There are some (somewhat dated) tutorials in this forum that show how to use JS on a web page to interact with Particle.function(), Particle.variable() and Particle.subscribe() on the device.
e.g.
Tutorial: Spark Variable and Function on One Web Page
While dated, the principle hasn’t changed a lot (replace Spark with Particle and consult the docs for new syntax - e.g. Particle.variable()).
You can also have a look at this
https://docs.particle.io/reference/SDKs/javascript/