In Javascript, how do I check if an array has duplicate values? [duplicate]

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Easiest way to find duplicate values in a javascript array

How do I check if an array has duplicate values?

If more than 1 element of the same exist, then return true. Otherwise, return false.

['hello','goodbye','hey'] //return false because no duplicates exist
['hello','goodbye','hello'] // return true because duplicaets exist

I don't want a list of duplicates removed. I just want to know true or false if a list has duplicates in it. — user847495, Sep 11 '11 at 6:08
The accepted answer for the exact same question you asked is your answer.stackoverflow.com/questions/840781/… — Brian Roach, Sep 11 '11 at 6:28
This question is not a duplicate. Since @user847495 simply wants to check if duplicates exists, the solution is faster/easier than what's needed to find all occurrences of duplicates. For example, you can do this:codr.io/v/bvzxhqm — alden, Sep 26 '15 at 16:32

Domenic · Accepted Answer · 2015-07-03 16:48:27Z

If you have an ES2015 environment (as of this writing: io.js, IE11, Chrome, Firefox, WebKit nightly), then the following will work, and will be fast (viz. O(n)):

function hasDuplicates(array) {
    return (new Set(array)).size !== array.length;
}

If you only need string values in the array, the following will work:

function hasDuplicates(array) {
    var valuesSoFar = Object.create(null);
    for (var i = 0; i < array.length; ++i) {
        var value = array[i];
        if (value in valuesSoFar) {
            return true;
        }
        valuesSoFar[value] = true;
    }
    return false;
}

We use a "hash table" valuesSoFar whose keys are the values we've seen in the array so far. We do a lookup using Object.prototype.hasOwnProperty.call to see if that value has been spotted already; if so, we bail out of the loop and return true. (We don't use valuesSoFar.hasOwnProperty directly because that would break if the array contained "hasOwnProperty" as a string.)

If you need a function that works for more than just string values, the following will work, but isn't as performant; it's O(n²) instead of O(n).

function hasDuplicates(array) {
    var valuesSoFar = [];
    for (var i = 0; i < array.length; ++i) {
        var value = array[i];
        if (valuesSoFar.indexOf(value) !== -1) {
            return true;
        }
        valuesSoFar.push(value);
    }
    return false;
}

The difference is simply that we use an array instead of a hash table for valuesSoFar, since JavaScript "hash tables" (i.e. objects) only have string keys. This means we lose the O(1) lookup time of hasOwnProperty, instead getting an O(n) lookup time of indexOf.

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In Javascript, how do I check if an array has duplicate values? [duplicate]

marked as duplicate by Brian Roach, mu is too short, Joseph Silber, Sasha Chedygov, Ben AlpertSep 11 '11 at 6:39

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