Most client-side JavaScript programs modify page contents imperatively, but Ur/Web is based on functional-reactive programming instead. Programs allocate data sources and then describe the page as a pure function of those data sources. When the sources change, the page changes automatically.
Here's an example where a button modifies a data source that affects some text on the page. The affected portion of the page is indicated with the pseudo-HTML tag dyn, whose signal attribute specifies one of these pure functions over mutable sources. A source containing data of type t has type source t and is created with the source operation within the transaction monad. Functions over sources are represented in the monad signal. Like in Haskell, we overload monad notations, so that the same return and bind operators can be used to write signals and transactions. The signal function coerces a source to a signal.