All Falsey Values in JavaScript

All falsey values in JavaScript

Falsey values in JavaScript

• false
• Zero of Number type: 0 and also -0, 0.0, and hex form 0x0 (thanks RBT)
• Zero of BigInt type: 0n and 0x0n (new in 2020, thanks GetMeARemoteJob)
• "", '' and `` - strings of length 0
• null
• undefined
• NaN
• document.all (in HTML browsers only)
  
  • This is a weird one. document.all is a falsey object, with typeof as undefined. It was a Microsoft-proprietory function in IE before IE11, and was added to the HTML spec as a "willful violation of the JavaScript specification" so that sites written for IE wouldn't break on trying to access, for example, document.all.something; it's falsy because if (document.all) used to be a popular way to detect IE, before conditional comments. See Why is document.all falsy? for details

"Falsey" simply means that JavaScript's internal ToBoolean function returns false. ToBoolean underlies !value, value ? ... : ...; and if (value). Here's its official specification (2020 working draft) (the only changes since the very first ECMAscript specification in 1997 are the addition of ES6's Symbols, which are always truthy, and BigInt, mentioned above:

Argument type	Result
Undefined	Return false.
Null	Return false.
Boolean	Return argument.
Number	If argument is +0, -0, or NaN, return false; otherwise return true.
String	If argument is the empty String (its length is zero), return false; otherwise return true.
BigInt	If argument is 0n, return false; otherwise return true.
Symbol	Return true.
Object	Return true.

Falsey values in JavaScript

One dangerous issue of falsey values you have to be aware of is when checking the presence of a certain property.

Suppose you want to test for the availability of a new property; when this property can actually have a value of 0 or "", you can't simply check for its availability using

if (!someObject.someProperty)
    /* incorrectly assume that someProperty is unavailable */

In this case, you must check for it being really present or not:

if (typeof someObject.someProperty == "undefined")
    /* now it's really not available */

Also be aware that NaN isn't equal to anything, even not to itself (NaN != NaN).

why all falsy values in javascript aren't equal to false when tested for equality

The == operator has its own semantics. You're comparing behaviors that are not defined to be the same.

If you want to see how the normal "truthy/falsy" evaluation works, you should use !value or !!value instead of value == false or value == true:

if (!null) console.log("hi");
if (!NaN) console.log("hi");

Removing all falsy values from a javascript array, including NaN but excluding 0 and empty strings

NaN is not equal to itself (i.e. NaN === NaN evaluates to false), thus using it with indexOf fails. Another approach that also conveys your goal ("filter() out all the falsy values except for '' and 0") better is the following:

const values = ['', 0, 'one', NaN, 1, 'two', 2, null, 'three', undefined, 3, false];
const filteredValues = values.filter(value => value || value === '' || value === 0);
console.log(filteredValues);

trouble with truthy/falsey

Edit:

The question was a bit confusing because of the snippet, I understood that you were trying to look for falsy values.

The reason why:

value => value == true

would print out false it's because none of the elements of the array is equal to true.

You are correct about what a falsy value is, but that doesn't mean that a truthy value would be == to true.

Here you can read more about it:

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Equality_comparisons_and_sameness

One way of checking for truthy values would be:

(values.some(value => value != false)) 

or

(values.some(value => !!value === true)) 

Old answer:

Because the method you use tests that at least one element in the array matches the condition.

You can read more here

If you want to check that all elements of the array matches the condition, then you can use .every()

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/every

let values = [11, NaN, [], ""]

function checkForFalsey() {
  if (values.every(value => value == false)) {
    console.log("All values are falsey");
  } else {
    console.log("NOT all values are falsey");
  }
}
checkForFalsey()

check for falsy values in javascript

The forEach function doesn't detect if you delete elements, so when you modify the array, it will skip elements. The best way to do what you want is:

g.filter(e => e);

Understanding JavaScript Truthy and Falsy

From my understanding, I believe that var a = 1; is the only truthy and the rest are falsy’s - is this correct?

No.

1. var a = 0;

   Number zero is falsy. However, note that the string zero "0" is truthy.

2. var a = 10 == 5;

   This is same as var a = (10 == 5);, so this is falsy.

3. var a = 1;

   var a = -1;

   Any non-zero number including negative numbers is truthy.

Quoting from MDN

In JavaScript, a truthy value is a value that translates to true when evaluated in a Boolean context. All values are truthy unless they are defined as falsy (i.e., except for false, 0, "", null, undefined, and NaN).

List of falsy values in JavaScript:^{From MDN}

1. false
2. null
3. undefined
4. 0
5. NaN
6. '', "", ``(Empty template string)
7. document.all
8. 0n: BigInt
9. -0

Check for falsy values on array

Try this code if you want to check for falsy values

Let me tell you that falsy values don't require to be boolean!

For Example, 0 is a falsy value however typeof(0) is number ..

The reason why your code returns an empty array is the if condition, you're checking if typeof(arr[i]) !== Boolean && arr[i]

this condition will always be false since typeof returns a string and Boolean is considered as an Object/ Constructor (which isn't a string)

function bouncer(arr) {
  let word = []
  for (let i = 0; i < arr.length; i++)
    if (arr[i]) {
      word.push(arr[i])
    }
  return word;
}

console.log(bouncer([false, null, 0, NaN, undefined, ""]));

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