Advanced way to specify if a function call might have side effects. #400

lib/compress.js

@@ -896,6 +896,7 @@ merge(Compressor.prototype, {
         def(AST_Call, function(compressor){
             var pure = compressor.option("pure_funcs");
             if (!pure) return true;
+            if (typeof pure == "function") return pure(this);
             return pure.indexOf(this.expression.print_to_string()) < 0;
         });