1. A computing device configured to: execute a first instance of a single-threaded script engine in a first thread associated with a first execution context, wherein the first instance of the single-threaded script engine accesses at least one shared script object through a first reference counted script base value object; and concurrently execute a second instance of the single-threaded script engine in a second thread associated with a second execution context, wherein the second instance of the single-threaded script engine accesses the at least one shared script object through a second reference counted script base value object, wherein the script engine does not switch between the execution contexts.