JavaScript Operators – Understanding the ! Operator

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I am now confused about ! operator in JavaScript. My understanding was ! operator operates only on boolean. But a comment to one of my answers says it can operate on anything and returns a boolean, which happened to be true after I did some tests.

alert(!undefined); //true
alert(!function(){}); //false
alert(!{}); //false
alert(!null); //true
alert(!()); //crash
alert(!"false"); //false
alert(!false); //true

Can somebody help me generalize the behavior of ! operator.

EDIT

Even more confusing stuff:

alert( new String() == ""); //true
alert(!""); //true
alert(! new String()); //false

How?

Best Answer

! does what you think: turns true to false and vice-versa. The weird behavior has to do with how Javascript can convert literally anything to true or false.

http://11heavens.com/falsy-and-truthy-in-javascript

Like in C (only worse) all values can be promoted to true or false. The googlable terms you want are "truthy" and "falsy," or "truthiness" and "falsiness." Truthy means something converts to true, falsy means something converts to false. All values are truthy except null, undefined, 0, "", NaN, and... false

This link has more fun examples:

http://www.sitepoint.com/javascript-truthy-falsy/

And this site really likes doing pathological things with the funny behavior here:

http://wtfjs.com

Also note that == really tries hard to make things comparable whereas === just returns false if the things aren't comparable. Crockford in Javascript: The Good Parts recommends not using == entirely.

Related Solutions

JavaScript !! Operator – What is the !! (Not Not) Operator?

It converts Object to boolean. If it was falsy (e.g., 0, null, undefined, etc.), it would be false, otherwise, true.

!object  // Inverted Boolean
!!object // Noninverted Boolean, so true Boolean representation

So !! is not an operator; it's just the ! operator twice.

It is generally simpler to do:

Boolean(object) // Boolean

Real World Example "Test IE version":

const isIE8 = !! navigator.userAgent.match(/MSIE 8.0/);
console.log(isIE8); // Returns true or false

If you ⇒

console.log(navigator.userAgent.match(/MSIE 8.0/));
// Returns either an Array or null

But if you ⇒

console.log(!!navigator.userAgent.match(/MSIE 8.0/));
// Returns either true or false

JavaScript – Meaning of ‘!’ Operator with Non-Boolean Variables

Any falsy value will satisfy the if(!insert_variable_here) condition, including:

false
null
undefined
The empty string ''
The number 0
NaN

If callback return evaluates any of those values, the condition will be satisfied.

Even though null != false, the following will give you an alert:

x = null;
if(!x) {
    alert('"!null" does evaluate to true');
}

Regardless of whether or not null != false makes sense to you or anyone else, the point is that in JavaScript null is a falsy value, and thus a value that would satisfy the condition in my first bit of code listed above. This, it seems, is the question you have asked--not, rather, if null should or should not == false.

Best Answer

Related Solutions

JavaScript !! Operator – What is the !! (Not Not) Operator?

JavaScript – Meaning of ‘!’ Operator with Non-Boolean Variables

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