Why is "" == [null] true in JavaScript?
I know JavaScript has lots of insane results with comparisons between types, though I don't fully understand why. Came across this one today.
Why does
"" == [null]
evaluate to true in JavaScript?
Some more Javascript equality amusement, thanks to @Qantas:
- Why does 2 == [2] in JavaScript?
- Why is 0 == "" true in JavaScript
- Why if([]) is validated while [] == false in javascript?
- Why does !{}[true] evaluate to true in JavaScript?
The "Abstract Equality Comparison Algorithm" has many parts, but the important one here is this:
If Type(x) is either String or Number and Type(y) is Object, return the result of the comparison x == ToPrimitive(y).
(There's a mirror-image of that too.) So, because "" is a string and [null] is an object, we've got to first convert [null] to a string by calling ToPrimitive([null]). That's an internal operation described as follows, when it's asked to convert an Object instance to a primitive value:
Return a default value for the Object. The default value of an object is retrieved by calling the [[DefaultValue]] internal method of the object, passing the optional hint PreferredType. The behaviour of the [[DefaultValue]] internal method is defined by this specification for all native ECMAScript objects in 8.12.8.
Now, the [[DefaultValue]] internal operation will call .toString() on the object and return that value. Try [null].toString() in your browser console:
> [null].toString()
""
And there you have it.
Edit: And why is [null].toString() an empty string? Because the .toString() operation on Array instances always just calls .join(), and that always yields an empty string for null and undefined values. Thus an array of one null ends up as just a single empty string.
It's according to the arcane type-conversion rules of Javascript. Rule #8:
If Type(x) is either String or Number and Type(y) is Object, return the result of the comparison x == ToPrimitive(y).
So the comparison is between x = "" and y = [null] converted to a string using ToPrimitive. Converting an array with one null element results in an empty string (because Array.toString() returns a comma-separated list of values), hence they evaluate to equal.