Because it is contained in just one .js file, Fleur can easily be used at client-side. Even if NodeJS is supporting some of latest ECMAScript syntax and new features, Fleur source is written for recent browsers.

Of course, client-side evaluation is also limited by security: accessing folders and files is restricted, cross-domain may not be allowed and it is not possible to run external processes.

Nevertheless, it is always a convenient way to try and debug more or less complex expressions, especially with result being serialized in XQuery notation. The browser debugger can help too but a good knowledge of Fleur sources and XQueryX is required to place breakpoints in instructions.