Compare arrays using Array.prototype.every() in JavaScript

We are required to write a JavaScript function that takes in two arrays of literals. Then our function should return true if all the elements of first array are included in the second array, irrespective of their count, false otherwise.

We have to use Array.prototype.every() method to make these comparisons.

Syntax

array.every(callback(element, index, array), thisArg)

How Array.every() Works

The every()true if all elements pass the test function, false if any element fails. It stops checking as soon as one element fails the test.

Example

The code for this will be:

const arr1 = [0, 2, 2, 2, 1];
const arr2 = [0, 2, 2, 2, 3];

const compareArrays = (arr1, arr2) => {
    const areEqual = arr1.every(el => {
        return arr2.includes(el);
    });
    return areEqual;
};

console.log(compareArrays(arr1, arr2));

Output

And the output in the console will be:

false

Why It Returns False

The function returns false because element 1 from arr1 is not present in arr2. The every() method checks each element of arr1 against arr2 using includes().

More Examples

// Example 1: All elements present
const fruits1 = ['apple', 'banana'];
const fruits2 = ['apple', 'banana', 'cherry'];
console.log(compareArrays(fruits1, fruits2)); // true

// Example 2: Missing element
const nums1 = [1, 2, 3];
const nums2 = [1, 2, 4];
console.log(compareArrays(nums1, nums2)); // false

// Example 3: Empty array
const empty = [];
const numbers = [1, 2, 3];
console.log(compareArrays(empty, numbers)); // true (vacuously true)

true
false
true

Optimized Version

We can simplify the function by directly returning the every() result:

const compareArraysOptimized = (arr1, arr2) => {
    return arr1.every(el => arr2.includes(el));
};

const test1 = [1, 2, 3];
const test2 = [1, 2, 3, 4, 5];
console.log(compareArraysOptimized(test1, test2));

true

Key Points

• every() returns true only if ALL elements pass the test
• It stops execution as soon as one element fails
• For empty arrays, every() returns true (vacuously true)
• This method checks inclusion, not equality of arrays

Conclusion

Using Array.prototype.every() with includes() provides an efficient way to check if all elements of one array exist in another. The method stops early when a mismatch is found, making it performant for large arrays.