In JavaScript, why is “0” equal to false, but when

Question

The following shows that "0" is false in Javascript:

>>> "0" == false
true

>>> false == "0"
true

So why does the following print "ha"?

>>> if ("0") console.log("ha")
ha

Answer 1

Your quotes around the 0 make it a string, which is evaluated as true.

Remove the quotes and it should work.

if (0) console.log("ha") 

Answer 2

if (x) 

coerces x using JavaScript's internal toBoolean (http://es5.github.com/#x9.2)

x == false

coerces both sides using internal toNumber coercion (http://es5.github.com/#x9.3) or toPrimitive for objects (http://es5.github.com/#x9.1)

For full details see http://javascriptweblog.wordpress.com/2011/02/07/truth-equality-and-javascript/

Answer 3

== Equality operator evaluates the arguments after converting them to numbers. So string zero "0" is converted to Number data type and boolean false is converted to Number 0. So

"0" == false // true

Same applies to `

false == "0" //true

=== Strict equality check evaluates the arguments with the original data type

"0" === false // false, because "0" is a string and false is boolean

Same applies to

false == "0" // false

In

if("0") console.log("ha");

The String "0" is not comparing with any arguments, and string is a true value until or unless it is compared with any arguments. It is exactly like

if(true) console.log("ha");

But

if (0) console.log("ha"); // empty console line, because 0 is false

`

Answer 4

It's PHP where the string "0" is falsy (false-when-used-in-boolean-context). In JavaScript, all non-empty strings are truthy.

The trick is that == against a boolean doesn't evaluate in a boolean context, it converts to number, and in the case of strings that's done by parsing as decimal. So you get Number 0 instead of the truthiness boolean true.

This is a really poor bit of language design and it's one of the reasons we try not to use the unfortunate == operator. Use === instead.

Answer 5

The reason is because when you explicitly do "0" == false, both sides are being converted to numbers, and then the comparison is performed.

When you do: if ("0") console.log("ha"), the string value is being tested. Any non-empty string is true, while an empty string is false.

Equal (==)

If the two operands are not of the same type, JavaScript converts the operands then applies strict comparison. If either operand is a number or a boolean, the operands are converted to numbers if possible; else if either operand is a string, the other operand is converted to a string if possible. If both operands are objects, then JavaScript compares internal references which are equal when operands refer to the same object in memory.

(From Comparison Operators in Mozilla Developer Network)

Answer 6

It's according to spec.

12.5 The if Statement 
.....

2. If ToBoolean(GetValue(exprRef)) is true, then 
a. Return the result of evaluating the first Statement. 
3. Else, 
....

ToBoolean, according to the spec, is

The abstract operation ToBoolean converts its argument to a value of type Boolean according to Table 11:

And that table says this about strings:

The result is false if the argument is the empty String (its length is zero); otherwise the result is true

Now, to explain why "0" == false you should read the equality operator, which states it gets its value from the abstract operation GetValue(lref) matches the same for the right-side.

Which describes this relevant part as:

if IsPropertyReference(V), then 
a. If HasPrimitiveBase(V) is false, then let get be the [[Get]] internal method of base, otherwise let get
be the special [[Get]] internal method defined below. 
b. Return the result of calling the get internal method using base as its this value, and passing 
GetReferencedName(V) for the argument

Or in other words, a string has a primitive base, which calls back the internal get method and ends up looking false.

If you want to evaluate things using the GetValue operation use ==, if you want to evaluate using the ToBoolean, use === (also known as the "strict" equality operator)