lib/compare.js
function firstDiff(exp, act) {
  var i;
  for (i = 0; i < exp.length && i < act.length; ++i) {
    if (exp[i] != act[i]) {
      break;
    }
  }
  return i;
}
function firstDiff(exp, act) {
  var i;
  for (i = 0; i < exp.length && i < act.length; ++i) {
    if (exp[i] != act[i]) {
      break;
    }
  }
  return i;
}
Given two arrays or strings, return the first element where they differ (shallow).