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Published at Jun 05 2021
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Instructions

Test suite

Solution

Given a string of digits, calculate the largest product for a contiguous substring of digits of length n.

For example, for the input `'1027839564'`

, the largest product for a
series of 3 digits is 270 (9 * 5 * 6), and the largest product for a
series of 5 digits is 7560 (7 * 8 * 3 * 9 * 5).

Note that these series are only required to occupy *adjacent positions*
in the input; the digits need not be *numerically consecutive*.

For the input `'73167176531330624919225119674426574742355349194934'`

,
the largest product for a series of 6 digits is 23520.

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 largest-series-product.js
```

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

A variation on Problem 8 at Project Euler http://projecteuler.net/problem=8

```
import { largestProduct } from './largest-series-product';
describe('Largest Series Product', () => {
test('finds the largest product if span equals length', () => {
expect(largestProduct('29', 2)).toEqual(18);
});
xtest('can find the largest product of 2 with numbers in order', () => {
expect(largestProduct('0123456789', 2)).toEqual(72);
});
xtest('can find the largest product of 2', () => {
expect(largestProduct('576802143', 2)).toEqual(48);
});
xtest('can find the largest product of 3 with numbers in order', () => {
expect(largestProduct('0123456789', 3)).toEqual(504);
});
xtest('can find the largest product of 3', () => {
expect(largestProduct('1027839564', 3)).toEqual(270);
});
xtest('can find the largest product of 5 with numbers in order', () => {
expect(largestProduct('0123456789', 5)).toEqual(15120);
});
xtest('can get the largest product of a big number', () => {
expect(
largestProduct('73167176531330624919225119674426574742355349194934', 6)
).toEqual(23520);
});
xtest('reports zero if the only digits are zero', () => {
expect(largestProduct('0000', 2)).toEqual(0);
});
xtest('reports zero if all spans include zero', () => {
expect(largestProduct('99099', 3)).toEqual(0);
});
xtest('rejects span longer than string length', () => {
expect(() => largestProduct('123', 4)).toThrow(
new Error('Span must be smaller than string length')
);
});
xtest('reports 1 for empty string and empty product (0 span)', () => {
expect(largestProduct('', 0)).toEqual(1);
});
xtest('reports 1 for nonempty string and empty product (0 span)', () => {
expect(largestProduct('123', 0)).toEqual(1);
});
xtest('rejects empty string and nonzero span', () => {
expect(() => largestProduct('', 1)).toThrow(
new Error('Span must be smaller than string length')
);
});
xtest('rejects invalid character in digits', () => {
expect(() => largestProduct('1234a5', 2)).toThrow(
new Error('Digits input must only contain digits')
);
});
xtest('rejects negative span', () => {
expect(() => largestProduct('12345', -1)).toThrow(
new Error('Span must be greater than zero')
);
});
});
```

```
/**
* Check out all my solutions to the Exercism JavaScript track:
* github.com/slaymance/exercism/tree/main/javascript
*/
// Helper functions for clarity
const containsNonDigits = string => /\D/.test(string);
const productBy = array => array.reduce((product, num) => product * num, 1);
/**
* These two functions are taken almost directly from my solution to Series exercise here on exercism
* https://exercism.io/tracks/javascript/exercises/series/solutions/40ab3d8550d345b580e1aa67a1fd08bf
*/
const pullDigits = series => [...series].map(Number);
const digitSpans = (digits, span) => Array.from(Array(digits.length - span + 1), (_, i) => digits.slice(i, i + span));
/**
* My solution uses the experimental pipeline operator which is a TC39 Stage 1 proposal to the JavaScript language. You
* can read more about it here:
* https://github.com/tc39/proposal-pipeline-operator
*
* To run this code you'll need to add the "@babel/plugin-proposal-pipeline-operator" to your dev dependencies and
* include the plugin in your babel.config file (the bigint plugin is already included in the project):
* plugins: ["@babel/plugin-syntax-bigint", ["@babel/plugin-proposal-pipeline-operator", { "proposal": "smart" }]]
*/
export const largestProduct = (series = '', span = 0) => {
if (span > series.length) throw new Error('Span must be smaller than string length');
if (span < 0) throw new Error('Span must be greater than zero');
if (containsNonDigits(series)) throw new Error('Digits input must only contain digits');
return series
|> pullDigits
|> digitSpans(#, span)
|> #.reduce((maxProduct, slice) => Math.max(maxProduct, productBy(slice)), 0);
};
```

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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