Reactive programming enables declarative descriptions of dependencies between and computations throughout signals, an abstraction of time-varying values. Signals have been extended to persistent signals (an abstraction of time-varying values with their execution histories) to enable them to go back in time with any given time. Currently, this feature is only supported by SignalJ, an extension to Java with signals. This limits the use of persistent signals only within the Java-based applications. This is an undesirable restriction, because mechanisms used in realizing persistent signals are actually language-independent. To tackle this problem, we propose an implementation of persistent signals in JavaScript, which makes application areas of persistent signals broader, including Web-frontend. To realize persistent signals in JavaScript, seamless connections between JavaScript programs that run on the restricted environments such as browsers and time-series databases that serve histories of persistent signals are required. It is also desirable to reuse existing JavaScript ecosystem. To address these problems, we design a relay-server-based architecture and realize persistent signals in JavaScript as a DSL library.