If two Objects added together equal NaN(not a number), which is technically of type number, then why does getting the type of two Objects added together result in "string"?
I will express this via the REPL:> {} + {}> NaN
ok. two objects added together creates NaN
> typeof(NaN)> "number"
ok. we know that NaN's type is "number"
> typeof({} + {})> "string"
wait. shouldn't this have been "number" also?
I'm aware that javascript has a less then desireable type system, but I'm confused as to what's happening here. Is the type being converted from number to string for some reason? Is number even a part of the type converting that goes on here? Or am I just using typeof wrong?
{} + {} is an empty block ({}) followed by a type conversion from object to number (+{}). It basically reads as
{} // empty block (a statement)
; // empty statement (just for clarity)
+{}; // expression statement (unary plus, object literal -> conversion to number)
However if you use typeof ({} + {}), then {} + {} will be evaluated as expression in which case both {} can only be object literals and the + is the concatenation operator.
You can also just use the grouping operator to force the construct to be evaluated as expression:
({} + {}) // expression statement (string concatenation with two objects)
See also Why {} + {} is NaN only on the client side? Why not in Node.js? and other questions related to [javascript] "{} + {}".
来源：https://stackoverflow.com/questions/25416725/why-is-typeofs-result-different-than-the-evaluated-result-of-the-expression-pas