luckyNumber.js

/*
Given an array of integers arr, a lucky integer is an integer which has a frequency in the array equal to its value.

Return a lucky integer in the array. If there are multiple lucky integers return the largest of them. If there is no lucky integer return -1.

 

Example 1:

Input: arr = [2,2,3,4]
Output: 2
Explanation: The only lucky number in the array is 2 because frequency[2] == 2.
Example 2:

Input: arr = [1,2,2,3,3,3]
Output: 3
Explanation: 1, 2 and 3 are all lucky numbers, return the largest of them.
Example 3:

Input: arr = [2,2,2,3,3]
Output: -1
Explanation: There are no lucky numbers in the array.
Example 4:

Input: arr = [5]
Output: -1
Example 5:

Input: arr = [7,7,7,7,7,7,7]
Output: 7
*/

//Answer//

/**
 * @param {number[]} arr
 * @return {number}
 */
var findLucky = function(arr) {
return Math.max(...arr.filter((x,_,a)=>a.filter(y=>y===x).length===x))===-Infinity?-1:Math.max(...arr.filter((x,_,a)=>a.filter(y=>y===x).length===x))
};