 # Lucky Numbers Learning Exercise

## Instructions

Your friend Kojo is a big fan of numbers. He has a small website for playing around with numbers: www.fun-with-numbers.com. Kojo is not that good at programming so he asked you for help.

You will build two helper functions for new number games on Kojos' website and a third one to validate some input the user can enter.

### 1. Calculate the sum for the numbers on the slot machine

One of the games on Kojos' website looks like a slot machine that shows two groups of wheels with digits on them. Each group of digits represents a number. For the game mechanics, Kojo needs to know the sum of those two numbers.

Write a function `twoSum` that accepts two arrays as parameters. Each array consists of one or more numbers between 0 and 9. The function should interpret each array as a number and return the sum of those two numbers.

``````twoSum([1, 2, 3], [0, 7]);
//=> 130

// [1, 2, 3] represents 123 and [0, 7] represents 7.
// 123 + 7 = 130
``````

### 2. Determine if a number is a palindrome

Another game on the website is a little quiz called "Lucky Numbers". A user can enter a number and then sees whether the number belongs to some secret sequence or pattern. The sequence or pattern of the "lucky numbers" changes each month and each user only has a limited number of tries to guess it.

This months' lucky numbers should be numbers that are palindromes. Palindromic numbers remain the same when the digits are reversed.

Implement the new `luckyNumber` function that accepts a number as parameter. The function should return `true` if the number is a palindrome and `false` otherwise. The input number will always be a positive integer.

``````luckyNumber(1441);
//=>  true

luckyNumber(123);
//=> false
``````

### 3. Generate an error message for invalid user input

In various places on the website, there are input fields where the users can enter numbers and click a button to trigger some action. Kojo wants to improve the user experience of his site. He wants to show an error message in case the user clicks the button but the field contains an invalid input value.

Here is some more information on how the value of an input field is provided.

• If the user types something into a field, the associated value is always a string even if the user only typed in numbers.
• If the user types something but deletes it again, the variable will be an empty string.
• Before the user even started typing, the variable can be `undefined` or `null`.

Write a function `errorMessage` that accepts the user input as parameter. If the user did not provide any input, `errorMessage` should return `'Required field'`. If the input does not represent a non-zero number (according to the JavaScript conversion rules), `'Must be a number besides 0'` should be returned. In all other cases you can assume the input is valid, the return value should be an empty string.

``````errorMessage('123');
// => ''

errorMessage('');
// => 'Required field'

errorMessage('abc');
// => 'Must be a number besides 0'
``````
Last updated 1 December 2021 Edit via GitHub    