swap.js

function swapETHToDai() public payable returns(uint[] memory) {
    // static array: address[k] memory array;
    // The following is the dynamic array way of initialization
    address[] memory _paths = new address[](2);
    // Also, push() is for storage array.
    _paths[0] = WETHAddress;
    _paths[1] = DaiAddress;
    return uniswapV2Router01.swapExactETHForTokens{value: msg.value}(0, _paths, msg.sender, now + 120);
}

function swapDaiForETH(uint _DaiAmount) public returns(uint[] memory) {
    require(Dai.transferFrom(msg.sender, address(this), _DaiAmount));
    address[] memory _paths = new address[](2);
    _paths[0] = DaiAddress;
    _paths[1] = WETHAddress;
    Dai.approve(address(uniswapV2Router01), _DaiAmount);
    return uniswapV2Router01.swapExactTokensForETH(_DaiAmount, 0, _paths, msg.sender, now + 120);
}