What does an exclamation mark before a variable mean in JavaScript

Question

I'm trying to learn JavaScript by going through some code in an application and I keep seeing !variable in if conditions. For example:

if (!variable.onsubmit || (variable.onsubmit() != false)) {

What is it? Some kind of test if the variable is empty?

Answer 1

! is the logical not operator in JavaScript.

Formally

!expression is read as:

• Take expression and evaluate it. In your case that's variable.onsubmit
• Case the result of that evaluation and convert it to a boolean. In your case since onsubmit is likely a function, it means - if the function is null or undefined - return false, otherwise return true.
• If that evaluation is true, return false. Otherwise return true.

In your case

In your case !variable.onsubmit means return true if there isn't a function defined (and thus is falsy), otherwise return false (since there is a function defined).

Simply put - !variable means take the truth value of variable and negate it.

Thus, if (!variable) { will enter the if clause if variable is false (or coerces to false)

In total

if (!variable.onsubmit || (variable.onsubmit() != false)) {

Means - check if variable.onsubmit is defined and truthy (thus true), then it checks if calling onsubmit returns a result that coerces to true. In a short line it checks if there is no onsubmit or it returns true.

Next time, how do I find this myself?

• MDN has a list of operators here.
• The language specification specifies such operators, though being the official specification it does contain some jargon which might be hard to understand.