I'm receiving json data that is aggregated by numerical indexes.
When I'm 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?
var a = [1, 2, 3], index = 2;
if ( a[index] !== void 0 ) { /* void 0 === undefined */
/* See concern about ``undefined'' below. */
/* index doesn't point to an undefined item. */
}
a is not defined it will fail just as badly when trying to access [index].undefined as its parameter name, he's worthy of being skinned alive regardless of programming language used, and worthy of being boiled in tar afterwards if he's coding in JS. FWIW, I wouldn't go with void 0 instead of undefined just for that reason - it's not "defensive coding", it's "paranoid coding" since ES5. void 0 is semantic noise, undefined is semantically correct. We should program with humans in mind, not computers, IMVHO.
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.
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
undefined, right?Array.lengthis broken) without you showing us any code. :/