Check if all Values in Array are Equal in JavaScript

# Check if all Values in an Array are Equal

To check if all values in an array are equal:

1. Use the Array.every() method to iterate over the array.
2. Check if each array element is equal to the first one.
3. The every() method will return true if all array elements are equal.

Copied!
const arr1 = [1, 1, 1];
const arr2 = [1, 1, 2];

function allAreEqual(array) {
  const result = array.every(element => {
    if (element === array[0]) {
      return true;
    }
  });

  return result;
}

console.log(allAreEqual(arr1)); // 👉️ true
console.log(allAreEqual(arr2)); // 👉️ false

if (allAreEqual(arr1)) {
  // 👇️ this runs
  console.log('All elements in the array are equal');
} else {
  console.log('Not all elements in the array are equal');
}

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

On each iteration, we check if the current element is equal to the element at position 0. If the condition is met for all elements then they are all equal.

Copied!
const arr1 = [];

function allAreEqual(array) {
  const result = array.every(element => {
    if (element === array[0]) {
      return true;
    }
  });

  return result;
}

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

If you consider an empty array one where all values are equal, you don't need to do anything.

However, if you consider an empty array one where not all elements are equal, use the following solution.

Copied!
const arr1 = [];
const arr2 = [1, 1, 1];

function allAreEqual(array) {
  if (array.length > 0) {
    const result = array.every(element => {
      if (element === array[0]) {
        return true;
      }
    });

    return result;
  }

  return false;
}

console.log(allAreEqual(arr1)); // 👉️ false
console.log(allAreEqual(arr2)); // 👉️ true

This is entirely use-case-specific. For example, you might want to check if the array has at least 2 elements to return true from the function.

An alternative approach is to use a Set object.

# Check if all Values in an Array are Equal using a Set object

This is a three-step process:

1. Pass the array to the Set() constructor and access the size property.
2. The Set object only stores unique values.
3. If the Set has a length of 1, then all array elements are equal or the array only contains 1 element.

Copied!
const arr1 = [1, 1, 1];
const arr2 = [1, 1, 2];

function allAreEqual(array) {
  const result = new Set(array).size === 1;

  return result;
}

console.log(allAreEqual(arr1)); // 👉️ true
console.log(allAreEqual(arr2)); // 👉️ false

The Set object allows us to store unique values and removes all duplicates automatically.

If we pass it an array containing the same value multiple times, it would only get added once to the Set.

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console.log(new Set([1, 1, 1])); // 👉️ { 1 }

The size property of the Set allows us to get the number of values stored in the Set.

If you only want to return true if the array contains at least 2 elements, add the following if statement.

Copied!
const arr1 = [1];
const arr2 = [1, 1, 1];

function allAreEqual(array) {
  if (array.length > 1) {
    const result = new Set(array).size === 1;

    return result;
  } else {
    return false;
  }
}

console.log(allAreEqual(arr1)); // 👉️ false
console.log(allAreEqual(arr2)); // 👉️ true

We first make sure the array contains more than 1 element.

If it does, we pass the array to the Set constructor.

In all other cases, we return false.

You can also use a for...of loop to check if all elements in an array are equal.

# Check if all Values in an Array are Equal using a for...of loop

This is a three-step process:

1. Use a for...of loop to iterate over the array.
2. Check if each element is not equal to the first array element.
3. If the condition is met, set a boolean variable to false and exit the loop.

Copied!
function allAreEqual(array) {
  let areEqual = true;

  for (const element of array) {
    if (element !== array[0]) {
      areEqual = false;
      break;
    }
  }

  return areEqual;
}

const arr = [0, 0, 0];
const arr1 = [1, 1, 2];
const arr2 = [1, 1, 1];

console.log(allAreEqual(arr)); // 👉️ true
console.log(allAreEqual(arr1)); // 👉️ false
console.log(allAreEqual(arr2)); // 👉️ true

The for...of statement is used to loop over iterable objects like arrays, strings, Map, Set and NodeList objects and generators.

On each iteration, we check if the current element is not equal to the first array element.

The for...of loop is quite efficient because it doesn't iterate needlessly once it finds a value that isn't equal to the other array elements.

You can also use a basic for loop to check if all elements in an array are equal.

# Check if all Values in an Array are Equal using a for loop

This is a three-step process:

1. Use a for loop to iterate over the array.
2. On each iteration, check if the current element is not equal to the first array element.
3. If the condition is never met, all array elements are equal.

Copied!
function allAreEqual(array) {
  let areEqual = true;

  for (let index = 0; index < array.length; index++) {
    if (array[index] !== array[0]) {
      areEqual = false;

      break;
    }
  }

  return areEqual;
}

const arr = [0, 0, 0];
const arr1 = [1, 1, 2];
const arr2 = [1, 1, 1];

console.log(allAreEqual(arr)); // 👉️ true
console.log(allAreEqual(arr1)); // 👉️ false
console.log(allAreEqual(arr2)); // 👉️ true

We used a basic for loop to iterate over the array.

On each iteration, we check if the element at the current index is not equal to the first array element.

If the condition is met, we set the areEqual variable to false and use the break statement to break out of the loop.

You can also use the Array.filter() method to check if all values in an array are equal.

# Check if all Values in an Array are Equal using Array.filter()

This is a three-step process:

1. Use the Array.filter() method to iterate over the array.
2. Check if each array element is equal to the first array element.
3. Check if the new array's length is equal to the original array's length.

Copied!
function allAreEqual(array) {
  return (
    array.filter(element => {
      return element === array[0];
    }).length === array.length
  );
}

const arr = [0, 0, 0];
const arr1 = [1, 1, 2];
const arr2 = [1, 1, 1];

console.log(allAreEqual(arr)); // 👉️ true
console.log(allAreEqual(arr1)); // 👉️ false
console.log(allAreEqual(arr2)); // 👉️ true

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

On each iteration, we check if the current element is equal to the first array element.

If the length of the new array is equal to the length of the original array, then all array elements are equal.

Note that this approach is not very efficient because you'd have to keep iterating over the array even if a non-matching value is found.

Which approach you pick is a matter of personal preference. I'd use the Array.every() method or the Set() constructor because they're quite performant and easy to read.

# Additional Resources

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

• Check if all Values in Array are True/False in JavaScript
• Check if Array has all Elements of Another Array - JavaScript
• Convert all Array Elements to Uppercase or Lowercase in JS
• Check if Array contains only Numbers in JavaScript