Question

Is this simply to shorten the lookup chain?

  var slice = Array.prototype.slice;
  var splice = Array.prototype.splice;

http://backbonejs.org/docs/backbone.html

jQuery does something similar:

core_push = Array.prototype.push,
core_slice = Array.prototype.slice,

http://code.jquery.com/jquery-1.8.2.js

Answer 1

For Backbone, that makes no sense at all. Both slice and splice are used exactly once, thus the var declarations crate unnecessary overhead.

For jQuery, different story. A local reference of something that is referenced numerous times, facilitates minification. So the code size of the production version decreases.

Yes, it’s a minor performance gain too, but nothing worth talking about.