Question

I know that Javascript’s eval can evaluate expressions like

(true) && (false)
(true) || (false) 
 etc.

However, does the specification of eval include evaluating statements involving unknowns like

(true) && (null)
(true) || (null)
(null) && (null)
(null) || (null)
 etc.

I have tried this in code and seen that it does it, but is this expected behavior? Do the Javascript eval specifications say this?

Answer 1

Does Javascript eval correctly evaluate tri-state boolean logic?

Define “correct”. Javascript defines the behavior. Eval in Javascript evaluates the string as a javascript code. SQL defines the behavior as well. The behavior is different in both.

In javascript, null acts like false in boolean expressions (is falsy).

0, NaN, "", null, undefined (and of course false) are all falsy. Objects, non-empty strings and non-zero numbers (and of course true) are all truthy.

&& returns the first falsy argument (if any) or the last argument (lazy AND) and does not evaluate the rest. null && "anything" is null. This can be used in statements like console && console.log && console.log().

|| returns the first truthy argument (if any) or the last argument (lazy OR) and does not evaluate the rest. null || "something" is "something". This can be used in statements like var xhr = XmlHttpRequest || ItsReplacementInOlderBrowsers

!null evaluates to true. if(null) ... evaluates the else branch. The same applies to anything falsy.

Technically, undefined variables are undefined, not null. Both are falsy, though.