The best replacement for backbone.js is node.js.

The best alternatives to backbone.js are rated as following:
★★★★★  node.js
★★★★  ember.js
★★  extjs
★  spine.js
★  meteor.js
★  yui
  angularjs
  knockout.js

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Speaking about backbone.js, one probably shouldn't forget about rails.

Other search terms related to backbone.js and their ratings are:
★★★★★ rails
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backbone.js
node.js
ember.js