Codility - Lesson 3: Time Complexity - PermMissingElem (https://codility.com/programmers/task/perm_missing_elem/)

perm-missing-elem.js

function solution(A) {
	A.push(0)
	return A.reduce(function(missing, value, i) {
		return missing + i + 1 - value
	}, 0)
}

var A = []
A[0] = 2
A[1] = 3
A[2] = 1
A[3] = 5

var test = solution(A)
console.log(test, test === 4)