We Keep Coding

What is wrong in this code, why does it print only the first code block [duplicate]

What are the values in JavaScript that are 'falsey', meaning that they evaluate as false in expressions like if(value), value ? and !value?
There are some discussions of the purpose of falsey values on Stack Overflow already, but no exhaustive complete answer listing what all the falsey values are.
I couldn't find any complete list on MDN JavaScript Reference, and I was surprised to find that the top results when looking for a complete, authoritative list of falsey values in JavaScript were blog articles, some of which had obvious omissions (for example, NaN), and none of which had a format like Stack Overflow's where comments or alternative answers could be added to point out quirks, surprises, omissions, mistakes or caveats. So, it seemed to make sense to make one.

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.
Comparisons with == (loose equality)
It's worth talking about falsy values' loose comparisons with ==, which uses ToNumber() and can cause some confusion due to the underlying differences. They effectively form three groups:
false, 0, -0, "", '' all match each other with ==
e.g. false == "", '' == 0 and therefore 4/2 - 2 == 'some string'.slice(11);
null, undefined match with ==
e.g. null == undefined but undefined != false
It's also worth mentioning that while typeof null returns 'object', null is not an object, this is a longstanding bug/quirk that was not fixed in order to maintain compatibility. It's not a true object, and objects are truthy (except for that "wilful violation" document.all when Javascript is implemented in HTML)
NaN doesn't match anything, with == or ===, not even itself
e.g. NaN != NaN, NaN !== NaN, NaN != false, NaN != null
With "strict equality" (===), there are no such groupings. Only false === false.
This is one of the reasons why many developers and many style guides (e.g. standardjs) prefer === and almost never use ==.
Truthy values that actually == false
"Truthy" simply means that JavaScript's internal ToBoolean function returns true. A quirk of Javascript to be aware of (and another good reason to prefer === over ==): it is possible for a value to be truthy (ToBoolean returns true), but also == false.
You might think if (value && value == false) alert('Huh?') is a logical impossibility that couldn't happen, but it will, for:
"0" and '0' - they're non-empty strings, which are truthy, but Javascript's == matches numbers with equivalent strings (e.g. 42 == "42"). Since 0 == false, if "0" == 0, "0" == false.
new Number(0) and new Boolean(false) - they're objects, which are truthy, but == sees their values, which == false.
0 .toExponential(); - an object with a numerical value equivalent to 0
Any similar constructions that give you a false-equaling value wrapped in a type that is truthy
[], [[]] and [0] (thanks cloudfeet for the JavaScript Equality Table link)
Some more truthy values
These are just a few values that some people might expect to be falsey, but are actually truthy.
-1 and all non-zero negative numbers
' ', " ", "false", 'null'... all non-empty strings, including strings that are just whitespace
Anything from typeof, which always returns a non-empty string, for example:
typeof null (returns a string 'object' due to a longstanding bug/quirk)
typeof undefined (returns a string 'undefined')
Any object (except that "wilful violation" document.all in browsers). Remember that null isn't really an object, despite typeof suggesting otherwise. Examples:
{}
[]
function(){} or () => {} (any function, including empty functions)
Error and any instance of Error
Any regular expression
Anything created with new (including new Number(0) and new Boolean(false))
Any Symbol
true, 1, "1" and [1] return true when compared to each other with ==.

Don't forget about the non-empty string "false" which evaluates to true

Just to add to #user568458's list of falsy values:
In addition to integer number 0, the decimal number 0.0, 0.00 or any such zeroish number is also a falsy value.
var myNum = 0.0;
if(myNum){
console.log('I am a truthy value');
}
else {
console.log('I am a falsy value');
}
Above code snippet prints I am a falsy value
Similarly hex representation of the number 0 is also a falsy value as shown in below code snippet:
var myNum = 0x0; //hex representation of 0
if(myNum){
console.log('I am a truthy value');
}
else {
console.log('I am a falsy value');
}
Above code snippet again prints I am a falsy value.

Addition to the topic, as of ES2020 we have a new value which is falsy, it's BigInt zero (0n):
0n == false // true
-0n == false // true
0n === false // false
-0n === false // false
So with this, we now have 7 "falsy" values in total (not including document.all as mentioned by user above since it's part of DOM and not JS).

When checking for undefined'ness in Javascript, should I use != or !==?

When I want to make sure a variable x is defined, before using it, I use:
if (typeof x !== "undefined") {
// do stuff with x
}
but I've noticed that others, e.g. in this question, use !== instead of != for the comparison. Which one should I be using, and why?
Note: I realize I can be using !==. The question is whether I should (and whether there will be any difference in behavior).

As noted in a comment by VLAZ, the typeof operator is guaranteed to return a string. If you compare the result to another string, then == and === (or != and !==) will do the exact same thing.

actually, the best approach would be to check if a value is falsy, and based on MDN this is the list of falsy values:
false The keyword false
0 The number zero
0n BigInt, when used as a boolean, follows the same rule as a Number. 0n is falsy.
"", '', ``
This is an empty string (the length of the string is zero). Strings in JavaScript can be defined with double quotes "", single quotes '', or Template literals ``.
null null - the absence of any value
undefined undefined - the primitive value
NaN NaN - not a number
so based on your code what you can do is simply:
if (!x) { // check for all the falsy values.
// do stuff with x
}
in the other hand, you ask for the difference of != and !==, well basically taking some examples you can see the difference:
0 == false // true, because false is equivalent of 0
0 === false // false, because both operands are of different type
2 == "2" // true, auto type coercion, string converted into number
2 === "2" // false, since both operands are not of same type
as mentioned by #VLAZ comment, these cases will only work if the variable x is defined, otherwise you will have the following error:
"Uncaught ReferenceError: x is not defined"
so on your case you could only use != because you will compare string vs string and you will avoid having to check if the variable was or not created.
if (typeof x != "undefined") {
// do stuff with x
}

Javascript "implicit" condition in IF statements [duplicate]

What are the values in JavaScript that are 'falsey', meaning that they evaluate as false in expressions like if(value), value ? and !value?
There are some discussions of the purpose of falsey values on Stack Overflow already, but no exhaustive complete answer listing what all the falsey values are.
I couldn't find any complete list on MDN JavaScript Reference, and I was surprised to find that the top results when looking for a complete, authoritative list of falsey values in JavaScript were blog articles, some of which had obvious omissions (for example, NaN), and none of which had a format like Stack Overflow's where comments or alternative answers could be added to point out quirks, surprises, omissions, mistakes or caveats. So, it seemed to make sense to make one.

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.
Comparisons with == (loose equality)
It's worth talking about falsy values' loose comparisons with ==, which uses ToNumber() and can cause some confusion due to the underlying differences. They effectively form three groups:
false, 0, -0, "", '' all match each other with ==
e.g. false == "", '' == 0 and therefore 4/2 - 2 == 'some string'.slice(11);
null, undefined match with ==
e.g. null == undefined but undefined != false
It's also worth mentioning that while typeof null returns 'object', null is not an object, this is a longstanding bug/quirk that was not fixed in order to maintain compatibility. It's not a true object, and objects are truthy (except for that "wilful violation" document.all when Javascript is implemented in HTML)
NaN doesn't match anything, with == or ===, not even itself
e.g. NaN != NaN, NaN !== NaN, NaN != false, NaN != null
With "strict equality" (===), there are no such groupings. Only false === false.
This is one of the reasons why many developers and many style guides (e.g. standardjs) prefer === and almost never use ==.
Truthy values that actually == false
"Truthy" simply means that JavaScript's internal ToBoolean function returns true. A quirk of Javascript to be aware of (and another good reason to prefer === over ==): it is possible for a value to be truthy (ToBoolean returns true), but also == false.
You might think if (value && value == false) alert('Huh?') is a logical impossibility that couldn't happen, but it will, for:
"0" and '0' - they're non-empty strings, which are truthy, but Javascript's == matches numbers with equivalent strings (e.g. 42 == "42"). Since 0 == false, if "0" == 0, "0" == false.
new Number(0) and new Boolean(false) - they're objects, which are truthy, but == sees their values, which == false.
0 .toExponential(); - an object with a numerical value equivalent to 0
Any similar constructions that give you a false-equaling value wrapped in a type that is truthy
[], [[]] and [0] (thanks cloudfeet for the JavaScript Equality Table link)
Some more truthy values
These are just a few values that some people might expect to be falsey, but are actually truthy.
-1 and all non-zero negative numbers
' ', " ", "false", 'null'... all non-empty strings, including strings that are just whitespace
Anything from typeof, which always returns a non-empty string, for example:
typeof null (returns a string 'object' due to a longstanding bug/quirk)
typeof undefined (returns a string 'undefined')
Any object (except that "wilful violation" document.all in browsers). Remember that null isn't really an object, despite typeof suggesting otherwise. Examples:
{}
[]
function(){} or () => {} (any function, including empty functions)
Error and any instance of Error
Any regular expression
Anything created with new (including new Number(0) and new Boolean(false))
Any Symbol
true, 1, "1" and [1] return true when compared to each other with ==.

Don't forget about the non-empty string "false" which evaluates to true

Just to add to #user568458's list of falsy values:
In addition to integer number 0, the decimal number 0.0, 0.00 or any such zeroish number is also a falsy value.
var myNum = 0.0;
if(myNum){
console.log('I am a truthy value');
}
else {
console.log('I am a falsy value');
}
Above code snippet prints I am a falsy value
Similarly hex representation of the number 0 is also a falsy value as shown in below code snippet:
var myNum = 0x0; //hex representation of 0
if(myNum){
console.log('I am a truthy value');
}
else {
console.log('I am a falsy value');
}
Above code snippet again prints I am a falsy value.

Addition to the topic, as of ES2020 we have a new value which is falsy, it's BigInt zero (0n):
0n == false // true
-0n == false // true
0n === false // false
-0n === false // false
So with this, we now have 7 "falsy" values in total (not including document.all as mentioned by user above since it's part of DOM and not JS).

Javascript falsy values (null, undefined, false, empty string: "" or '' and 0) and comparison(==) operator [duplicate]

This question already has answers here:
JavaScript: What is the difference between `if (!x)` and `if (x == null)`?
(5 answers)
Closed 9 years ago.
When I am using any one of values(null, undefined, false, '', 0) in a if statement, it is always evaluated as fallacy(false). Also, the negation of these values((null, undefined, false, '', 0) in a if statement always evaluated as tautology(true).
if(null){
}else{
}
if(undefined){
}else{
}
if(false){
}else{
}
if(''){
}else{
}
if(0){
}else{
}
In all the above cases, if statement is evaluated as false & else statement executes.
However, when I am comparing these fallacy values with == operator, it is not returning true always. Surprisingly, it is always returning true values when I am comparing the negation of these values.
if double equalto (==) operator checks/compares for values & not strictly for types, then why:
null == false // returns false
null == 0 // returns false
null == '' // returns false
But,
!null == !false // returns true
!null == !0 // returns true
!false == !undefined // returns true
And,
null == undefined // returns true
false == 0 // returns true
I appreciate if any one can clarify the behavior or relationship among these values(null, undefined, false, '', 0).

A common misconception
"...If double equalto (==) operator only checks/compares for values & not for types..."
That's an incorrect assumption, though it's often repeated by people. In reality, the == does check types, and in fact pays far more attention to the types than a === comparison does.
See Abstract Equality Comparison Algorithm.
A == comparison doesn't do a simple toBoolean conversion. Rather it walks through a somewhat complex recursive algorithm, which, after checking the types, attempts to coerce the operands to the same type if they don't match.
The type coercion that it performs is very specific to the types of the operands. A different sequence of coercions can take place for different type pairs. Usually (but not always) it ends up ultimately coercing the operands down to number types.
Why !ing the operands changes things
When you manually coerce both operands using !, you're now doing a simple toBoolean conversion causing the types to match, which avoids the type coercive part of the algorithm, making it behave essentially like the Strict Equality Comparison Algorithm.
So the only way to predict the outcome of a == comparison when the types don't match is to understand that Abstract algorithm.
Don't forget about NaN
And FYI, there's one more "falsey" value to consider, NaN. Its == comparison will always be false, no matter what. Even when comparing to another NaN value, it'll be false.

undefined: means a variable was declared but has no value assigned
null: the value of null has been assigned, which means it has no value
false, '' and 0 I think you can probably work out what these mean.

NULL is different from false (NULL is of type object and false is of type of boolean), null is different from 0 (0 is of type integer), null is also different from '' ('' is of type of string). but they are all falsy values. The ! operator negates a boolean value. If ! is used on falsy values, it results to conversion of falsy values to an object of type boolean.

Why does "undefined equals false" return false?

When I compare undefined and null against Boolean false, the statement returns false:
undefined == false;
null == false;
It return false. Why?

With the original answer pointing to the spec being deleted, I'd like to provide a link and short excerpt from the spec here.
http://www.ecma-international.org/ecma-262/5.1/#sec-11.9.3
The ECMA spec doc lists the reason that undefined == false returns false. Although it does not directly say why this is so, the most important part in answering this question lies in this sentence:
The comparison x == y, where x and y are values, produces true or false.
If we look up the definition for null, we find something like this:
NULL or nil means "no value" or "not applicable".
In Javascript, undefined is treated the same way. It is without any value. However, false does have a value. It is telling us that something is not so. Whereas undefined and null are not supposed to be giving any value to us. Likewise, there is nothing it can convert to for its abstract equality comparison therefore the result would always be false. It is also why null == undefined returns true (They are both without any value). It should be noted though that null === undefined returns false because of their different types. (Use typeof(null) and typeof(undefined) in a console to check it out)
What I'm curious of though, is that comparing NaN with anything at all will always return false. Even when comparing it to itself. [NaN == NaN returns false]
Also, another odd piece of information: [typeof NaN returns "number"]
Strict Equality
If possible, you should avoid using the == operator to compare two values. Instead use === to truly see if two values are equal to each other. == gives the illusion that two values really are exactly equal when they may not be by using coercion. Examples:
5 == "5" is true
5 === "5" is false
"" == false is true
"" === false is false
0 == false is true
0 === false is false

From the incomparable MDN, sponsored by the company of JavaScript's creator.
JavaScript provides three different value-comparison operations:
strict equality (or "triple equals" or "identity") using ===,
loose equality ("double equals") using ==,
and Object.is (new in ECMAScript > 6).
The choice of which operation to use depends on what sort of
comparison you are looking to perform.
Briefly, double equals will perform a type conversion when comparing
two things; triple equals
will do the same comparison without type conversion (by simply always
returning false if the types differ); and Object.is will behave the
same way as triple equals, but with special handling for NaN and -0
and +0 so that the last two are not said to be the same, while
Object.is(NaN, NaN) will be true. (Comparing NaN with NaN
ordinarily—i.e., using either double equals or triple equals—evaluates
to false, because IEEE 754 says so.) Do note that the distinction
between these all have to do with their handling of primitives; none
of them compares whether the parameters are conceptually similar in
structure. For any non-primitive objects x and y which have the same
structure but are distinct objects themselves, all of the above forms
will evaluate to false.
For a visual overview of the whole picture of equality in JavaScript:
https://dorey.github.io/JavaScript-Equality-Table/
The truth is, this seemingly "bad" aspect of JavaScript is a source of power when you understand how it works.

So undefined really means undefined. Not False, not True, not 0, not empty string. So when you compare undefined to anything, the result is always false, it is not equal to that.

You question is half, as we compare undefined/ null to any other types.
we will have false return.
There is no coercion happening, even we are using == operator.

This is so because it is so. :)
Read the ECMA standards here: https://www.ecma-international.org/ecma-262/5.1/#sec-11.9.3

Undefined is not the same thing as false, false is a boolean object (which has a value of 0 therefore it is indeed defined).
An example:
var my_var;
var defined = (my_var === undefined)
alert(defined); //prints true. It is true that my_var is undefined
my_var = 22;
defined = (my_var === undefined)
alert(defined); //prints false. my_var is now defined
defined = (false === undefined)
alert(defined); //prints false, false is defined
defined = (true === undefined)
alert(defined); //prints false, true is defined

According to the specification flow:
If Type(y) is Boolean, return the result of the comparison x ==
ToNumber(y).
1) undefined == false; -> undefined == Number(false); -> undefined == 0;
2) null == false; -> null == Number(false); -> null == 0;
There is no rule for these cases, so go with the behavior in the last step:
Return false.