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# slaymance's solution

## to Armstrong Numbers in the JavaScript Track

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.

## Setup

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

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

## Requirements

Please `cd` into exercise directory before running all below commands.

Install assignment dependencies:

``````\$ npm install
``````

## Making the test suite pass

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`.

## Submitting Solutions

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

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

## Submitting Incomplete Solutions

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

## Exercise Source Credits

### armstrong-numbers.spec.js

``````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;``````

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.

Solution Author
commented 112 days ago

@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:

``````.reduce((sum, digit, _, srcArray) => sum + digit ** srcArray.length, 0)
``````
Solution Author
commented 109 days ago

@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

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