profile a function in console

profile.js

measure = function (subj) { 
  var t0 = performance.now()
  subj()
  var t1 = performance.now()
  return (t1 - t0) + 'ms'
}

arr = _.times(10000, function (i) { return { id: i, name: 'Foo ' + i } })
index = _.keyBy(arr, 'id')
arr2 = _.times(5000, function (i) { return { id: i + 1, name: 'Foo ' + i +1 } })

measure(function () {
  index2 = _.keyBy(arr2, 'id')
  _.chain(index).clone().merge(index2).value()
})