Break Javascript array of timestamps into smaller arrays by hour, day

Question

I have an array of objects each containing an ID and timestamp like so:

var data = [
{
   id: 1,
   date: '2015-01-01T00:00:00Z'
},
{
   id: 2,
   date: '2015-01-03T03:00:00Z'
},
{
   id: 3,
   date: '2015-01-03T09:00:00Z'
},
{
   id: 4,
   date: '2015-01-05T00:00:00Z'
},
]

I want to chart the quantity of these objects by hour and day. So I need to know how many have a timestamp between 00:00 and 01:00, 01:00 and 02:00, etc. Likewise, I need to find how many have a timestamp between 2015-01-03 and 2015-01-04, etc.

My first instinct is to loop over the hours of the day/days of the week and within that, loop again over the rows in the table and

if (val.date() >> hourIterator && val.date() << (hourIterator + 1)) {
    new_array.push(val)
}

Am I going about this the right way?

Answer 1

JavaScript date instance have a function .getTime() to get number of milliseconds, for more http://www.w3schools.com/jsref/jsref_gettime.asp

Answer 2

Generally it's best to deal with dates as Date objects, however you can get by with treating them as strings in some cases.

The following counts particular parts within a range (inclusive of the limit), but considers only one factor:

var data = [
            {id: 1, date: '2015-01-01T00:00:00Z'},
            {id: 2, date: '2015-01-03T03:00:00Z'},
            {id: 3, date: '2015-01-03T09:00:00Z'},
            {id: 4, date: '2015-01-05T00:00:00Z'},
           ];

/*  @param {string} part        - name of part to compare, e.g. 'day', 'hour'
**  @param {string/number} low  - value for start of range
**  @param {string/number} high - value for end of range
**  @returns {number}           - count of values in range, inclusive
*/
function countInRange(part, low, high) {
  var partNames = {year:0, month:1, day:2, hour:3, min:4, sec:5};
  var n = partNames[part];
  var count = 0;
  data.forEach(function(obj) {
    var b = obj.date.split(/\D/);
    if (b[n] >= low && b[n] <= high) {
      count++;
    }
  });
  return count;
}

console.log(countInRange('day',1,2));   // 1
console.log(countInRange('day',1,3));   // 3
console.log(countInRange('day',4,6));   // 1
console.log(countInRange('min',0,10));  // 4

If you want to consider other factors (e.g. between 02:00 and 06:00 on a particular day or date range) then converting them to Dates may be more suitable.