Connect Four is a game in which two players take turns dropping red or yellow colored discs into a vertically suspended 7 x 6 grid. Discs fall to the bottom of the grid, occupying the next available space. in this function we'll resolve if the game is done/won given array

connect-four-who-won-function.js

/**
array of game for example:

[['-','-','-','-','-','-','-'],
 ['-','-','-','-','-','-','-'],
 ['-','-','-','R','R','R','R'],
 ['-','-','-','Y','Y','R','Y'],
 ['-','-','-','Y','R','Y','Y'],
 ['-','-','Y','Y','R','R','R']];
 **/

function checkList(list = []) {
  return list.map((k,i) => (k === list[i+1] && k === list[i+2] && k === list[i+3]) && k !== '-' ? k : null).filter(k => k)[0] || null;
}

function connectFour(board) {
  let winner = null;
  let horizontalList = [];
  let diagonallyBottomRight = [];
  let diagonallyBottomLeft = [];
  let firstLineIsEmpty = board[0].filter(value => value === '-').length >= 7;
  let nextRow = [];
  let currentRow = []
  let horizontal = '';

  for(let a=0; a<=board.length;a++) {
    currentRow = board[a] || [];

    winner = checkList(currentRow);
    horizontalList = [];

    if(winner) {
      break;
    }
    
    for(let b=0;b<7;b++) {
      horizontal = board[b] ? board[b][a] : null;

      if(horizontal) {
        horizontalList.push(horizontal);
      }

      let cell = currentRow[b];
      
      diagonallyBottomRight = [];
      diagonallyBottomLeft = [];

      if(cell != '-') {
        
        for(let c=0;c<6;c++) {  
          nextRow = board[a+c] || null;

          if(!nextRow) {
            continue;
          }

          diagonallyBottomRight.push(nextRow[b+c]);
          diagonallyBottomLeft.push(nextRow[b-c]);
        }

        winner = checkList(diagonallyBottomLeft) || checkList(diagonallyBottomRight);

        if(winner) {
          break;
        }
      }
    }

    if(winner) {
      break;
    }

    winner = checkList(horizontalList);

    if(winner) {
      break;
    }
  }

  if(winner) {
    return winner;
  }

  if(!winner && firstLineIsEmpty) {
    return 'in progress';
  }

  return winner || 'draw';
}