Why does !!1=="1" equal true and !!2=="2" equal false?

Question

As the title states, why does:

> !!1=="1" 

equal

True 

and

> !!2=="2" 

equal:

False 

Likewise, why does > "1"==true equal true and > "2"==true equal false

I'm baffled. Are these just bugs in JS or what's going on here?

Answer 1

As per the Operator precedence rules, logical ! has higher priority over ==. So, in both the cases, !! is evaluated first.

Note: Truthiness of various objects have been explained in this answer of mine.

First Case

!!1 == "1" 

!1 will be evaluated to false, since 1 is considered Truthy. Negating again we get true. So the expression becomes

true == "1" 

Now, the coercion rules kick in as you have used == operator, which evaluates as per the The Abstract Equality Comparison Algorithm defined in ECMAScript 5.1 Specification,

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

So, true will be converted to a number, which is 1 as per ToNumber algorithm for Boolean values. Now the expression becomes

1 == "1" 

Now,

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

So, "1" will be converted to a number and that will give 1, as per the ToNumber algorithm. That is why it shows true in the first case.

Second Case

The same rules are applied here.

!!2 == "2" 

becomes

true == "2" 

then

1 == "2" 

which becomes

1 == 2 

which is not true, that is why the second case prints false.

Answer 2

tldr; this is due to the [ToNumber] conversions in the == operator algorithm.

The first step is to simplify the expression. Since !!x=="x" is parsed like (!!x)=="x" and !!a_truthy_expression -> true, the actual relevant expression for the equality is

!!1=="2" -> true=="1" -> Boolean==String !!2=="2" -> true=="2" -> Boolean==String 

So then looking at the rules for 11.9.3 The Abstract Equality Comparison Algorithm and following along with the application yields

Rule 6 - If Type(x) is Boolean, return the result of the comparison ToNumber(x) == y.

which results in Number==String or 1=="1" and 1=="2", respectively¹. Then the rule

Rule 7 - If Type(x) is Number and Type(y) is String, return the result of the comparison x == ToNumber(y).

is applied which results in Number==Number or 1==1 and 1==2, respectively¹; the latter is clearly false.

Rule 1 - If Type(x) is the same as Type(y), then [by c.iii.] If x is the same Number value as y, return true [else return false].

(The same algorithm explains the String==Boolean case when the complementing rules are applied.)

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¹To see the [ToNumber] rule applied, consider:

+false -> 0 +true  -> 1 +"1"   -> 1 +"2"   -> 2