---
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: <a href="https://sudoku-solver.freecodecamp.rocks/" target="_blank" rel="noopener noreferrer nofollow">https://sudoku-solver.freecodecamp.rocks/</a>. Working on this project will involve you writing your code using one of the following methods:

-   Clone <a href="https://github.com/freecodecamp/boilerplate-project-sudoku-solver" target="_blank" rel="noopener noreferrer nofollow">this GitHub repo</a> and complete your project locally.
-   Use <a href="https://replit.com/github/freeCodeCamp/boilerplate-project-sudoku-solver" target="_blank" rel="noopener noreferrer nofollow">our Replit starter project</a> 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.

If you use Replit, follow these steps to set up the project:

-   Start by importing the project on Replit.
-   Next, you will see a `.replit` window.
-   Select `Use run command` and click the `Done` button.

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 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 inputs = [
    '..9..5.1.85.4....2432......1...69.83.9.....6.62.71...9......1945....4.37.4.3..6.',
    '..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';
  for (const input of inputs) {
    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 inputs = [
    '..9..5.1.85.4....2432......1...69.83.9.....6.62.71...9......1945....4.37.4.3..6.',
    '..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';
  for (const input of inputs) {
    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 inputs = [
    {
      puzzle: '..9..5.1.85.4....2432......1...69.83.9.....6.62.71...9......1945....4.37.4.3..6..',
      value: '1',
    },
    {
      puzzle: '..9..5.1.85.4....2432......1...69.83.9.....6.62.71...9......1945....4.37.4.3..6..',
      coordinate: 'A1',
    },
    {
      coordinate: 'A1',
      value: '1'
    }
  ];
  for (const input of inputs) {
    const output = 'Required field(s) missing';
    const data = await fetch(getUserInput('url') + '/api/check', {
      method: 'POST',
      headers: { 'Content-Type': 'application/json' },
      body: JSON.stringify(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 coordinates = ['A0', 'A10', 'J1', 'A', '1', 'XZ18'];
  const value = '7';
  for (const coordinate of coordinates) {
    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 value 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 values = ['0', '10', 'A'];
  for (const value of values) {
    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.

```js
async (getUserInput) => {
  try {
    const getTests = await $.get(getUserInput('url') + '/_api/get-tests');
    assert.isArray(getTests);
    const unitTests = getTests.filter((test) => {
      return !!test.context.match(/Unit\s*Tests/gi);
    });
    assert.isAtLeast(unitTests.length, 12, 'At least 12 tests passed');
    unitTests.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.

```js
async (getUserInput) => {
  try {
    const getTests = await $.get(getUserInput('url') + '/_api/get-tests');
    assert.isArray(getTests);
    const functTests = getTests.filter((test) => {
      return !!test.context.match(/Functional\s*Tests/gi);
    });
    assert.isAtLeast(functTests.length, 14, 'At least 14 tests passed');
    functTests.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.
*/
```
