Smart contracts in plain JavaScript.

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.20;

import "@openzeppelin/contracts/token/ERC20/IERC20.sol";
import "@chainlink/contracts/src/v0.8/AutomationCompatible.sol";

contract Subscriptions is AutomationCompatibleInterface {
    IERC20 public immutable token;
    address public immutable merchant;

    struct Sub { uint256 amount; uint256 nextCharge; }
    mapping(address => Sub) public subs;
    address[] public subscribers;
    mapping(address => bool) private known;

    event Subscribed(address indexed user, uint256 amount);
    event Charged(address indexed user, uint256 amount);

    constructor(IERC20 _token, address _merchant) {
        token = _token; merchant = _merchant;
    }

    function subscribe(uint256 amountPerWeek) external {
        subs[msg.sender] = Sub(amountPerWeek, block.timestamp + 7 days);
        if (!known[msg.sender]) {
            known[msg.sender] = true;
            subscribers.push(msg.sender);
        }
        emit Subscribed(msg.sender, amountPerWeek);
    }

    function checkUpkeep(bytes calldata) external view
        returns (bool upkeepNeeded, bytes memory)
    {
        for (uint i = 0; i < subscribers.length; i++) {
            if (block.timestamp >= subs[subscribers[i]].nextCharge)
                return (true, "");
        }
        return (false, "");
    }

    function performUpkeep(bytes calldata) external {
        for (uint i = 0; i < subscribers.length; i++) {
            address u = subscribers[i];
            Sub storage s = subs[u];
            if (block.timestamp >= s.nextCharge) {
                token.transferFrom(u, merchant, s.amount);
                s.nextCharge += 7 days;
                emit Charged(u, s.amount);
            }
        }
    }
}
// + register with Chainlink Automation (fund LINK)

// A subscription that fires itself.
// Customer signs once. Cron does the rest.

function subscribe(amountPerWeek) {
  storage.set('sub:' + msg.sender, {
    amount:     amountPerWeek,
    nextCharge: chain.timestamp + 7 * 86400,
  });
  cron.schedule(this.address, 'chargeAll', '@daily');
  emit('Subscribed', { user: msg.sender, amount: amountPerWeek });
}

// Auto-called every day by the chain itself.
function chargeAll() {
  for (const [addr, sub] of storage.entries('sub:')) {
    if (chain.timestamp >= sub.nextCharge) {
      E(AEV).transferFrom(addr, storage.merchant, sub.amount);
      sub.nextCharge += 7 * 86400;
    }
  }
}