[javascript] how to prevent adding duplicate keys to a javascript array

I found a lot of related questions with answers talking about for...in loops and using hasOwnProperty but nothing I do works properly. All I want to do is check whether or not a key exists in an array and if not, add it.

I start with an empty array then add keys as the page is scrubbed with jQuery.

Initially, I hoped that something simple like the following would work: (using generic names)

if (!array[key])
   array[key] = value;

No go. Followed it up with:

for (var in array) {
   if (!array.hasOwnProperty(var))
      array[key] = value;
}

Also tried:

if (array.hasOwnProperty(key) == false)
   array[key] = value;

None of this has worked. Either nothing is pushed to the array or what I try is no better than simply declaring array[key] = value Why is something so simple so difficult to do. Any ideas to make this work?

The logic is wrong. Consider this:

x = ["a","b","c"]
x[0]     // "a"
x["0"]   // "a"
0 in x   // true
"0" in x // true
x.hasOwnProperty(0)   // true
x.hasOwnProperty("0") // true

There is no reason to loop to check for key (or indices for arrays) existence. Now, values are a different story...

Happy coding

You can try this:

var names = ["Mike","Matt","Nancy","Adam","Jenny","Nancy","Carl"];
var uniqueNames = [];
$.each(names, function(i, el){
if($.inArray(el, uniqueNames) === -1) uniqueNames.push(el);
});

Easiest way to find duplicate values in a JavaScript array

var a = [1,2,3], b = [4,1,5,2];

b.forEach(function(value){
  if (a.indexOf(value)==-1) a.push(value);
});

console.log(a);
// [1, 2, 3, 4, 5]

For more details read up on Array.indexOf.

If you want to rely on jQuery, instead use jQuery.inArray:

$.each(b,function(value){
  if ($.inArray(value,a)==-1) a.push(value);
});

If all your values are simply and uniquely representable as strings, however, you should use an Object instead of an Array, for a potentially massive speed increase (as described in the answer by @JonathanSampson).

function check (list){
    var foundRepeatingValue = false;
    var newList = [];
    for(i=0;i<list.length;i++){
        var thisValue = list[i];
        if(i>0){
            if(newList.indexOf(thisValue)>-1){
                foundRepeatingValue = true;
                console.log("getting repeated");
                return true;
            }
       } newList.push(thisValue);
    } return false;
}

var list1 = ["dse","dfg","dse"];
check(list1);

Output:

getting repeated
true

A better alternative is provided in ES6 using Sets. So, instead of declaring Arrays, it is recommended to use Sets if you need to have an array that shouldn't add duplicates.

var array = new Set();
array.add(1);
array.add(2);
array.add(3);

console.log(array);
// Prints: Set(3) {1, 2, 3}

array.add(2); // does not add any new element

console.log(array);
// Still Prints: Set(3) {1, 2, 3}