check if a piece can win a connect 4 game

connect4.js

const COLUMNS = 7;
const ROWS = 6;

const grid = [];
const height = [];  // next available row at column j

for (let j = 0; j < COLUMNS; j++) {
    height.push(0);
    for (let i = 0; i < ROWS; i++) {
        grid.push('-');
    }
}

const convertRowColToIdx = (row, col) => {
    if (row >= 0 && row < ROWS && col >= 0 && col < COLUMNS) {
        return row * COLUMNS + col;
    }
    return -1;
}

const canPlay = (column) => {
    if (column < 0 || column >= COLUMNS) {
        return false;
    }
    return height[column] < ROWS;
}

const play = (column, player) => {
    const idx = convertRowColToIdx(height[column], column);
    grid[idx] = player;
    height[column]++;
}

// console.log(grid);

console.log(canPlay(0));
console.log(canPlay(1));
console.log(canPlay(2));
console.log(canPlay(3));
console.log(canPlay(4));
console.log(canPlay(5));

grid[3] = grid[10] = grid[17] = grid[24] = grid[31] = grid[35] = '1';
console.log(canPlay(3));

const isWinningPiece = (col, player) => {
    if (!canPlay(col)) {
        return false;
    }
    // check vertical
    if (height[col] >= 3) {
        if (height[col - 1] === player || height[col - 2] === player || height[col - 3] === player) {
            return true;
        }
    }

    // check horizontal
    for (let x = 3; x >= 0; x--) {
        let pieces = 0;
        for (let delta = -3; delta <= 0; delta++) {
            if (delta !== x) {
                let colIdx = col + delta + x;
                if (colIdx >= 0 && colIdx < COLUMNS) {
                    let idx = convertRowColToIdx(height[col], colIdx);
                    if (grid[idx] === player) {
                        pieces += 1;
                    }    
                }            
            }
        }
        if (pieces === 3) {
            return true;
        }
    }

    // check left diagonal
    for (let x = 3; x >= 0; x--) {
        let pieces = 0;
        for (let delta = -3; delta <= 0; delta++) {
            if (delta !== x) {
                let colIdx = col + delta + x;
                let rowIdx = height[col] - (delta + x);                    
                if (colIdx >= 0 && colIdx < COLUMNS) {
                    let idx = convertRowColToIdx(height[col] - colIdx, colIdx);
                    if (grid[idx] === player) {
                        pieces += 1;
                    }    
                }            
            }
        }
        if (pieces === 3) {
            return true;
        }
    }

    // check right diagonal
    return false;
}