Objects don't have a .length property.

A simple solution if you know you don't have to worry about hasOwnProperty checks, would be to do this:

Object.keys(data).length;

If you have to support IE 8 or lower, you'll have to use a loop, instead:

var length= 0;
for(var key in data) {
    if(data.hasOwnProperty(key)){
        length++;
    }
}

You have spelling mistake

 var test = cars.length;

Note: Array indexes are nothing but properties of Array objects.

Quoting MDN's Relationship between length and numerical properties section,

When setting a property on a JavaScript array when the property is a valid array index and that index is outside the current bounds of the array, the engine will update the array's length property accordingly.

Quoting ECMA Script 5 Specification of Array Objects,

whenever a property is added whose name is an array index, the length property is changed, if necessary, to be one more than the numeric value of that array index; and whenever the length property is changed, every property whose name is an array index whose value is not smaller than the new length is automatically deleted

So, when you set a value at index 5, JavaScript engine adjusts the length of the Array to 6.

Quoting ECMA Script 5 Specification of Array Objects,

A property name P (in the form of a String value) is an array index if and only if ToString(ToUint32(P)) is equal to P and ToUint32(P) is not equal to 2³²−1.

So, in your case 2 and 4 are valid indexes but only 2 is defined in the array. You can confirm that like this

arr.hasOwnProperty(2)

The other indexes are not defined in the array yet. So, your array object is called a sparse array object.

So why arr[2] is counted in for..in loop and not arr[4] is not counted?

The for..in enumerates all the valid enumerable properties of the object. In your case, since only 2 is a valid property in the array, it will be counted.

But, when you print arr[4], it prints undefined, because JavaScript will return undefined, if you try to access a property which is not defined in an object. For example,

console.log({}['name']);
// undefined

Similarly, since 4 is not yet defined in the arr, undefined is returned.

While we are on this subject, you might want to read these answers as well,

• Why doesn't the length of the array change when I add a new property?

• JavaScript 'in' operator for undefined elements in Arrays

Arrays are zero-based indexing. Which means The first element of the array is indexed by subscript of 0 and last element will be length - 1

const arr = [2 , 3, 6, 8];
const end = arr[ arr.length - 1 ];
console.log(end);

You can create an array using array syntax:

    array = ["first item", {second:'item'}, 3];
    array.def = {something:"else"};
    alert(array.length);

And here's how you create a multidimensional array using an array of arrays:

    array = [
        [1,2,3],
        [4,5,6],
        [7,8,9]
    ];

    alert(array[0][2]); // alerts "3"

The reason is that you're not logging superArray in your fetch promise. If you add the call to console.log() in your last then() call it works. The reason for this is that fetch is executed asynchronously, which means that any additional code that comes after the fetch call is executed long before the fetch has returned anything.

The reason you can see a full log of superArray even when doing it outside the fetch is a special console behaviour.

const URL1 = "https://www.politifact.com/api/statements/truth-o-meter/people/barack-obama/json/?n=50";
const URL2 = "https://www.politifact.com/api/statements/truth-o-meter/people/hillary-clinton/json/?n=210";
const URL3 = "https://www.politifact.com/api/statements/truth-o-meter/people/bernie-s/json/?n=70";
var superArray = [];
fetch("https://cors-anywhere.herokuapp.com/" + URL1)
  .then(results => {
    return results.json();
  })
  .then(data => {
    data = data.filter(function(item) {
      return item.speaker.name_slug == "barack-obama" && item.statement_type.statement_type !== "Flip";
    });
    for (var i = 0; i < 10; i++) {
      superArray.push(data.splice(Math.random() * data.length, 1)[0]);
    }
  })
fetch("https://cors-anywhere.herokuapp.com/" + URL2)
  .then(results => {
    return results.json();
  })
  .then(data => {
    data = data.filter(function(item) {
      return item.speaker.name_slug == "hillary-clinton" && item.statement_type.statement_type !== "Flip";
    });
    for (var i = 0; i < 10; i++) {
      superArray.push(data.splice(Math.random() * data.length, 1)[0]);
    }
  })
fetch("https://cors-anywhere.herokuapp.com/" + URL3)
  .then(results => {
    return results.json();
  })
  .then(data => {
    data = data.filter(function(item) {
      return item.speaker.name_slug == "bernie-s" && item.statement_type.statement_type !== "Flip";
    });
    for (var i = 0; i < 10; i++) {
      superArray.push(data.splice(Math.random() * data.length, 1)[0]);
    }
    console.log(superArray[0]);
  })

lenght = length!!

So there you go!

Arrays can grow and shrink dynamically. So from a certain point of view, they are already of undefined length. You can always add new objects to it if you want to.

You can also create a helper function which checks first if an object exists at a certain position and if not, creates a new one.

You mentioned array[2].value = 'foo' as an example. Here is a helper function that you could use:

function getObjectAtIndex(arr, index) {
    return arr[index] || (arr[index] = {});
}

and then, instead of writing array[2].value = 'foo', you'd write:

getObjectAtIndex(array, 2).value = 'foo'

It is spelled length not lenght.

It should be:

var rcount = ary.length;