Javascript for loop through JSON values

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;
}

Have you defined isset anywhere? isset exists as a function in PHP, but not JavaScript. — ajm
– ajm, Commented Jun 13, 2011 at 15:23
Consider looping through the existing properties instead of looping through all possible property names (see my answer below). — Ateş Göral
– Ateş Göral, Commented Jun 13, 2011 at 15:28

Ateş Göral · Accepted Answer · 2011-06-13 15:23:20Z

10

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.

answered Jun 13, 2011 at 15:23

Ateş Göral

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Comments

Jeremy · Accepted Answer · 2011-06-13 15:22:20Z

3

You are looking for a property called datestring when you should be doing r[i][datestring] or r[i]['date'+j]. This will read the value of the variable and look a property with that name.

answered Jun 13, 2011 at 15:22

Jeremy

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3 Comments

JaredMcAteer Over a year ago

Just to elaborate r[i].datestring === r[i]['datestring'] and r[i][datestring] === r[i]['dateX'] (X being whatever iteration of the loop)

Timothée HENRY Over a year ago

Using r[i][datestring] I am getting an error "TypeError Object Expected"

Jeremy Over a year ago

Where is it coming from? I don't know what your isset function is doing.

castis · Accepted Answer · 2011-06-22 15:49:31Z

disclaimer: if you came here looking for an example of looping through JSON values in the same method that the original poster is asking, Ates answer below mine is a much much better method, please do not use this one.

Im not sure why you'd want to do whatever you're trying because you're going to be overwriting values in value like crazy but you probably know more about what you're doing than we do so heres a working example based on your code.

var 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"
}
];


var max = 10; // guess

var value = {};

for(var i in r)
{
    for (j=0; j<=max; j++)
    {
        datestring = 'date'+j;

        if (undefined != r[i][datestring])
        {
            value[j] = r[i][datestring];
        }
        else
        {
            value[j] = null;
        }
    }
}

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