从JavaScript ES5中的数组获取未定义的项目

Question

I'm retrieving a list of items from the server side in JSON format and storing them into a javascript array. The list of items retrieved from the server can contain duplicate elements but I need that the javascript array only contains unique elements.

In order to do that, I'm using the id field from each item (an integer value) as the array index, so that I can check if the item is already in the array by checking the array value at that position.

Due to that the resulting array can contain only elements at indexes 0 and 5 (just an example), and I'm facing problems when iterating it with jQuery's $.each() method, because it traverses every array position from 0 to 5, being undefined the values in positions 1-4.

I can of course just check that the item is not undefined when traversing, but I would like to know if there is some native or straightforward way to obtain the values of the array from those positions that aren't undefined.

var x = [];
x[0] = "some value";
x[5] = "another value";

$.each(x, function(index, value) {
   console.log(index + " - " + value);    
});

// This results in:
// 0 - some value
// 1 - undefined
// 2 - undefined
// 3 - undefined
// 4 - undefined
// 5 - another value

I also accept suggestions to perform this process in a different way, instead of using the item id as the array index to check uniqueness.

Answer 1

Use an object instead:

 var x = {}; x[0] = "some value"; x[5] = "another value"; $.each(x, function(key, value) { console.log(key + " - " + value); }); 

 <script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script> 

Answer 2

Just use the array's own forEach , which skips non-existent entries:

 var x = []; x[0] = "some value"; x[5] = "another value" x.forEach(function(value, index) { console.log(index + " - " + value); }); 

Note that the order of the parameters to the callback is reversed.

If you really want to use $.each , do a check to see if the property exists:

 var x = []; x[0] = "some value"; x[5] = "another value" $.each(x, function(index, value) { if (x.hasOwnProperty(index)) { console.log(index + " - " + value); } }); 

 <script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>