Get the max n values from an array in JavaScript

We are required to write a JavaScript function that takes in an array of Numbers as the first argument and a number, say n, as the second argument.

Our function should then pick the n greatest numbers from the array and return a new array consisting of those numbers.

Using Sort and Slice

The most straightforward approach is to sort the array in descending order and take the first n elements:

const arr = [3, 4, 12, 1, 0, 5, 22, 20, 18, 30, 52];

const pickGreatest = (arr = [], num = 1) => {
    if(num > arr.length){
        return [];
    }
    const sorter = (a, b) => b - a;
    const descendingCopy = arr.slice().sort(sorter);
    return descendingCopy.slice(0, num);
};

console.log(pickGreatest(arr, 3));
console.log(pickGreatest(arr, 4));
console.log(pickGreatest(arr, 5));

[ 52, 30, 22 ]
[ 52, 30, 22, 20 ]
[ 52, 30, 22, 20, 18 ]

Alternative: Using Math.max() with Loop

For smaller arrays or when you need to preserve original order of max values:

const arr = [3, 4, 12, 1, 0, 5, 22, 20, 18, 30, 52];

const getMaxN = (arr, n) => {
    if (n > arr.length) return [];
    
    const result = [];
    const tempArr = [...arr];
    
    for (let i = 0; i < n; i++) {
        const maxValue = Math.max(...tempArr);
        result.push(maxValue);
        const index = tempArr.indexOf(maxValue);
        tempArr.splice(index, 1);
    }
    
    return result;
};

console.log(getMaxN(arr, 3));

[ 52, 30, 22 ]

Performance Comparison

Edge Cases

const testArray = [1, 2, 3];

console.log(pickGreatest([], 2));      // Empty array
console.log(pickGreatest(testArray, 0)); // Zero elements
console.log(pickGreatest(testArray, 5)); // More than array length

[]
[]
[]

Conclusion

The sort-based approach is generally preferred for its simplicity and consistent O(n log n) performance. Use the Math.max() method when you need to maintain the original relative order of maximum values.