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Falsey values in JavaScript
(6 answers)
Convert truthy or falsy to an explicit boolean, i.e. to True or False
(2 answers)
Closed 3 years ago.
While programming with javascript, I come in some situations that I have a condition an wonder what values will pass that condition (if it will be truthy).
The solution I came up with is open the console in chrome and type if(foo) {true} and if the foo is truthy, it returns true, otherwise false.
An example of that is when I have some expression (foo) that won't return only true/false values. Inside the if it can return alot of things depending on the input (sometimes string, or number or maybe NaN).
But I fell that writing a if for that is too much.
Is there a easiest way of checking the truthy of a value in javascript?
Edit:
I'm looking for the easiest way of checking the truthy of a value, not just how to check and as I said in my question, I already do if(foo) {true} to check, but I'm looking for a easiest way
I would use a ternary operator to shorthand the if-statement.
The result is something like this:
const foo = true;
console.log(foo ? 'truthy' : 'falsy'); // returns truthy
Related
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Why do empty JavaScript arrays evaluate to true in conditional structures?
(7 answers)
Closed 5 years ago.
I am going through JavaScript: The Definitive Guide. In it it says
Boolean([]) // => true
But I don't understand the logic behind this. Why is the boolean of an empty array true?
The ECMAScript specification defines how values are cast to booleans, per the abstract ToBoolean operation: https://www.ecma-international.org/ecma-262/6.0/#sec-toboolean
That operations includes a single entry for object input:
Object: Return true.
Thus, when you supply any object to Boolean, including an array (even an empty one), you'll get a true value back,
Array is considered as an object, even if it's empty. That's why the Boolean has a value, means it's true.
Only false, null or undefined are values which will return false.
JavaScript (and other languages) have a concept of 'truthy' and 'falsey' values.
You said you're from a C++ background, so we can make it analogous to something like this in C++:
if (ptr) { }
which is falsey if ptr is null, and truthy otherwise.
It just so happens that in JavaScript, arrays - even empty ones, among many other things - are considered to be truthy.
This question already has answers here:
Why if([]) is validated while [] == false in javascript?
(3 answers)
Closed 6 years ago.
I think in every language I know
if(a)
is the same thing as
if(a == true)
Turns out in JavaScript it isn't true, because:
if([])
Seems to act as if the condition is fulfilled, but:
if([] == true)
Does the opposite thing.
I can't really find any possible explanation, especially that this problem doesn't occur with empty string for example (which is == true, but isn't === true, same as empty array). Is this a bug in JavaScript or what?
In JavaScript, there is a concept of truthy and falsey values. if statements test the truthiness or falsiness of a given value rather than strict equality to true or false.
true is obviously truthy. false is obviously falsey. The rest can be a little tricky. MDN has possibly the clearest documentation about which values evaluate to falsey: https://developer.mozilla.org/en-US/docs/Glossary/Truthy
In this case [] is a truthy value so the condition passes and the code is executed.
This question already has answers here:
How do you check that a number is NaN in JavaScript?
(33 answers)
Closed 7 years ago.
What is the best way to check a given variable is NaN or not?
(only using pure Javascript without any libraries such as Underscore)
JavaScript has function isNaN for that purpose. As described in w3schools:
The isNaN() function determines whether a value is an illegal number
(Not-a-Number).
This function returns true if the value is NaN, and false if not.
For example:
isNaN(123) // returns false;
isNaN("Hello") //returns true
You can use isNaN(x) which returns a boolean value (true if x is NaN, false otherwise).
This question already has answers here:
Why is the !! preferable in checking if an object is true? [duplicate]
(3 answers)
Closed 7 years ago.
Note: it's actually a duplicate of What is the difference between if(!!condition) and if(condition)
While I understand what the !! means (double not), for this same reason it doesn't make sense to me its use in the MDN documentation:
if (!!window.Worker) {
...
}
Isn't this exactly the same as this for this situation?
if (window.Worker) {
...
}
The casting to boolean makes no sense for me since the if will only be executed if the window.Worker exists. To say that it's True or Object for an if() conditional (I think) is the same.
So, why is the !! used here? Or, why is the window.Worker casted to boolean inside an if()?
Yes it is exactly the same. Nothing to add.
It might have been used to emphasize that the window.Worker property - expected to be a function - is cast to a boolean for detecting its presence, instead of looking like a forgotten () call. Regardless, it is now gone.
This question already has answers here:
Closed 11 years ago.
Possible Duplicate:
What is the !! operator in JavaScript?
What is the difference between these two operators? Does !! have special meaning, or does it simply mean you are doing two '!' operations. I know there are "Truth" and "Truthy" concepts in Javascript, but I'm not sure if !! is meant for "Truth"
!! is just double !
!true // -> false
!!true // -> true
!! is a common way to cast something to boolean value
!!{} // -> true
!!null // -> false
Writing !! is a common way of converting a "truthy" or "falsey" variable into a genuine boolean value.
For example:
var foo = null;
if (!!foo === true) {
// Code if foo was "truthy"
}
After the first ! is applied to foo, the value returned is true. Notting that value again makes it false, meaning the code inside the if block is not entered.