← Blog## JavaScript Katas: Missing values

## Intro 🌐

## Understanding the Exercise❗

### Today’s exercise

## Thinking about the Solution 💭

## Implementation ⛑

### Result

## Playground ⚽

## Next Part ➡️

## Further Reading 📖

## Questions ❔

August, 2020

Problem solving is an important skill, for your career and your life in general.

That’s why I take interesting katas of all levels, customize them and explain how to solve them.

First, we need to understand the exercise! If you don’t understand it, you can’t solve it!.

My personal method:

- Input: What do I put in?
- Output: What do I want to get out?

Today, another `7 kyu`

kata,
meaning we slightly increase the difficulty.

Source: Codewars

Write a function `missingValues`

, that accepts one parameter: `myArray`

.

Given an array of number, e.g. `[1, 1, 1, 2, 2, 3]`

,

find:

- the number
`x`

, that appears once, e.g.`3`

- the number
`y`

, that appears twice, e.g.`2`

and return the product `x`

* `x`

* `y`

, e.g. `18`

(=> `3 x 3 x 2`

).

Input: an array of numbers.

Output: a number.

I think I understand the exercise (= what I put into the function and what I want to get out of it).

Now, I need the specific steps to get from input to output.

I try to do this in small baby steps:

- Find the number that appears once,
`x`

- Find the number that appears twice,
`y`

- Return the product of
`x * x * y`

Example:

- Input:
`[1, 1, 1, 2, 2, 3]`

- Find the number that appears once,
`x`

:`3`

- Find the number that appears twice,
`y`

:`2`

- Return the product of
`x * x * y`

:`18`

(=>`3 x 3 x 2`

) - Output:
`18`

✅

```
function missingValues(myArray) {
// count amount of each number
const count = myArray.reduce(
(acc, cur) =>
acc.hasOwnProperty(cur)
? { ...acc, [cur]: acc[cur] + 1 }
: { ...acc, [cur]: 1 },
{}
);
// helper function to find the object key (= our number) that appears [amount] times
const appears = (amount) =>
Object.entries(count)
.filter(([key, value]) => value === amount)
.map((entry) => entry[0]);
return appears(1) * appears(1) * appears(2);
}
```

```
console.log([1, 1, 1, 2, 2, 3]);
// 18 ✅
console.log([6, 5, 4, 100, 6, 5, 4, 100, 6, 5, 4, 200]);
// 4000000 ✅
```

You can play around with the code here

Great work!

We learned how to use `reduce`

, `filter`

, `map`

, `hasOwnProperty`

, `Object.entries`

.

I hope you can use your new learnings to solve problems more easily!

Next time, we’ll solve another interesting kata. Stay tuned!

If I should solve a specific kata, shoot me a message here.

If you want to read my latest stuff, get in touch with me!

- How often do you do katas?
- Which implementation do you like more? Why?
- Any alternative solution?

Hi! I'm Michael 👋 I'm a Mentor & Educator & Senior Web Developer - I help you to reach your (career) goals.