Check if an Array contains Duplicates in JavaScript

# Table of Contents

1. Check if an Array contains Duplicates in JavaScript
2. Check if an Array contains Duplicates with some()
3. Check if an array contains duplicate objects using Set()
4. Check if an array contains duplicate objects using Array.map()

# Check if an Array contains Duplicates in JavaScript

To check if an array contains duplicates:

1. Pass the array to the Set() constructor and access the size property.
2. Compare the size of the Set to the array's length.
3. If the Set contains as many values as the array, the array doesn't contain duplicates.

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const arr1 = ['a', 'a'];
const arr2 = ['a', 'b'];

function containsDuplicates(array) {
  if (array.length !== new Set(array).size) {
    return true;
  }

  return false;
}

console.log(containsDuplicates(arr1)); // 👉️ true

console.log(containsDuplicates(arr2)); // 👉️ false

We used the Set object to remove all duplicates from the array.

When an array containing duplicates is passed to the Set() constructor, all duplicates get removed.

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console.log(new Set(['a', 'a', 'a'])); // 👉️ { 'a' }

The size property on the Set returns the number of values the Set contains.

If the array's length is not equal to the length of the Set object, the array contains duplicates.

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const arr1 = ['a', 'a'];
const arr2 = ['a', 'b'];

function containsDuplicates(array) {
  if (array.length !== new Set(array).size) {
    return true;
  }

  return false;
}

console.log(containsDuplicates(arr1)); // 👉️ true

console.log(containsDuplicates(arr2)); // 👉️ false

You can also use the Array.some() method to check if an array contains duplicates.

# Check if an Array contains Duplicates with some()

This is a three-step process:

1. Use the Array.some() method to iterate over the array.
2. Check if the index of the first occurrence of the current value is NOT equal to the index of its last occurrence.
3. If the condition is met, then the array contains duplicates.

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const arr1 = ['a', 'a'];
const arr2 = ['a', 'b'];

function containsDuplicates(array) {
  const result = array.some(element => {
    if (array.indexOf(element) !== array.lastIndexOf(element)) {
      return true;
    }

    return false;
  });

  return result;
}

console.log(containsDuplicates(arr1)); // 👉️ true

console.log(containsDuplicates(arr2)); // 👉️ false

The function we passed to the Array.some() method gets called with each element in the array until it returns a truthy value or iterates over the entire array.

If the function returns true at least once, the some() method short-circuits returning true.

If the first and last indexes of an element in an array are different, the value is contained at least twice in the array.

If the condition is met at least once, the some() method short-circuits returning true.

This is good for performance because we don't need to needlessly loop after we have the solution.

You can check if an array contains duplicate objects using the Set() constructor.

# Check if an array contains duplicate objects using Set()

This is a three-step process:

1. Use the Array.map() method to get an array of the values of the relevant property.
2. Pass the array to the Set() constructor to remove the duplicates.
3. Check if the length of the Set is less than the length of the array.

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const arr = [
  {id: 1, name: 'Alice'},
  {id: 2, name: 'Bob'},
  {id: 1, name: 'Carl'},
];

const uniqueValues = new Set(
  arr.map(obj => {
    return obj.id;
  }),
);
console.log(uniqueValues); // 👉️ Set(2) { 1, 2 }

const hasDuplicates = uniqueValues.size < arr.length;
console.log(hasDuplicates); // 👉️ true

The function we passed to the Array.map() method gets called with each element (object) in the array.

We used the Array.map() method to get an array of all of the values of the id property.

The next step is to pass the array to the Set() constructor to remove all of the duplicates.

You can also use the Array.map() and Array.some() methods to check if an array contains duplicate objects.

# Check if an array contains duplicate objects using Array.map()

This is a three-step process:

1. Use the Array.map() method to get an array of the values of the relevant object property.
2. Use the Array.some() method to check if each value is contained multiple times in the array.
3. If the condition is met, the array contains duplicate objects.

Copied!
const arr = [
  {id: 1, name: 'Alice'},
  {id: 2, name: 'Bob'},
  {id: 1, name: 'Carl'},
];

const values = arr.map(o => o.id);
console.log(values); // 👉️ [1, 2, 1]

const hasDuplicates = values.some((item, index) => {
  return values.indexOf(item) !== index;
});

console.log(hasDuplicates); // 👉️ true

if (hasDuplicates) {
  console.log('The array has duplicate objects');
} else {
  console.log('The array does NOT have duplicate objects');
}

The function we passed to the Array.map() method gets called with each element (object) in the array.

We returned the value of the id property on each iteration, but the property you use to check for uniqueness might be different.

At this point, we have an array that stores all of the values of the relevant property.

Copied!
const arr = [
  {id: 1, name: 'Alice'},
  {id: 2, name: 'Bob'},
  {id: 1, name: 'Carl'},
];

const values = arr.map(o => o.id);
console.log(values); // 👉️ [1, 2, 1]

const hasDuplicates = values.some((item, index) => {
  return values.indexOf(item) !== index;
});

The next step is to use the Array.some() method to iterate over the array.

On each iteration, we use the indexOf() method to get the index of the first occurrence of the value in the array.

If the hasDuplicates variable stores a true value, the array contains duplicate objects, otherwise, no duplicate objects are contained in the array.

Here is another similar example of checking if an array contains duplicate objects by using the Array.some() method with a Set object.

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const arr = [
  {id: 1, name: 'Alice'},
  {id: 2, name: 'Bob'},
  {id: 1, name: 'Carl'},
];

const seen = new Set();

const hasDuplicates = arr.some(obj => {
  return seen.size === seen.add(obj.id).size;
});

console.log(seen); // 👉️ Set(2) { 1, 2 }
console.log(hasDuplicates); // 👉️ true

We declared a seen variable that stores an empty Set object and used the Array.some() method to iterate over the array of objects.

On each iteration, we compare the length of the Set to the length of the Set after adding the value of the id property of the current object to the Set.

If the expression evaluates to true, then we've added a duplicate value to the Set, so its length didn't get updated.

If the expression returns true, the array contains duplicate objects, otherwise, the array doesn't contain any duplicate objects.

# Additional Resources

You can learn more about the related topics by checking out the following tutorials:

• Check if Array Doesn't contain a Value in JavaScript
• Check if Array Contains an Object in JavaScript
• Check if Array contains only Numbers in JavaScript
• Check if Array contains any element of another Array in JS