`. This can be useful as a type guard, as for example, `[1, null].filter(Boolean)` does not always type-guard correctly.