Pretender is a mock server library for XMLHttpRequest and Fetch, that comes with an express/sinatra style syntax for defining routes and their handlers.
Packs ECMAScript/CommonJs/AMD modules for the browser. Allows you to split your codebase into multiple bundles, which can be loaded on demand. Supports loaders to preprocess files, i.e. json, jsx, es7, css, less, ... and your custom stuff.