Sudoku Solver, recursion

Hi all, I am a bit stuck on the Sudoku solver recursion part. Recursion has always been a pain for me. I sort of get it, but not quite. Watched this video Python Sudoku Solver - Computerphile - YouTube, and I think I understand the concept, but when I try to do it, it does not work. So obviously I don’t.

In theory, I loop over all rows/cols. When I find an empty field I check if any value from 1-9 is allowed in that field, by running the 3 helpers. If it is allowed, I set that specific row/col value of the puzzle string to the current value.

Then I should run the solve() on the updated string. If I run into a dead-end I should return, reset the row/col back to “.” and continue?

What am I doing wrong here?

Your code so far
Here is my code:
https://repl.it/@Aigarsss/boilerplate-project-sudoku-solver

  solve(puzzleString) {

    let puzzleArray = puzzleString.split("");

    for (let i = 0; i < 9; i++){

      let row = i + 1;

      for (let x = 0; x < 9; x++){
        let currentPos = i * 9 + x
        let col = x + 1;

        if (puzzleArray[currentPos] == "."){

          // loop through numbers 1-9
          for (let y = 1; y <= 9; y++){
            //do all 3 checks on each number. checkRowPlacement, checkColPlacement, checkRegionPlacement
            let checkRow = this.checkRowPlacement(puzzleString, row, col, y);
            let checkCol = this.checkColPlacement(puzzleString, row, col, y);
            let checkRegion = this.checkRegionPlacement(puzzleString, row, col, y);

              if (checkRow && checkCol && checkRegion) {
                console.log(`got here on row ${row}, col ${col}, num ${y}`)
                puzzleArray[currentPos] = y.toString();
                this.solve(puzzleArray.join(""))
                puzzleArray[currentPos] = "."
              }

            // console.log(`Check for row ${row}, col ${col}, num ${y}:
            // checkRow = ${checkRow}
            // checkCol = ${checkCol}
            // checkRegion = ${checkRegion}`)
          }
          return
          }
      }
    }

    return puzzleArray.join("");

  }

Challenge: Sudoku Solver

Link to the challenge:

Here is the solution if anyone has this question. I modified the original code a bit, to make it a little simpler. Make sure all the helpers are doing what you expect them to do.

solve(puzzleString) {

    let nextBlankPosition = this.getNextEmptyCell(puzzleString);
    let blankRow = nextBlankPosition === -1 ? -1 : Math.floor(nextBlankPosition / 9) + 1;
    let blankCol = nextBlankPosition === -1 ? -1 : (nextBlankPosition % 9) + 1;

    // Base case
    if (blankCol === -1){
      return puzzleString;
    }

    let puzzleArray;

    for (let y = 1; y <= 9; y++){
      if (this.comboPlacementCheck(puzzleString, blankRow, blankCol, y)) {
        puzzleArray = puzzleString.split("");
        puzzleArray[nextBlankPosition] = y.toString();
        puzzleString = puzzleArray.join("");
        //this actually sets the puzzleString to the base case value
        puzzleString = this.solve(puzzleString)
      }

    }

    if (this.getNextEmptyCell(puzzleString) !== -1){
      puzzleArray = puzzleString.split("");
      puzzleArray[nextBlankPosition] = ".";
      puzzleString = puzzleArray.join("");
    }

    return puzzleString
  }