Why are JavaScript negative numbers not always true or false?
bitwise-operators, boolean-logic, javascript
Solution
In the first two cases, the boolean is cast to a number - 1 for true and 0 for false. In the final case, it is a number that is cast to a boolean and any number except for 0 and NaN will cast to true. So your test cases are really more like this:
-1 == 1; // false
-1 == 0; // false
true ? true : false; // true
The same would be true of any number that isn't 0 or 1.
For more detail, read the ECMAScript documentation. From the 3rd edition [PDF], section 11.9.3 The Abstract Equality Comparison Algorithm:
19. If Type(y) is Boolean, return the result of the comparison x == ToNumber(y).
It's worth giving the full algorithm a read because other types can cause worse gotchas.
Problem
``` -1 == true; // false -1 == false // false -1 ? true : false; // true ``` Can anyone explain the above output? I know I could work round this by comparing to 0 but I'm interested. I'd expect at least one of the sloppy equals statements to be true as they do implicit type conversion, and I certainly didn't expect the ternary to come up with a totally different result.