JavaScript - Find the smallest missing positive integer from an unsorted integer array

readme.txt

This problem was asked by Stripe.

Given an array of integers, find the first missing positive integer in linear time and constant space. 
In other words, find the lowest positive integer that does not exist in the array. The array can contain duplicates and 
negative numbers as well.

For example, the input [3, 4, -1, 1] should give 2. The input [1, 2, 0] should give 3.

You can modify the input array in-place.



Source: Daily Coding Problem (dailycodingproblem.com)

index.js

const test1 = [1, 2, 0] // Output: 3
const test2 = [3, 4, -1, 1] // Output: 2
const test3 = [7,8,9,11,12] // Output: 1

const firstMissingPositive = (inputArray) => {
  // Re-order array lowest to highest (mutation OK)
  inputArray.sort((a, b) => a - b)
  
  // Remove negative numbers
  const cleanArray = inputArray.filter((n) => n > 0)
  
  // Check for an index where array[i] != i + 1, if it exists than the answer is i + 1
  for (i = 0; i < cleanArray.length; i++) {    
    if (cleanArray[i] != i + 1) {
      return i + 1
    }
  }
  
  // Otherwise answer is array length + 1
  return cleanArray.length + 1
}

const result = firstMissingPositive(test1)
document.write("The lowest possible number is: " + result)