Parse a timestamp or duration like `3h2m1s` to total seconds.

parseTimestamp.js

  /*
  * Parses a human readable duration on the form
  * and return the total duration in seconds.
  *
  * @param {string} - Timestamp on the form XhYmZs. Unit
  *                   parts are optional: 3h2s, 3m1s, 4h2m
  *                   are all valid.
  */
  function parseTimestamp(timestamp) {

    /**
    * Wrapper around `parseInt`, but returns false
    * if result is NaN.
    */
    function parse(value) {
      var intVal = parseInt(value)
      return _.isNaN(intVal) ? false : intVal
    }

    return _.chain(timestamp.match(/(\d+h)?(\d+m)?(\d+s)?/i))
      .last(3)    // regex match result is in last three elements in array
      .reverse()  // reverse indexes (see below)
      .reduce(function(seconds, value, index) {
        /*
          `value` is a string: '2h', etc. `index` will
          signify the time unit (hour=2, min=1, seconds=0).
          Use that to convert from hours/minutes to seconds:
        */
        var intVal = parse(value)
        // If the value is a parsable number, convert to
        // seconds. Otherwise do nothing with end result `seconds`.
        seconds += intVal ? intVal*(Math.pow(60, index)) : 0

        return seconds
      }, 0)
    .value()
  }