Extending native prototypes creates other less obvious hurdles for libraries too. Craft.js, es5-shim, Modernizr, MooTools, Prototype.js, and Sugar.js, to name a

Extending native prototypes creates other less obvious hurdles for libraries too. Craft.js, es5-shim, Modernizr, MooTools, Prototype.js, and Sugar.js, to name a few, have all, at one time or another, added incorrect shims to native prototypes. While Underscore is in a better position than those that extend native prototypes, regarding api/environment conflicts, it can still be tripped up by poor shims because it defers to many ES5 methods if they exist. For example, if Prototype 1.6.0 and Underscore.js are included on a page Underscore's `_.reduce` method won't work properly. This is one of the reasons why libs/frameworks like Dojo, Ember, Lo-Dash, RequireJS, Sizzle, and YUI do native checks too.