Validates a Sudoku game. The input (vals) is assumed to be an array of arrays with numerical values from 1 through 9.

validoku.js

/**
 * Validates a Sudoku game.
 * The input <vals> is assumed to be an array of arrays with numerical values from 1 through 9.
 */
function validoku(vals) {
	var val,
		x, y, blockX,
		uniqRow = {},
		uniqCols = [{}, {}, {}, {}, {}, {}, {}, {}, {}], // this construct could be "built up" on each loop iteration, but it's done this way for clarity of example
		uniqBlocks = [{}, {}, {}];

	// iterate through the rows
	for (y = 0; y < vals.length; ++y) {
		// reset
		if (y % 3 === 0) {
			uniqBlocks = [{}, {}, {}];
		}

		// iterate through the columns
		for (x = 0; x < vals[y].length; ++x) {
			val = vals[y][x];
			blockX = (x / 3) | 0; // bitwise operation to ensure integer - equivalent to Math.floor()

			// check the validity of the row so far
			if (uniqRow[val] === true) {
				console.log(val + " is invalid for the row at (" + x + "," + y + ")");
				return false;
			}
			uniqRow[val] = true;

			// check the validity of the column so far
			if (uniqCols[x][val] === true) {
				console.log(val + " is invalid for the column at (" + x + "," + y + ")");
				return false;
			}
			uniqCols[x][val] = true;

			// check the validity of the current square
			if (uniqBlocks[blockX][val] === true) {
				console.log(val + " is invalid for the block at (" + x + "," + y + ")");
				return false;
			}
			uniqBlocks[blockX][val] = true;
		}

		// reset
		uniqRow = {};
	}

	return true;
}

Here are some sample inputs for the function

var valid = [
    [8,3,5,4,1,6,9,2,7],
    [2,9,6,8,5,7,4,3,1],
    [4,1,7,2,9,3,6,5,8],
    [5,6,9,1,3,4,7,8,2],
    [1,2,3,6,7,8,5,4,9],
    [7,4,8,5,2,9,1,6,3],
    [6,5,2,7,8,1,3,9,4],
    [9,8,1,3,4,5,2,7,6],
    [3,7,4,9,6,2,8,1,5]
];

var invalidRow = [
    [8,3,5,4,1,6,9,2,7],
    [2,9,6,8,5,7,4,8,1], // (7,1)
    [4,1,7,2,9,3,6,5,8],
    [5,6,9,1,3,4,7,8,2],
    [1,2,3,6,7,8,5,4,9],
    [7,4,8,5,2,9,1,6,3],
    [6,5,2,7,8,1,3,9,4],
    [9,8,1,3,4,5,2,7,6],
    [3,7,4,9,6,2,8,1,5]
];

var invalidCol = [
    [8,3,5,4,1,6,9,2,7],
    [2,9,6,8,5,7,4,3,1],
    [4,1,7,2,9,3,6,5,8],
    [5,1,9,1,3,4,7,8,2], // (1,3)
    [1,2,3,6,7,8,5,4,9],
    [7,4,8,5,2,9,1,6,3],
    [6,5,2,7,8,1,3,9,4],
    [9,8,1,3,4,5,2,7,6],
    [3,7,4,9,6,2,8,1,5]
];

var invalidBlock = [
    [8,3,5,4,1,6,9,2,7],
    [2,9,6,8,5,7,4,3,1],
    [4,1,7,2,9,3,6,5,8],
    [5,6,9,1,3,4,7,8,2],
    [9,2,3,6,7,8,5,4,1], // (0,4)
    [7,4,8,5,2,9,1,6,3],
    [6,5,2,7,8,1,3,9,4],
    [9,8,1,3,4,5,2,7,6],
    [3,7,4,9,6,2,8,1,5]
];

> validoku(valid)
  true

> validoku(invalidRow)
  8 is invalid for the row at (7,1)
  false

> validoku(invalidCol)
  1 is invalid for the column at (1,3)
  false

> validoku(invalidBlock);
  9 is invalid for the block at (0,4)
  false