underscore.bind as partial application

gistfile1.js

var add = function(a, b) {
  return a + b;
};

add(20, 30); // 50

// Twenty is so special for our use case that we want it to always be included
var addTwenty = _.bind(add, {}, 20);
                     //fn, this, args...
                     
addTwenty(30); // 50

// if we wrote this by hand instead:
var addTwenty = function(b) {
  this = {};
  var a = 20;
  return a + b;
}

addTwenty(30); // 50