What does an exclamation mark before a variable mean in JavaScript

The selected answer already answers the question. One thing to add in this is that ! operator can be used in a rather interesting fashion.

obj = {}
if (!!obj) {console.log('works')} // !!obj = true
obj = undefined
if (!!obj) {console.log('does not work')} // !!obj = false

So double !! will change any expression to a boolean value with no exceptions.

This has a very peculiar use case in some cases.

• Lets say there is a function that returns either true or false and nothing else. In that case one could just change return returnValue to return !!returnValue for extra caution (although not need in most of the cases)

• Conversely, if a function only accepts true or false and nothing else, one could just call the function as functionA(!!parameter) instead of functionA(parameter), which again is not needed in most of the cases, but could ensure extra security

! 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.

It is a negation operator used for truth tests on a variable.

var myVariable = 1;

if ( ! myVariable )
{
    // myVariable evaluates as false
}

if ( myVariable )
{
    // myVariable evaluates as true
}