0 vs '0' with Boolean operators

Question

0 == false and '0' == false are true

However (true && 0) is false, and (true && '0') is true.

Why?

Answer 1

The rules for abstract comparison ( == ) are described in ES5 11.9.3 , while the rules for logical operators ( && ) are described in ES5 11.11 .

In short, == simply more complicated than && . Where && simply uses the internal ToBoolean() to evaluate its operands, == has various conditions that may lead to the use of ToBoolean() , ToNumber() and / or ToPrimitive() .

• (0 == false) == true :

  7. If Type (y) is boolean, return the result of the comparison x == ToNumber (y)

  ToNumber(false) === 0 , therefore 0 == 0 , therefore true .

• ('0' == false) == true :

  This also goes through step 7, resulting in '0' == 0 .

  Then, starting from above, it reaches step 5:

  5. If Type (x) is String and Type (y) is Number, returns the result of the comparison ToNumber (x) == y.

  ToNumber('0') === 0 , so again 0 == 0 and again true .

• !!(true && 0) == false

  && simply returns the 1st operand if it is false ( ToBoolean(...) === false ) or the second operand.

  This is strictly (true && 0) === 0 .

  And when used as an if condition, the result ( 0 ) will also be passed through ToBoolean(...) and ToBoolean(0) === false .

• !!(true && '0') == true

  Again, this returns the 2nd operand, '0' .

  This time, however, ToBoolean('0') === true as '0' is a non-empty string, making it true.

Also, if you need simpler comparison rules, use strict comparison ( === , 11.9.6 ).

Answer 2

'0' (or any non-empty string) is the "truth" in JS. The == operator, however, does some weird type coercion, so many of JS's outstanding figures, including Crockford, are very discouraged. This is a good example of why you should avoid it, it takes the string "0" and forces it to a false value.

Here is a link that explains this process:

http://webreflection.blogspot.com/2010/10/javascript-coercion-demystified.html

 If Type(x) is Boolean, return the result of the comparison ToNumber(x) == y: false == 0 and true == 1 but true != 2 

So even weirder than your example:

 ('1' == true) // true ('2' == true) // false!