What is the !! (not not) operator in JavaScript?
JavascriptOperatorsJavascript Problem Overview
I saw some code that seems to use an operator I don't recognize, in the form of two exclamation points, like so: !!
. Can someone please tell me what this operator does?
The context in which I saw this was,
this.vertical = vertical !== undefined ? !!vertical : this.vertical;
Javascript Solutions
Solution 1 - Javascript
Converts Object
to boolean
. If it was falsey (e.g. 0
, null
, undefined
, etc.), it will be false
, otherwise, true
.
!object // inverted boolean
!!object // non inverted boolean so true boolean representation
So !!
is not an operator, it's just the !
operator twice.
It may be 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
Solution 2 - Javascript
It's a horribly obscure way to do a type conversion.
!
means NOT. So !true
is false
, and !false
is true
. !0
is true
, and !1
is false
.
So you're converting a value to a boolean, then inverting it, then inverting it again.
// Maximum Obscurity:
val.enabled = !!userId;
// Partial Obscurity:
val.enabled = (userId != 0) ? true : false;
// And finally, much easier to understand:
val.enabled = (userId != 0);
// Or just
val.enabled = Boolean(userId);
Note: the latter two expressions aren't exactly equivalent to the first expression when it comes to some edge case (when userId
is []
, for example), due to the way the !=
operator works and what values are considered truthy.
Solution 3 - Javascript
!!expr
(two !
operators followed by an expression) returns a Boolean value (true
or false
) depending on the truthiness of the expression. It makes more sense when used on non-boolean types. Consider these examples, especially the 3rd example and onward:
!!false === false
!!true === true
!!0 === false
!!parseInt("foo") === false // NaN is falsy
!!1 === true
!!-1 === true // -1 is truthy
!!(1/0) === true // Infinity is truthy
!!"" === false // empty string is falsy
!!"foo" === true // non-empty string is truthy
!!"false" === true // ...even if it contains a falsy value
!!window.foo === false // undefined value is falsy
!!undefined === false // undefined primitive is falsy
!!null === false // null is falsy
!!{} === true // an (empty) object is truthy
!![] === true // an (empty) array is truthy; PHP programmers beware!
Solution 4 - Javascript
Brew some tea:
!!
is not an operator. It is the double-use of !
-- which is the logical "not" operator.
In theory:
!
determines the "truth" of what a value is not:
-
The truth is that
false
is nottrue
(that's why!false
results intrue
) -
The truth is that
true
is notfalse
(that's why!true
results infalse
)
!!
determines the "truth" of what a value is not not:
-
The truth is that
true
is not nottrue
(that's why!!true
results intrue
) -
The truth is that
false
is not notfalse
(that's why!!false
results infalse
)
What we wish to determine in the comparison is the "truth" about the value of a reference, not the value of the reference itself. There is a use-case where we might want to know the truth about a value, even if we expect the value to be false
(or falsey), or if we expect the value not to be typeof boolean
.
In practice:
Consider a concise function which detects feature functionality (and in this case, platform compatibility) by way of dynamic typing (aka "duck typing"). We want to write a function that returns true
if a user's browser supports the HTML5 <audio>
element, but we don't want the function to throw an error if <audio>
is undefined; and we don't want to use try ... catch
to handle any possible errors (because they're gross); and also we don't want to use a check inside the function that won't consistently reveal the truth about the feature (for example, document.createElement('audio')
will still create an element called <audio>
even if HTML5 <audio>
is not supported).
Here are the three approaches:
// this won't tell us anything about HTML5 `<audio>` as a feature
var foo = function(tag, atr) { return document.createElement(tag)[atr]; }
// this won't return true if the feature is detected (although it works just fine)
var bar = function(tag, atr) { return !document.createElement(tag)[atr]; }
// this is the concise, feature-detecting solution we want
var baz = function(tag, atr) { return !!document.createElement(tag)[atr]; }
foo('audio', 'preload'); // returns "auto"
bar('audio', 'preload'); // returns false
baz('audio', 'preload'); // returns true
Each function accepts an argument for a <tag>
and an attribute
to look for, but they each return different values based on what the comparisons determine.
But wait, there's more!
Some of you probably noticed that in this specific example, one could simply check for a property using the slightly more performant means of checking if the object in question has a property. There are two ways to do this:
// the native `hasOwnProperty` method
var qux = function(tag, atr) { return document.createElement(tag).hasOwnProperty(atr); }
// the `in` operator
var quux = function(tag, atr) { return atr in document.createElement(tag); }
qux('audio', 'preload'); // returns true
quux('audio', 'preload'); // returns true
We digress...
However rare these situations may be, there may exist a few scenarios where the most concise, most performant, and thus most preferred means of getting true
from a non-boolean, possibly undefined value is indeed by using !!
. Hopefully this ridiculously clears it up.
Solution 5 - Javascript
!!
converts the value to the right of it to its equivalent boolean value. (Think poor man's way of "type-casting"). Its intent is usually to convey to the reader that the code does not care what value is in the variable, but what it's "truth" value is.
Solution 6 - Javascript
!!foo
applies the unary not operator twice and is used to cast to boolean type similar to the use of unary plus +foo
to cast to number and concatenating an empty string ''+foo
to cast to string.
Instead of these hacks, you can also use the constructor functions corresponding to the primitive types (without using new
) to explicitly cast values, ie
Boolean(foo) === !!foo
Number(foo) === +foo
String(foo) === ''+foo
Solution 7 - Javascript
So many answers doing half the work. Yes, !!X
could be read as "the truthiness of X [represented as a boolean]". But !!
isn't, practically speaking, so important for figuring out whether a single variable is (or even if many variables are) truthy or falsy. !!myVar === true
is the same as just myVar
. Comparing !!X
to a "real" boolean isn't really useful.
The only thing you gain with !!
is the ability to check the truthiness of multiple variables against each other in a repeatable, standardized (and JSLint friendly) fashion.
Simply casting :(
That is...
0 === false
isfalse
.!!0 === false
istrue
.
The above's not so useful. if (!0)
gives you the same results as if (!!0 === false)
. I can't think of a good case for casting a variable to boolean and then comparing to a "true" boolean.
See "== and !=" from JSLint's directions (note: Crockford is moving his site around a bit; that link is liable to die at some point) for a little on why:
> The == and != operators do type coercion before comparing. This is bad because it causes ' \t\r\n' == 0 to be true. This can mask type errors. JSLint cannot reliably determine if == is being used correctly, so it is best to not use == and != at all and to always use the more reliable === and !== operators instead.
>
> If you only care that a value is truthy or falsy, then use the short form. Instead of
> (foo != 0)
>
> just say
> (foo)
>
> and instead of
> (foo == 0)
>
> say
> (!foo)
Note that there are some unintuitive cases where a boolean will be cast to a number (true
is cast to 1
and false
to 0
) when comparing a boolean to a number. In this case, !!
might be mentally useful. Though, again, these are cases where you're comparing a non-boolean to a hard-typed boolean, which is, imo, a serious mistake. if (-1)
is still the way to go here.
╔═══════════════════════════════════════╦═══════════════════╦═══════════╗ ║ Original ║ Equivalent ║ Result ║ ╠═══════════════════════════════════════╬═══════════════════╬═══════════╣ ║ if (-1 == true) console.log("spam") ║ if (-1 == 1) ║ undefined ║ ║ if (-1 == false) console.log("spam") ║ if (-1 == 0) ║ undefined ║ ║ Order doesn't matter... ║ ║ ║ ║ if (true == -1) console.log("spam") ║ if (1 == -1) ║ undefined ║ ╠═══════════════════════════════════════╬═══════════════════╬═══════════╣ ║ if (!!-1 == true) console.log("spam") ║ if (true == true) ║ spam ║ better ╠═══════════════════════════════════════╬═══════════════════╬═══════════╣ ║ if (-1) console.log("spam") ║ if (truthy) ║ spam ║ still best ╚═══════════════════════════════════════╩═══════════════════╩═══════════╝
And things get even crazier depending on your engine. WScript, for instance, wins the prize.
function test()
{
return (1 === 1);
}
WScript.echo(test());
Because of some historical Windows jive, that'll output -1 in a message box! Try it in a cmd.exe prompt and see! But WScript.echo(-1 == test())
still gives you 0, or WScript's false
. Look away. It's hideous.
Comparing truthiness :)
But what if I have two values I need to check for equal truthi/falsi-ness?
Pretend we have myVar1 = 0;
and myVar2 = undefined;
.
myVar1 === myVar2
is0 === undefined
and is obviously false.!!myVar1 === !!myVar2
is!!0 === !!undefined
and is true! Same truthiness! (In this case, both "have a truthiness of falsy".)
So the only place you'd really need to use "boolean-cast variables" would be if you had a situation where you're checking if both variables have the same truthiness, right? That is, use !!
if you need to see if two vars are both truthy or both falsy (or not), that is, of equal (or not) truthiness.
I can't think of a great, non-contrived use case for that offhand. Maybe you have "linked" fields in a form?
if (!!customerInput.spouseName !== !!customerInput.spouseAge ) {
errorObjects.spouse = "Please either enter a valid name AND age "
+ "for your spouse or leave all spouse fields blank.";
}
So now if you have a truthy for both or a falsy for both spouse name and age, you can continue. Otherwise you've only got one field with a value (or a very early arranged marriage) and need to create an extra error on your errorObjects
collection.
Though even in this case, the !!
really is superfluous. One !
was enough to cast to a boolean, and you're just checking equality.
EDIT 24 Oct 2017, 6 Feb 19:
3rd party libraries that expect explicit Boolean values
Here's an interesting case... !!
might be useful when 3rd party libs expect explicit Boolean values.
React
For instance, False in JSX (React) has a special meaning that's not triggered on simple falsiness. If you tried returning something like the following in your JSX, expecting an int in messageCount
...
{messageCount && <div>You have messages!</div>}
... you might be surprised to see React render a 0
when you have zero messages. You have to explicitly return false for JSX not to render. The above statement returns 0
, which JSX happily renders, as it should. It can't tell you didn't have Count: {messageCount}
.
-
One fix involves the bangbang, which coerces
0
into!!0
, which isfalse
:
{!!messageCount && <div>You have messages!</div>}
-
JSX' docs suggest you be more explicit, write self-commenting code, and use a comparison to force to a Boolean.
{messageCount > 0 && <div>You have messages!</div>}
-
I'm more comfortable handling falsiness myself with a ternary --
{messageCount ? <div>You have messages!</div> : false}
Typescript
Same deal in Typescript: If you have a function that returns a boolean (or you're assigning a value to a boolean variable), you [usually] can't return/assign a boolean-y value; it has to be a strongly typed boolean. This means, iff myObject
is strongly typed, return !myObject;
works for a function returning a boolean, but return myObject;
doesn't. You have to return !!myObject
(or cast to the proper boolean another way) to match Typescript's expectations.
> The exception for Typescript? If myObject
was an any
, you're back in JavaScript's Wild West and can return it without !!
, even if your return type is a boolean.
Keep in mind that these are JSX & Typescript conventions, not ones inherent to JavaScript.
But if you see strange 0
s in your rendered JSX, think loose falsy management.
Solution 8 - Javascript
It's just the logical NOT operator, twice - it's used to convert something to boolean, e.g.:
true === !!10
false === !!0
Solution 9 - Javascript
It converts the suffix to a Boolean value.
Solution 10 - Javascript
It's a double not
operation. The first !
converts the value to boolean and inverts its logical value. The second !
inverts the logical value back.
Solution 11 - Javascript
It seems that the !!
operator results in a double negation.
var foo = "Hello World!";
!foo // Result: false
!!foo // Result: true
Solution 12 - Javascript
It simulates the behavior of the Boolean()
casting function.
The first NOT
returns a Boolean value no matter what operand it is given. The second NOT
negates that Boolean
value and so gives the true
Boolean value of a variable. The end result is the same as using the Boolean()
function on a value.
Solution 13 - Javascript
!!
it's using NOT
operation twice together, !
convert the value to a boolean
and reverse it, so using it twice, showing the boolean(false or true) of that value. here is a simple example to see how !!
works:
At first, the place you have:
var zero = 0;
Then you do !0
, it will be converted to boolean and be evaluated to true
, because 0 is falsy
, so you get the reversed value and converted to boolean, so it gets evaluated to true
.
!zero; //true
but we don't want the reversed boolean version of the value, so we can reverse it again to get our result! That's why we use another !
.
Basically, !!
make us sure, the value we get is boolean, not falsy, truthy or string etc...
So it's like using Boolean
function in javascript, but easy and shorter way to convert a value to boolean:
var zero = 0;
!!zero; //false
Solution 14 - Javascript
! is "boolean not", which essentially typecasts the value of "enable" to its boolean opposite. The second ! flips this value. So, !!enable
means "not not enable," giving you the value of enable
as a boolean.
Solution 15 - Javascript
I think worth mentioning is, that a condition combined with logical AND/OR will not return a boolean value but last success or first fail in case of && and first success or last fail in case of || of condition chain.
res = (1 && 2); // res is 2
res = (true && alert) // res is function alert()
res = ('foo' || alert) // res is 'foo'
In order to cast the condition to a true boolean literal we can use the double negation:
res = !!(1 && 2); // res is true
res = !!(true && alert) // res is true
res = !!('foo' || alert) // res is true
Solution 16 - Javascript
It's not a single operator, it's two. It's equivalent to the following and is a quick way to cast a value to boolean.
val.enabled = !(!enable);
Solution 17 - Javascript
The !!
construct is a simple way of turning any JavaScript expression into
its Boolean equivalent.
For example: !!"he shot me down" === true
and !!0 === false
.
Solution 18 - Javascript
This question has been answered quite thoroughly, but I'd like to add an answer that I hope is as simplified as possible, making the meaning of !! as simple to grasp as can be.
Because javascript has what are called "truthy" and "falsey" values, there are expressions that when evaluated in other expressions will result in a true or false condition, even though the value or expression being examined is not actually true
or false
.
For instance:
if (document.getElementById('myElement')) {
// code block
}
If that element does in fact exist, the expression will evaluate as true, and the code block will be executed.
However:
if (document.getElementById('myElement') == true) {
// code block
}
...will NOT result in a true condition, and the code block will not be executed, even if the element does exist.
Why? Because document.getElementById()
is a "truthy" expression that will evaluate as true in this if()
statement, but it is not an actual boolean value of true
.
The double "not" in this case is quite simple. It is simply two not
s back to back.
The first one simply "inverts" the truthy or falsey value, resulting in an actual boolean type, and then the second one "inverts" it back again to it's original state, but now in an actual boolean value. That way you have consistency:
if (!!document.getElementById('myElement')) {}
and
if (!!document.getElementById('myElement') == true) {}
will BOTH return true, as expected.
Solution 19 - Javascript
I suspect this is a leftover from C++ where people override the ! operator but not the bool operator.
So to get a negative(or positive) answer in that case you would first need to use the ! operator to get a boolean, but if you wanted to check the positive case would use !!.
Solution 20 - Javascript
The if
and while
statements and the ?
operator use truth values to determine which branch of code to run. For example, zero and NaN numbers and the empty string are false, but other numbers and strings are true. Objects are true, but the undefined value and null
are both false.
The double negation operator !!
calculates the truth value of a value. It's actually two operators, where !!x
means !(!x)
, and behaves as follows:
- If
x
is a false value,!x
istrue
, and!!x
isfalse
. - If
x
is a true value,!x
isfalse
, and!!x
istrue
.
When used at the top level of a Boolean context (if
, while
, or ?
), the !!
operator is behaviorally a no-op. For example, if (x)
and if (!!x)
mean the same thing.
Practical uses
However it has several practical uses.
One use is to lossily compress an object to its truth value, so that your code isn't holding a reference to a big object and keeping it alive. Assigning !!some_big_object
to a variable instead of some_big_object
lets go of it for the garbage collector. This is useful for cases that produce either an object or a false value such as null
or the undefined value, such as browser feature detection.
Another use, which I mentioned in an answer about C's corresponding !!
operator, is with "lint" tools that look for common typos and print diagnostics. For example, in both C and JavaScript, a few common typos for Boolean operations produce other behaviors whose output isn't quite as Boolean:
if (a = b)
is assignment followed by use of the truth value ofb
;if (a == b)
is an equality comparison.if (a & b)
is a bitwise AND;if (a && b)
is a logical AND.2 & 5
is0
(a false value);2 && 5
is true.
The !!
operator reassures the lint tool that what you wrote is what you meant: do this operation, then take the truth value of the result.
A third use is to produce logical XOR and logical XNOR. In both C and JavaScript, a && b
performs a logical AND (true if both sides are true), and a & b
performs a bitwise AND. a || b
performs a logical OR (true if at least one are true), and a | b
performs a bitwise OR. There's a bitwise XOR (exclusive OR) as a ^ b
, but there's no built-in operator for logical XOR (true if exactly one side is true). You might, for example, want to allow the user to enter text in exactly one of two fields. What you can do is convert each to a truth value and compare them: !!x !== !!y
.
Solution 21 - Javascript
Double boolean negation. Often used to check if value is not undefined.
Solution 22 - Javascript
I just wanted to add that
if(variableThing){
// do something
}
is the same as
if(!!variableThing){
// do something
}
But this can be an issue when something is undefined.
// a === undefined, b is an empty object (eg. b.asdf === undefined)
var a, b = {};
// Both of these give error a.foo is not defined etc.
// you'd see the same behavior for !!a.foo and !!b.foo.bar
a.foo
b.foo.bar
// This works -- these return undefined
a && a.foo
b.foo && b.foo.bar
b && b.foo && b.foo.bar
The trick here is the chain of &&
s will return the first falsey value it finds -- and this can be fed to an if statement etc. So if b.foo is undefined, it will return undefined and skip the b.foo.bar
statement, and we get no error.
The above return undefined but if you have an empty string, false, null, 0, undefined those values will return and soon as we encounter them in the chain -- []
and {}
are both "truthy" and we will continue down the so-called "&& chain" to the next value to the right.
P.S. Another way of doing the above (b && b.foo
) is (b || {}).foo
. Those are equivalent, because if b is undefined then b || {}
will be {}
, and you'll be accessing a value in an empty object (no error) instead of trying to access a value within "undefined" (causes an error).
So, (b || {}).foo
is the same as b && b.foo
and ((b || {}).foo || {}).bar
is the same as b && b.foo && b.foo.bar
.
Solution 23 - Javascript
!!x
is shorthand for Boolean(x)
The first bang forces the js engine to run Boolean(x)
but also has the side effect of inverting the value. So the second bang undoes the side effect.
Solution 24 - Javascript
It forces all things to boolean.
For example:
console.log(undefined); // -> undefined
console.log(!undefined); // -> true
console.log(!!undefined); // -> false
console.log('abc'); // -> abc
console.log(!'abc'); // -> false
console.log(!!'abc'); // -> true
console.log(0 === false); // -> undefined
console.log(!0 === false); // -> false
console.log(!!0 === false); // -> true
Solution 25 - Javascript
Tons of great answers here, but if you've read down this far, this helped me to 'get it'. Open the console on Chrome (etc), and start typing:
!(!(1))
!(!(0))
!(!('truthy'))
!(!(null))
!(!(''))
!(!(undefined))
!(!(new Object())
!(!({}))
woo = 'hoo'
!(!(woo))
...etc, etc, until the light goes on ;)
Naturally, these are all the same as merely typing !!someThing, but the added parentheses might help make it more understandable.
Solution 26 - Javascript
After seeing all these great answers, I would like to add another reason for using !!
. Currenty I'm working in Angular 2-4 (TypeScript) and I want to return a boolean as false
when my user is not authenticated. If he isn't authenticated, the token-string would be null
or ""
. I can do this by using the next block of code:
public isAuthenticated(): boolean {
return !!this.getToken();
}
Solution 27 - Javascript
here is a piece of code from angular js
var requestAnimationFrame = $window.requestAnimationFrame ||
$window.webkitRequestAnimationFrame ||
$window.mozRequestAnimationFrame;
var rafSupported = !!requestAnimationFrame;
their intention is to set rafSupported to true or false based on the availability of function in requestAnimationFrame
it can be achieved by checking in following way in general:
if(typeof requestAnimationFrame === 'function')
rafSupported =true;
else
rafSupported =false;
the short way could be using !!
rafSupported = !!requestAnimationFrame ;
so if requestAnimationFrame was assigned a function then !requestAnimationFrame would be false and one more ! of it would be true
if requestAnimationFrame was assinged undefined then !requestAnimationFrame would be true and one more ! of it would be false
Solution 28 - Javascript
Use logical not operator two times
it means !true = false
and !!true = true
Solution 29 - Javascript
Returns boolean value of a variable.
Instead, Boolean
class can be used.
(please read code descriptions)
var X = "test"; // X value is "test" as a String value
var booleanX = !!X // booleanX is `true` as a Boolean value beacuse non-empty strings evaluates as `true` in boolean
var whatIsXValueInBoolean = Boolean(X) // whatIsXValueInBoolean is `true` again
console.log(Boolean(X) === !!X) // writes `true`
Namely, Boolean(X) = !!X
in use.
Please check code snippet out below ↓
let a = 0
console.log("a: ", a) // writes a value in its kind
console.log("!a: ", !a) // writes '0 is NOT true in boolean' value as boolean - So that's true.In boolean 0 means false and 1 means true.
console.log("!!a: ", !!a) // writes 0 value in boolean. 0 means false.
console.log("Boolean(a): ", Boolean(a)) // equals to `!!a`
console.log("\n") // newline
a = 1
console.log("a: ", a)
console.log("!a: ", !a)
console.log("!!a: ", !!a) // writes 1 value in boolean
console.log("\n") // newline
a = ""
console.log("a: ", a)
console.log("!a: ", !a) // writes '"" is NOT true in boolean' value as boolean - So that's true.In boolean empty strings, null and undefined values mean false and if there is a string it means true.
console.log("!!a: ", !!a) // writes "" value in boolean
console.log("\n") // newline
a = "test"
console.log("a: ", a) // writes a value in its kind
console.log("!a: ", !a)
console.log("!!a: ", !!a) // writes "test" value in boolean
console.log("Boolean(a) === !!a: ", Boolean(a) === !!a) // writes true
Solution 30 - Javascript
It is important to remember the evaluations to true
and false
in JavaScript:
-
Everything with a "Value" is
true
(namely truthy), for example:101
,3.1415
,-11
,"Lucky Brain"
,new Object()
- and, of course,
true
-
Everything without a "Value" is
false
(namely falsy), for example:0
,-0
,""
(empty string),undefined
,null
,NaN
(not a number)- and, of course,
false
Applying the "logical not" operator (!
) evaluates the operand, converting it to boolean
and then negating it. Applying it twice will negate the negation, effectively converting the value to boolean
. Not applying the operator will just be a regular assignment of the exact value. Examples:
var value = 23; // number
var valueAsNegatedBoolean = !value; // boolean falsy (because 23 is truthy)
var valueAsBoolean = !!value; // boolean truthy
var copyOfValue = value; // number 23
var value2 = 0;
var value2AsNegatedBoolean = !value2; // boolean truthy (because 0 is falsy)
var value2AsBoolean = !!value2; // boolean falsy
var copyOfValue2 = value2; // number 0
value2 = value;
assigns the exact objectvalue
even if it is notboolean
hencevalue2
won't necessarily end up beingboolean
.value2 = !!value;
assigns a guaranteedboolean
as the result of the double negation of the operandvalue
and it is equivalent to the following but much shorter and readable:
if (value) {
value2 = true;
} else {
value2 = false;
}
Solution 31 - Javascript
Some operators in JavaScript perform implicit type conversions, and are sometimes used for type conversion.
The unary !
operator converts its operand to a boolean and negates it.
This fact lead to the following idiom that you can see in your source code:
!!x // Same as Boolean(x). Note double exclamation mark
Solution 32 - Javascript
!!
is similar to using the Boolean constructor, or arguably more like the Boolean function.
console.log(Boolean(null)); // Preferred over the Boolean object
console.log(new Boolean(null).valueOf()); // Not recommended for coverting non-boolean values
console.log(!!null); // A hacky way to omit calling the Boolean function, but essentially does the same thing.
// The context you saw earlier (your example)
var vertical;
function Example(vertical)
{
this.vertical = vertical !== undefined ? !!vertical :
this.vertical;
// Let's break it down: If vertical is strictly not undefined, return the boolean value of vertical and set it to this.vertical. If not, don't set a value for this.vertical (just ignore it and set it back to what it was before; in this case, nothing).
return this.vertical;
}
console.log( "\n---------------------" )
// vertical is currently undefined
console.log(new Example(vertical).vertical); // The falsey or truthy value of this.vertical
console.log(!!new Example(vertical).vertical); // Coerced value of this.vertical
vertical = 12.5; // set vertical to 12.5, a truthy value.
console.log(new Example(vertical).vertical); // The falsey or truthy value of this.vertical which happens to be true anyway
console.log(!!new Example(vertical).vertical); // Coerced value of this.vertical
vertical = -0; // set vertical to -0, a falsey value.
console.log(new Example(vertical).vertical); // The falsey or truthy value of this.vertical which happens to be false either way
console.log(!!new Example(vertical).vertical); // Coerced value of this.vertical
Falsey values in javascript coerce to false, and truthy values coerce to true. Falsey and truthy values can also be used in if
statements and will essentially "map" to their corresponding boolean value. However, you will probably not find yourself having to use proper boolean values often, as they mostly differ in output (return values).
>Although this may seem similar to casting, realistically this is likely a mere coincidence and is not 'built' or purposely made for and like a boolean cast. So let's not call it that.
To be concise, it looks something like this: Why and how it works
! ( !null )
. Whereas, null
is falsey, so !null
would be true. Then !true
would be false and it would essentially invert back to what it was before,
except this time as a proper boolean value (or even vice versa with truthy values like {}
or 1
).
Overall, the context that you saw simply adjusts Going back to your example
this.vertical
depending on whether or not vertical
is defined, and if so; it will be set to the resulting boolean value of vertical, otherwise it will not change. In other words, if vertical
is defined; this.vertical
will be set to the boolean value of it, otherwise, it will stay the same. I guess that in itself is an example of how you would use !!
, and what it does.
Run this example and fiddle around with the vertical value in the input. See what the result coerces to so that you can fully understand your context's code. In the input, enter any valid javascript value. Remember to include the quotations if you are testing out a string. Don't mind the CSS and HTML code too much, simply run this snippet and play around with it. However, you might want to take a look at the non-DOM-related javascript code though (the use of the Example constructor and the vertical variable). Vertical I/O Example
var vertical = document.getElementById("vertical");
var p = document.getElementById("result");
function Example(vertical)
{
this.vertical = vertical !== undefined ? !!vertical :
this.vertical;
return this.vertical;
}
document.getElementById("run").onclick = function()
{
p.innerHTML = !!( new Example(eval(vertical.value)).vertical );
}
input
{
text-align: center;
width: 5em;
}
button
{
margin: 15.5px;
width: 14em;
height: 3.4em;
color: blue;
}
var
{
color: purple;
}
p {
margin: 15px;
}
span.comment {
color: brown;
}
<!--Vertical I/O Example-->
<h4>Vertical Example</h4>
<code id="code"><var class="var">var</var> vertical = <input type="text" id="vertical" maxlength="9" />; <span class="comment">// enter any valid javascript value</span></code>
<br />
<button id="run">Run</button>
<p id="result">...</p>
Solution 33 - Javascript
To cast your JavaScript variables to boolean,
var firstname = "test";
//type of firstname is string
var firstNameNotEmpty = !!firstname;
//type of firstNameNotEmpty is boolean
javascript false for "",0,undefined and null
javascript is true for number other then zero,not empty strings,{},[] and new Date() so,
!!("test") /*is true*/
!!("") /*is false*/
Solution 34 - Javascript
a = 1;
alert(!a) // -> false : a is not not defined
alert(!!a) // -> true : a is not not defined
For !a
, it checks whether a
is NOT defined, while !!a
checks if the variable is defined.
!!a
is the same as !(!a)
. If a
is defined, a
is true
, !a
is false
, and !!a
is true
.
Solution 35 - Javascript
!! is not an operator, it's just the ! operator twice
But with javascript, apply !! for convert Object to boolean is redundancy and verbose in the most case because: > Any object of which the value is not undefined or null, including a > Boolean object whose value is false, evaluates to true when passed to > a conditional statement
Ex: if ({}) { console.log("{} is true")} // logs: "{} is true"
Solution 36 - Javascript
!! is simply NOT operator twice. net effect is just convert anything to ensure boolean datatype. eg.
!!undefined is false !!0 is false !!null is false !!anyobject is true !!true is true !!false is false !0 is true !1 is false
Solution 37 - Javascript
Sometimes it is necessary to check whether we have a value in the function or not, and the amount itself is not important to us, but whether or not it matters. for example we want to check ,if user has major or not and we have a function just like:
hasMajor(){return this.major}//it return "(users major is)Science"
but the answer is not important to us we just want to check it has a major or not and we need a boolean value(true or false) how we get it:
just like this:
hasMajor(){ return !(!this.major)}
or as the same
hasMajor(){return !!this.major)}
if this.major has a value then !this.major
return false but because the value has exits and we need to return true we use ! twice to return the correct answer !(!this.major)
Solution 38 - Javascript
const foo = 'bar';
console.log(!!foo); // Boolean: true
! negates (inverts) a value AND always returns/ produces a boolean. So !'bar' would yield false (because 'bar' is truthy => negated + boolean = false). With the additional ! operator, the value is negated again, so false becomes true.
Solution 39 - Javascript
This is a really handy way to check for undefined, "undefined", null, "null", ""
if (!!var1 && !!var2 && !!var3 && !!var4 ){
//... some code here
}