Mini Sudoku Game with 24 Levels – Free HTML, CSS & JS

Mini Sudoku is a 6x6 version of the classic number puzzle: every row, every column and every 3x2 box must contain the digits 1 to 6 exactly once. This version is a full game rather than a demo. It has 24 levels in four difficulty tiers, a Daily Puzzle with a streak, three lives, hints that explain themselves, pencil notes, undo, stars and saved progress. It is written in plain HTML, CSS and JavaScript, with no libraries, no images and no audio files.

A generator that guarantees one solution

Every puzzle starts from a complete grid built with randomised backtracking (fillGrid()). carve() then removes digits one at a time in random order. After each removal, countSolutions() runs a depth-first search that always branches on the cell with the fewest candidates and stops after finding two solutions. If the puzzle now has more than one solution, the digit is put back. The result is a puzzle with exactly one answer, so a player can always reason their way to it instead of guessing.

Difficulty from data, not guesses

The TIERS array holds four tiers (Warm-up, Sharp, Brain-burner, Grandmaster), each with a clue count and the hardest technique it may require. solvableBy() plays a puzzle using only naked singles (a cell with one candidate left) or also hidden singles (a digit that fits only one cell in a row, column or box). Warm-up levels must be solvable with naked singles alone, the middle tiers need hidden singles, and Grandmaster only guarantees a unique solution with as few as 11 clues. If a generated puzzle is too hard for its tier, the generator tries the next seed.

Seeded, so levels are the same for everyone

A small seeded random generator (mulberry32) drives the whole pipeline. Level number i always uses the seed 1000 + i * 7919, so level 7 is the same puzzle for every player and every visit. The Daily Puzzle uses a seed derived from the day number, so everyone gets the same puzzle on the same day, and clearing it on consecutive days builds a streak.

Hints that teach

useHint() looks at the board (ignoring wrong digits) and prefers a naked single, then a hidden single. It fills the cell and explains why in plain words: "R3C4 can only be 5: 1, 2, 3, 4 and 6 are already in its row, column or box", or "In row 2, only R2C5 can hold a 3". The player learns the technique instead of just seeing an answer, and hints reduce the star rating.

Lives, notes and undo

A digit that does not match the solution costs one of three lives, shakes the lives counter and turns the cell red. Pencil notes are stored as bit masks, one per cell, and a correct digit removes itself from the notes of every cell in its row, column and box. Undo restores a snapshot of the grid, the notes and the hint cells. Placing the last needed digit of a row, column or box sends a green wave across it.

Stars, saved progress and feel

A solved level earns up to three stars: one is lost for any mistake and one for using a hint. A level unlocks when the one before it has at least one star. Stars, best times, the streak and the daily result are saved to localStorage inside try/catch, so the game also runs in a sandboxed iframe. The board highlights the selected cell's row, column and box and every cell with the same digit, the number pad shows how many of each digit are left, and a synthesized sound and confetti mark a win. The timer pauses with P, when you open the level menu, and when the tab is hidden.