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Published at Jun 05 2021
·
5 comments

Instructions

Test suite

Solution

An Armstrong number is a number that is the sum of its own digits each raised to the power of the number of digits.

For example:

- 9 is an Armstrong number, because
`9 = 9^1 = 9`

- 10 is
*not*an Armstrong number, because`10 != 1^2 + 0^2 = 1`

- 153 is an Armstrong number, because:
`153 = 1^3 + 5^3 + 3^3 = 1 + 125 + 27 = 153`

- 154 is
*not*an Armstrong number, because:`154 != 1^3 + 5^3 + 4^3 = 1 + 125 + 64 = 190`

Write some code to determine whether a number is an Armstrong number.

Go through the setup instructions for Javascript to install the necessary dependencies:

https://exercism.io/tracks/javascript/installation

Please `cd`

into exercise directory before running all below commands.

Install assignment dependencies:

```
$ npm install
```

Execute the tests with:

```
$ npm test
```

In the test suites all tests but the first have been skipped.

Once you get a test passing, you can enable the next one by changing `xtest`

to
`test`

.

Once you have a solution ready, you can submit it using:

```
exercism submit armstrong-numbers.js
```

It's possible to submit an incomplete solution so you can see how others have completed the exercise.

```
import { isArmstrongNumber } from './armstrong-numbers';
describe('Armstrong Numbers', () => {
test('Zero is an Armstrong number', () => {
expect(isArmstrongNumber(0)).toEqual(true);
});
xtest('Single digit numbers are Armstrong numbers', () => {
expect(isArmstrongNumber(5)).toEqual(true);
});
xtest('There are no 2 digit Armstrong numbers', () => {
expect(isArmstrongNumber(10)).toEqual(false);
});
xtest('Three digit number that is an Armstrong number', () => {
expect(isArmstrongNumber(153)).toEqual(true);
});
xtest('Three digit number that is not an Armstrong number', () => {
expect(isArmstrongNumber(100)).toEqual(false);
});
xtest('Four digit number that is an Armstrong number', () => {
expect(isArmstrongNumber(9474)).toEqual(true);
});
xtest('Four digit number that is not an Armstrong number', () => {
expect(isArmstrongNumber(9475)).toEqual(false);
});
xtest('Seven digit number that is an Armstrong number', () => {
expect(isArmstrongNumber(9926315)).toEqual(true);
});
xtest('Seven digit number that is not an Armstrong number', () => {
expect(isArmstrongNumber(9926314)).toEqual(false);
});
});
```

```
/**
* Check out all my solutions to the Exercism JavaScript track:
* github.com/slaymance/exercism/tree/main/javascript
*/
export const isArmstrongNumber = num => [...`${num}`]
.reduce((sum, digit, _, { length }) => sum + digit ** length, 0) === num;
```

A huge amount can be learned from reading other peopleâ€™s code. This is why we wanted to give exercism users the option of making their solutions public.

Here are some questions to help you reflect on this solution and learn the most from it.

- What compromises have been made?
- Are there new concepts here that you could read more about to improve your understanding?

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## Community comments

Brilliant use of reduce! I am trying to find the documentation for the use of

`{ length }`

array as the 4th parameter. I understand that it works, but curious what`length`

is applied to in this case, would appreciate any reference.@danchenkov Glad you like the solution! The fourth parameter to

`.reduce`

is always a reference to the source array upon which the reduce method is being called. In this case, I'm using object destructuring to get the`length`

property of the source array to aid in the calculation of Armstrong numbers. It's functionally equivalent to:@JakubBorowki917 It looks like your solution is working with one minor bug that causes the whole thing to break: the

`.reduce`

callback needs an initial value. If no initial value is provided to`.reduce`

then the first value in the array is used as the accumulator and the callback logic begins on the second element in the array.So in your solution, the first element of the array you create is never being raised to the length of the number passed into the function.

oh my god! you are very clever