Question

I have a json object r:

[ 
  {
    "id"     : "3443",
    "name"   : "Joe",
    "date1"  : "254",
    "date4"  : "261"
  },
  { "id"     : "50942",
    "name"   : "John",
    "date2"  : "192",
    "date4"  : "195"
  },
  { "id"     : "2524",
    "name"   : "Mary",
    "date1"  : "153",
    "date4"  : "163"
  }
]

and I wish to have a Javascript For loop to go through the dateX values.
I know that X is between 1 and a Max value.

But following code does not work:

for (j=1; j<=Max; j=j+1) {
  datestring = 'date' + j;
  if (isset(r[i].datestring)) value[j] = r[i].datestring;
  else value[j] = null; 
}

---

Forgot to explain that I defined the isset function as:

function isset(variable) {
  return typeof(variable) != "undefined" && variable !== null;
} 

Answer 1

First iterate through the array of objects with an ordinary for loop. Then iterate through the properties of each object with for (var p in obj) and check the value of p. Something like:

for (var i = 0; i < arr.length; i++) {
    var obj = arr[i];

    for (var p in obj) {
        if (/^date\d+$/.test(p)) {
            var value = obj[p];
            // p is a dateX property name and value is the value
        }
    }
}

It’s counter-intuitive and inefficient to loop through all possible property values to check their existence. Instead, loop through the actual properties and test their names against your pattern.