Appendix B. Bad Parts
And, I pray thee now, tell me for which of my bad parts didst thou first fall in love with me?
!==, and their evil twins
The good ones work the way you would expect. If the two operands are of the same
type and have the same value, then
false. The evil twins
do the right thing when the operands are of the same type, but if they are of
different types, they attempt to coerce the values. The rules by which they do that
are complicated and unmemorable. These are some of the interesting cases:
'' == '0' // false 0 == '' // true 0 == '0' // true false == 'false' // false false == '0' // true false == undefined // false false == null // false null == undefined // true ' \t\r\n ' == 0 // true
The lack of transitivity is alarming. My advice is to never use the evil twins.
Instead, always use
!==. All of the comparisons just shown produce
false with the