To facilitate better optimization in the common case the Closure Compiler assumes toString and valueOf implementation are effectively side-effect free.

In EcmaScript, toString and valueOf are implicitly called when type conversions are necessary, for example '' + x is equivalent to '' + x.valueOf() or '' + x.toString() if valueOf is not defined. A valueOf or toString with side-effects can be called in surprising places. A toString implementation such as Foo.prototype.toString = function() {alert("hi")} is a useful learning exercise but allowing for such behavior prohibit many basic optimizations, such as: 'a' + 'b' + 'c' ==> 'abc'