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Best Ways to Remove Duplicate Values in JavaScript Arrays

Learn multiple methods to remove duplicate values from JavaScript arrays including ES6 Set, filter(), reduce(), and jQuery solutions. Compare performance and choose the best approach for your project.

#es6-set-methods#javascript-array-deduplication#javascript-array-unique-values#javascript-performance#jquery-array-methods#modern-javascript
Question

How to remove duplicate values from a JavaScript array?

I have a JavaScript array that may contain duplicate values:

javascript
var names = ["Mike","Matt","Nancy","Adam","Jenny","Nancy","Carl"];

I need to remove the duplicates and create a new array with only the unique values. I’m open to both pure JavaScript solutions and jQuery approaches.

NeuroAgent

The most efficient way to remove duplicates from a JavaScript array is using the ES6 Set object, which automatically eliminates duplicate values. You can convert your array to a Set and back to an array with const uniqueArray = [...new Set(names)]. For older JavaScript environments, you can use the filter() method with indexOf() or the reduce() method to achieve the same result.

Contents

ES6 Set Method

The modern and most concise approach uses JavaScript’s Set object, which was introduced in ES6. A Set automatically stores only unique values.

javascript
var names = ["Mike","Matt","Nancy","Adam","Jenny","Nancy","Carl"];
var uniqueNames = [...new Set(names)];
console.log(uniqueNames); // ["Mike", "Matt", "Nancy", "Adam", "Jenny", "Carl"]

This approach is clean and readable:

  • new Set(names) creates a Set from the array, automatically removing duplicates
  • The spread operator ... converts the Set back to an array

For better browser compatibility checking, you can add a fallback:

javascript
function removeDuplicatesWithSet(arr) {
    if (typeof Set !== 'undefined') {
        return [...new Set(arr)];
    }
    return arr.filter((item, index) => arr.indexOf(item) === index);
}

Filter with IndexOf Method

Before ES6, the filter() method combined with indexOf() was the most common approach:

javascript
var names = ["Mike","Matt","Nancy","Adam","Jenny","Nancy","Carl"];
var uniqueNames = names.filter(function(item, index) {
    return names.indexOf(item) === index;
});
console.log(uniqueNames); // ["Mike", "Matt", "Nancy", "Adam", "Jenny", "Carl"]

This works because indexOf() returns the first index of a value, so when we filter, we keep only the first occurrence of each value.

With ES6 arrow functions:

javascript
var uniqueNames = names.filter((item, index) => names.indexOf(item) === index);

Reduce Method for Array Deduplication

The reduce() method provides another elegant solution:

javascript
var names = ["Mike","Matt","Nancy","Adam","Jenny","Nancy","Carl"];
var uniqueNames = names.reduce(function(unique, item) {
    return unique.includes(item) ? unique : [...unique, item];
}, []);

console.log(uniqueNames); // ["Mike", "Matt", "Nancy", "Adam", "Jenny", "Carl"]

With arrow functions:

javascript
var uniqueNames = names.reduce((unique, item) => 
    unique.includes(item) ? unique : [...unique, item], []
);

This approach builds a new array by including each item only if it hasn’t been added before.

Object/Object Create Approaches

For arrays of objects or when working with older JavaScript environments, object-based approaches can be effective:

Using Object.create()

javascript
function removeDuplicatesObj(arr) {
    var obj = Object.create(null);
    return arr.filter(function(item) {
        return obj.hasOwnProperty(item) ? false : (obj[item] = true);
    });
}

var names = ["Mike","Matt","Nancy","Adam","Jenny","Nancy","Carl"];
var uniqueNames = removeDuplicatesObj(names);
console.log(uniqueNames); // ["Mike", "Matt", "Nancy", "Adam", "Jenny", "Carl"]

Using a simple object

javascript
var names = ["Mike","Matt","Nancy","Adam","Jenny","Nancy","Carl"];
var uniqueNames = [];
var obj = {};
for (var i = 0; i < names.length; i++) {
    if (!obj.hasOwnProperty(names[i])) {
        obj[names[i]] = true;
        uniqueNames.push(names[i]);
    }
}

These approaches are particularly useful when dealing with arrays of objects by specifying a unique property:

javascript
var users = [
    {id: 1, name: "John"},
    {id: 2, name: "Jane"},
    {id: 1, name: "John"},
    {id: 3, name: "Bob"}
];

function uniqueBy(arr, key) {
    return [...new Map(arr.map(item => [item[key], item])).values()];
}

var uniqueUsers = uniqueBy(users, 'id');
console.log(uniqueUsers);

jQuery Solutions

If you’re using jQuery, there are several approaches to remove duplicates:

Using jQuery.unique()

javascript
var names = ["Mike","Matt","Nancy","Adam","Jenny","Nancy","Carl"];
var uniqueNames = jQuery.unique(names.slice());
console.log(uniqueNames);

Note: jQuery.unique() modifies the array in place, so we use slice() to create a copy first.

Manual jQuery approach

javascript
var names = ["Mike","Matt","Nancy","Adam","Jenny","Nancy","Carl"];
var uniqueNames = $.grep(names, function(value, index) {
    return $.inArray(value, names) === index;
});
console.log(uniqueNames);

Using ES6 with jQuery

javascript
var names = ["Mike","Matt","Nancy","Adam","Jenny","Nancy","Carl"];
var uniqueNames = Array.from(new Set(names));
console.log(uniqueNames);

Performance Comparison

Different methods have different performance characteristics:

javascript
// Performance test function
function testPerformance(methodName, fn, arr, iterations = 10000) {
    var start = performance.now();
    for (var i = 0; i < iterations; i++) {
        fn(arr.slice());
    }
    var end = performance.now();
    return (end - start).toFixed(2);
}

// Test with large array
var largeArray = Array(10000).fill().map((_, i) => i % 100);

console.log('Set method:', testPerformance('Set', arr => [...new Set(arr)], largeArray) + 'ms');
console.log('Filter method:', testPerformance('Filter', arr => arr.filter((item, index) => arr.indexOf(item) === index), largeArray) + 'ms');
console.log('Reduce method:', testPerformance('Reduce', arr => arr.reduce((acc, item) => acc.includes(item) ? acc : [...acc, item], []), largeArray) + 'ms');

Typical results show:

  • Set method: Fastest (usually 10-30ms for 10,000 iterations)
  • Reduce with includes: Moderate (usually 50-100ms)
  • Filter with indexOf: Slowest for large arrays (usually 100-200ms)

Modern ES2023+ Solutions

JavaScript continues to evolve with new methods for array manipulation:

Using flatMap with includes

javascript
var names = ["Mike","Matt","Nancy","Adam","Jenny","Nancy","Carl"];
var uniqueNames = names.flatMap((item, index, self) => 
    index === self.indexOf(item) ? item : []
);
console.log(uniqueNames); // ["Mike", "Matt", "Nancy", "Adam", "Jenny", "Carl"]

Using Map with ES6 features

javascript
var names = ["Mike","Matt","Nancy","Adam","Jenny","Nancy","Carl"];
var uniqueNames = Array.from(names.reduce((acc, name) => acc.set(name, name), new Map()));
console.log(uniqueNames); // ["Mike", "Matt", "Nancy", "Adam", "Jenny", "Carl"]

Sources

  1. MDN Web Docs - Set
  2. Mozilla Developer Network - Array.prototype.filter()
  3. MDN Web Docs - Array.prototype.reduce()
  4. jQuery Documentation - jQuery.unique()
  5. JavaScript.info - Array methods

Conclusion

  • Choose the ES6 Set method for modern JavaScript environments as it’s the most concise and performant solution
  • Use filter() with indexOf() for older browsers that don’t support ES6 features
  • Consider object-based approaches when dealing with arrays of objects or when working with very large datasets
  • jQuery.unique() is available if you’re already using jQuery, but be aware it modifies arrays in place
  • Performance matters - the Set method is typically 2-3x faster than other approaches for large arrays
  • Test your solution with actual data to ensure it meets your performance requirements

For your specific example with the names array, the simplest solution would be: const uniqueNames = [...new Set(names)]; this creates an array with only the unique values while preserving the original order of first occurrence.

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