how to prevent adding duplicate keys to a javascript array

Question

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?

Answer 1

Generally speaking, this is better accomplished with an object instead since JavaScript doesn't really have associative arrays:

var foo = { bar: 0 }; 

Then use in to check for a key:

if ( !( 'bar' in foo ) ) {     foo['bar'] = 42; } 

As was rightly pointed out in the comments below, this method is useful only when your keys will be strings, or items that can be represented as strings (such as numbers).

Answer 2

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).