---
id: 5e601bf95ac9d0ecd8b94afd
title: Sudoku Solver
challengeType: 4
forumTopicId: 462357
dashedName: sudoku-solver
---

# --description--

Build a full stack JavaScript app that is functionally similar to this: <https://sudoku-solver.freecodecamp.rocks/>. Working on this project will involve you writing your code using one of the following methods:

-   Clone [this GitHub repo](https://github.com/freecodecamp/boilerplate-project-sudoku-solver) and complete your project locally.
-   Use [our Replit starter project](https://replit.com/github/freeCodeCamp/boilerplate-project-sudoku-solver) to complete your project.
-   Use a site builder of your choice to complete the project. Be sure to incorporate all the files from our GitHub repo.

When you are done, make sure a working demo of your project is hosted somewhere public. Then submit the URL to it in the `Solution Link` field. Optionally, also submit a link to your project's source code in the `GitHub Link` field.

# --instructions--

- All puzzle logic can go into `/controllers/sudoku-solver.js`
  - The `validate` function should take a given puzzle string and check it to see if it has 81 valid characters for the input.
  - The `check` functions should be validating against the *current* state of the board.
  - The `solve` function should handle solving any given valid puzzle string, not just the test inputs and solutions. You are expected to write out the logic to solve this.
- All routing logic can go into `/routes/api.js`
- See the `puzzle-strings.js` file in `/controllers` for some sample puzzles your application should solve
- To run the challenge tests on this page, set `NODE_ENV` to `test` without quotes in the `.env` file
- To run the tests in the console, use the command `npm run test`. To open the Replit console, press Ctrl+Shift+P (Cmd if on a Mac) and type "open shell"

Write the following tests in `tests/1_unit-tests.js`:

-   Logic handles a valid puzzle string of 81 characters
-   Logic handles a puzzle string with invalid characters (not 1-9 or `.`)
-   Logic handles a puzzle string that is not 81 characters in length
-   Logic handles a valid row placement
-   Logic handles an invalid row placement
-   Logic handles a valid column placement
-   Logic handles an invalid column placement
-   Logic handles a valid region (3x3 grid) placement
-   Logic handles an invalid region (3x3 grid) placement
-   Valid puzzle strings pass the solver
-   Invalid puzzle strings fail the solver
-   Solver returns the the expected solution for an incomplete puzzle

Write the following tests in `tests/2_functional-tests.js`

-   Solve a puzzle with valid puzzle string: POST request to `/api/solve`
-   Solve a puzzle with missing puzzle string: POST request to `/api/solve`
-   Solve a puzzle with invalid characters: POST request to `/api/solve`
-   Solve a puzzle with incorrect length: POST request to `/api/solve`
-   Solve a puzzle that cannot be solved: POST request to `/api/solve`
-   Check a puzzle placement with all fields: POST request to `/api/check`
-   Check a puzzle placement with single placement conflict: POST request to `/api/check`
-   Check a puzzle placement with multiple placement conflicts: POST request to `/api/check`
-   Check a puzzle placement with all placement conflicts: POST request to `/api/check`
-   Check a puzzle placement with missing required fields: POST request to `/api/check`
-   Check a puzzle placement with invalid characters: POST request to `/api/check`
-   Check a puzzle placement with incorrect length: POST request to `/api/check`
-   Check a puzzle placement with invalid placement coordinate: POST request to `/api/check`
-   Check a puzzle placement with invalid placement value: POST request to `/api/check`

# --hints--

You should provide your own project, not the example URL.

```js
(getUserInput) => {
  const url = getUserInput('url');
  assert(!/.*\/sudoku-solver\.freecodecamp\.rocks/.test(getUserInput('url')));
};
```

You can `POST` `/api/solve` with form data containing `puzzle` which will be a string containing a combination of numbers (1-9) and periods `.` to represent empty spaces. The returned object will contain a `solution` property with the solved puzzle.

```js
async (getUserInput) => {
  const input =
    '..9..5.1.85.4....2432......1...69.83.9.....6.62.71...9......1945....4.37.4.3..6..';
  const output =
    '769235418851496372432178956174569283395842761628713549283657194516924837947381625';
  const data = await fetch(getUserInput('url') + '/api/solve', {
    method: 'POST',
    headers: { 'Content-Type': 'application/json' },
    body: JSON.stringify({ puzzle: input })
  });
  const parsed = await data.json();
  assert.property(parsed, 'solution');
  assert.equal(parsed.solution, output);
};
```

If the object submitted to `/api/solve` is missing `puzzle`, the returned value will be `{ error: 'Required field missing' }`

```js
async (getUserInput) => {
  const input =
    '..9..5.1.85.4....2432......1...69.83.9.....6.62.71...9......1945....4.37.4.3..6..';
  const output = 'Required field missing';
  const data = await fetch(getUserInput('url') + '/api/solve', {
    method: 'POST',
    headers: { 'Content-Type': 'application/json' },
    body: JSON.stringify({ notpuzzle: input })
  });
  const parsed = await data.json();
  assert.property(parsed, 'error');
  assert.equal(parsed.error, output);
};
```

If the puzzle submitted to `/api/solve` contains values which are not numbers or periods, the returned value will be `{ error: 'Invalid characters in puzzle' }`

```js
async (getUserInput) => {
  const input =
    'AA9..5.1.85.4....2432......1...69.83.9.....6.62.71...9......1945....4.37.4.3..6..';
  const output = 'Invalid characters in puzzle';
  const data = await fetch(getUserInput('url') + '/api/solve', {
    method: 'POST',
    headers: { 'Content-Type': 'application/json' },
    body: JSON.stringify({ puzzle: input })
  });
  const parsed = await data.json();
  assert.property(parsed, 'error');
  assert.equal(parsed.error, output);
};
```

If the puzzle submitted to `/api/solve` is greater or less than 81 characters, the returned value will be `{ error: 'Expected puzzle to be 81 characters long' }`

```js
async (getUserInput) => {
  const input =
    '9..5.1.85.4....2432......1...69.83.9.....6.62.71...9......1945....4.37.4.3..6..';
  const output = 'Expected puzzle to be 81 characters long';
  const data = await fetch(getUserInput('url') + '/api/solve', {
    method: 'POST',
    headers: { 'Content-Type': 'application/json' },
    body: JSON.stringify({ puzzle: input })
  });
  const parsed = await data.json();
  assert.property(parsed, 'error');
  assert.equal(parsed.error, output);
};
```

If the puzzle submitted to `/api/solve` is invalid or cannot be solved, the returned value will be `{ error: 'Puzzle cannot be solved' }`

```js
async (getUserInput) => {
  const input =
    '9.9..5.1.85.4....2432......1...69.83.9.....6.62.71...9......1945....4.37.4.3..6..';
  const output = 'Puzzle cannot be solved';
  const data = await fetch(getUserInput('url') + '/api/solve', {
    method: 'POST',
    headers: { 'Content-Type': 'application/json' },
    body: JSON.stringify({ puzzle: input })
  });
  const parsed = await data.json();
  assert.property(parsed, 'error');
  assert.equal(parsed.error, output);
};
```

You can `POST` to `/api/check` an object containing `puzzle`, `coordinate`, and `value` where the `coordinate` is the letter A-I indicating the row, followed by a number 1-9 indicating the column, and `value` is a number from 1-9.

```js
async (getUserInput) => {
  const input =
    '..9..5.1.85.4....2432......1...69.83.9.....6.62.71...9......1945....4.37.4.3..6..';
  const coordinate = 'A1';
  const value = '7';
  const data = await fetch(getUserInput('url') + '/api/check', {
    method: 'POST',
    headers: { 'Content-Type': 'application/json' },
    body: JSON.stringify({ puzzle: input, coordinate, value })
  });
  const parsed = await data.json();
  assert.property(parsed, 'valid');
  assert.isTrue(parsed.valid);
};
```

The return value from the `POST` to `/api/check` will be an object containing a `valid` property, which is `true` if the number may be placed at the provided coordinate and `false` if the number may not. If false, the returned object will also contain a `conflict` property which is an array containing the strings `"row"`, `"column"`, and/or `"region"` depending on which makes the placement invalid.

```js
async (getUserInput) => {
  const input =
    '..9..5.1.85.4....2432......1...69.83.9.....6.62.71...9......1945....4.37.4.3..6..';
  const coordinate = 'A1';
  const value = '1';
  const conflict = ['row', 'column'];
  const data = await fetch(getUserInput('url') + '/api/check', {
    method: 'POST',
    headers: { 'Content-Type': 'application/json' },
    body: JSON.stringify({ puzzle: input, coordinate, value })
  });
  const parsed = await data.json();
  assert.property(parsed, 'valid');
  assert.isFalse(parsed.valid);
  assert.property(parsed, 'conflict');
  assert.include(parsed.conflict, 'row');
  assert.include(parsed.conflict, 'column');
};
```

If `value` submitted to `/api/check` is already placed in `puzzle` on that `coordinate`, the returned value will be an object containing a `valid` property with `true` if `value` is not conflicting.

```js
async (getUserInput) => {
  const input =
  '..9..5.1.85.4....2432......1...69.83.9.....6.62.71...9......1945....4.37.4.3..6..';
  const coordinate = 'C3';
  const value = '2';
  const data = await fetch(getUserInput('url') + '/api/check', {
    method: 'POST',
    headers: { 'Content-Type': 'application/json' },
    body: JSON.stringify({ puzzle: input, coordinate, value })
  });
  const parsed = await data.json();
  assert.property(parsed, 'valid');
  assert.isTrue(parsed.valid);
};
```

If the puzzle submitted to `/api/check` contains values which are not numbers or periods, the returned value will be `{ error: 'Invalid characters in puzzle' }`

```js
async (getUserInput) => {
  const input =
    'AA9..5.1.85.4....2432......1...69.83.9.....6.62.71...9......1945....4.37.4.3..6..';
  const coordinate = 'A1';
  const value = '1';
  const output = 'Invalid characters in puzzle';
  const data = await fetch(getUserInput('url') + '/api/check', {
    method: 'POST',
    headers: { 'Content-Type': 'application/json' },
    body: JSON.stringify({ puzzle: input, coordinate, value })
  });
  const parsed = await data.json();
  assert.property(parsed, 'error');
  assert.equal(parsed.error, output);
};
```

If the puzzle submitted to `/api/check` is greater or less than 81 characters, the returned value will be `{ error: 'Expected puzzle to be 81 characters long' }`

```js
async (getUserInput) => {
  const input =
    '9..5.1.85.4....2432......1...69.83.9.....6.62.71...9......1945....4.37.4.3..6..';
  const coordinate = 'A1';
  const value = '1';
  const output = 'Expected puzzle to be 81 characters long';
  const data = await fetch(getUserInput('url') + '/api/check', {
    method: 'POST',
    headers: { 'Content-Type': 'application/json' },
    body: JSON.stringify({ puzzle: input, coordinate, value })
  });
  const parsed = await data.json();
  assert.property(parsed, 'error');
  assert.equal(parsed.error, output);
};
```

If the object submitted to `/api/check` is missing `puzzle`, `coordinate` or `value`, the returned value will be `{ error: Required field(s) missing }`

```js
async (getUserInput) => {
  const input =
    '..9..5.1.85.4....2432......1...69.83.9.....6.62.71...9......1945....4.37.4.3..6..';
  const output = 'Required field(s) missing';
  const data = await fetch(getUserInput('url') + '/api/check', {
    method: 'POST',
    headers: { 'Content-Type': 'application/json' },
    body: JSON.stringify({ puzzle: input })
  });
  const parsed = await data.json();
  assert.property(parsed, 'error');
  assert.equal(parsed.error, output);
};
```

If the coordinate submitted to `api/check` does not point to an existing grid cell, the returned value will be `{ error: 'Invalid coordinate'}`

```js
async (getUserInput) => {
  const input =
    '..9..5.1.85.4....2432......1...69.83.9.....6.62.71...9......1945....4.37.4.3..6..';
  const output = 'Invalid coordinate';
  const coordinate = 'XZ18';
  const value = '7';
  const data = await fetch(getUserInput('url') + '/api/check', {
    method: 'POST',
    headers: { 'Content-Type': 'application/json' },
    body: JSON.stringify({ puzzle: input, coordinate, value })
  });
  const parsed = await data.json();
  assert.property(parsed, 'error');
  assert.equal(parsed.error, output);
};
```

If the `value` submitted to `/api/check` is not a number between 1 and 9, the returned values will be `{ error: 'Invalid value' }`

```js
async (getUserInput) => {
  const input =
    '..9..5.1.85.4....2432......1...69.83.9.....6.62.71...9......1945....4.37.4.3..6..';
  const output = 'Invalid value';
  const coordinate = 'A1';
  const value = 'X';
  const data = await fetch(getUserInput('url') + '/api/check', {
    method: 'POST',
    headers: { 'Content-Type': 'application/json' },
    body: JSON.stringify({ puzzle: input, coordinate, value })
  });
  const parsed = await data.json();
  assert.property(parsed, 'error');
  assert.equal(parsed.error, output);
};
```

All 12 unit tests are complete and passing. See `/tests/1_unit-tests.js` for the expected behavior you should write tests for.

```js
async (getUserInput) => {
  try {
    const getTests = await $.get(getUserInput('url') + '/_api/get-tests');
    assert.isArray(getTests);
    const units = getTests.filter((el) => el.context.includes('UnitTests'));
    assert.isAtLeast(units.length, 12, 'At least 12 tests passed');
    units.forEach((test) => {
      assert.equal(test.state, 'passed', 'Test in Passed State');
      assert.isAtLeast(
        test.assertions.length,
        1,
        'At least one assertion per test'
      );
    });
  } catch (err) {
    throw new Error(err.responseText || err.message);
  }
};
```

All 14 functional tests are complete and passing. See `/tests/2_functional-tests.js` for the functionality you should write tests for.

```js
async (getUserInput) => {
  try {
    const getTests = await $.get(getUserInput('url') + '/_api/get-tests');
    assert.isArray(getTests);
    const funcs = getTests.filter((el) =>
      el.context.includes('Functional Tests')
    );
    assert.isAtLeast(funcs.length, 14, 'At least 14 tests passed');
    funcs.forEach((test) => {
      assert.equal(test.state, 'passed', 'Test in Passed State');
      assert.isAtLeast(
        test.assertions.length,
        1,
        'At least one assertion per test'
      );
    });
  } catch (err) {
    throw new Error(err.responseText || err.message);
  }
};
```

# --solutions--

```js
/**
  Backend challenges don't need solutions,
  because they would need to be tested against a full working project.
  Please check our contributing guidelines to learn more.
*/
```