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up vote 0 down vote favorite

Im receiving json data that is aggregated by numerical indexes. When Im in my forloop, for example, the index might start at 1, which means in my forloop an error would occur because 0 doesnt exist. How do I check if a numerical index exists in the javascript array?

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closed as not a real question by JE SUIS CHARLIE, Alp, Stewie, WATTO Studios, Roman C Jun 22 '13 at 12:21

It's difficult to tell what is being asked here. This question is ambiguous, vague, incomplete, overly broad, or rhetorical and cannot be reasonably answered in its current form. For help clarifying this question so that it can be reopened, visit the help center. If this question can be reworded to fit the rules in the help center, please edit the question.
    
Lets see an example of this json data –  Musa Jun 21 '13 at 22:48
1  
How could an index not exist in an array? What you mean is it's value is undefined, right? –  acdcjunior Jun 21 '13 at 22:49
1  
Please post an example. Descriptions of problems in questions are often difficult to answer without concrete examples. –  CodeMonkeyKing Jun 21 '13 at 22:51
    
You haven't posted any code for us to see why you would have an error, but as a suggestion, have you considered using Array.prototyp.forEach? This will most likely solve your problem (unless Array.length is broken) without you showing us any code. :/ –  Xotic750 Jun 21 '13 at 23:31
    
This is a valid question. Arrays in JavaScript can be sparse. They're basically Objects with some special properties. –  squint Sep 29 '14 at 15:28

4 Answers 4

up vote 3 down vote accepted 
var a = [1, 2, 3],
    index = 2;

if ( a[index] !== undefined ) {
    // index doesn't point to an undefined item.
}
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Or if (index in a) would technically check for existence, though your !== undefined check will most likely suffice. –  squint Sep 28 '14 at 16:39
up vote 3 down vote

You should be able to use for(key in data)

var data = [];
data[1] = 'a';
data[3] = 'b';

for(var index in data) {
  console.log(index+":"+data[index]);
}
//Output:
// 1-a
// 3-b

Which will loop over each key item in data if the indexes aren't contiguous.

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up vote 0 down vote

If what you are actually describing is an Object rather than an Array, but is array like in the fact that it has properties that are of uint32_t but does not have essential length property present. Then you could convert it to a real array like this. Browser compatibility wise this requires support of hasOwnProperty

Javascript

function toArray(arrayLike) {
    var array = [],
        i;

    for (i in arrayLike) {
        if (Object.prototype.hasOwnProperty.call(arrayLike, i) && i >= 0 && i <= 4294967295 && parseInt(i) === +i) {
            array[i] = arrayLike[i];
        }
    }

    return array;
}

var object = {
    1: "a",
    30: "b",
    50: "c",
},
array = toArray(object);

console.log(array);

Output

[1: "a", 30: "b", 50: "c"]`

On jsfiddle

Ok, now you have a sparsely populated array and want to use a for loop to do something.

Javascript

var array = [],
    length,
    i;

array[1] = "a";
array[30] = "b";
array[50] = "c";

length = array.length;
for (i = 0; i < length; i += 1) {
    if (Object.prototype.hasOwnProperty.call(array, i)) {
        console.log(i, array[i]);
    }
}

Ouput

1 "a"
30 "b"
50 "c"

On jsfiddle

Alternatively, you can use Array.prototype.forEach if your browser supports it, or the available shim as given on the MDN page that I linked, or es5_shim

Javascript

var array = [];

array[1] = "a";
array[30] = "b";
array[50] = "c";

array.forEach(function (element, index) {
    console.log(index, element);
});

Output

1 "a"
30 "b"
50 "c"

On jsfiddle

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Car to explain the down vote? –  Xotic750 Jun 23 '13 at 11:44
up vote -1 down vote

You can check to see if the index is undefined

var arr = [];
arr[1] = "hello";
arr[2] = "world";

if (arr[0] === undefined) {
  // arr[0] is not set
}

If you're looping like this, you're going to have a bad time because your array does not start with a 0-based index.

for (var i=0, len=arr.length; i<len; i++) {
  // ...
}

The pattern above works for most arrays, but it seems like your array is not like most arrays!

for (var i in arr) {
  console.log(arr[i]);
}

//=> hello
//=> world
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