Log the time of JS operations

logtime.js

var logtime = (function() {
  var ids = {};

  return function(id) {
    if (!ids[id]) {
      ids[id] = +new Date();
      return;
    }

    var time = +new Date() - ids[id];
    delete ids[id];

    console.log(id + ': ' + time + 'ms');
    return time;
  }
}());

usage.js

logtime('foo');
doSomethingExpensive();
logtime('foo'); // console logs the time between calls to logtime('foo')