So based on the edit the array with actual information we want is the value of the Items key.

myArr doesn't seem to be an array, but it's actually a key-value pair based on the edit as well.

Here is my attempt:

var myArr = JSON.parse(this.responseText);

//console.log(myArr.Items)
//Declare and initialize k-v pair.
var results = {};
for (var i = 0, len = myArr.Items.length; i < len; i++) {
    var id_art = myArr.Items[i].id_a;
    if (id_art == 123456) {
        //console.log(i, myArr.Items[i]);
        //Create the key-value pair and store the index information in the key.
        results[i] = myArr.Items[i];
    }
}

console.log(results);

Output:

{
  '3': {
    type: 'Image',
    id_a: '123456',
    value: 'moto_guzzi_v100_mandello.jpg',
    number: 3,
    id: 3
  },
  '4': {
    type: 'Text',
    id_a: '123456',
    value: 'The star of the Piaggio group stand is without doubt ... Discover all his secrets with our video',
    number: 2,
    id: 2
  }
}

Explanation: Since you have specified you do not care how the data is stored. I have put it in the key of a new key-value Object in Javascript (do they call these dictionaries or hashmaps here?).

Thus, you can access them now by cycling through the elements in the dictionary, or getting a list of just keys in the dictionary and choosing which value you would like to access.

For instance, to print out a list of keys that exist in this object, you might use:

for (let value of Object.keys(results)) {
    console.log(value)
}

Let me know if I have understood you correctly.

Edit

To get the output mentioned in your comment, now using while:

var x = 0;
while (x < Object.entries(results).length) {
    //console.log(Object.entries(results)[x][1]);
    if (Object.entries(results)[x][1].type == "Image"){
        console.log("The index is: ", Object.entries(results)[x][0]);
        console.log("The value is: ", Object.entries(results)[x][1].value);
    }
    x++;

}

Output: