This is a simple diagram showcasing a simple overview of how it works:
The Implementation consists a server and a user, the server in this case being a firebase function node.js instance, but you could copy-paste the code into any nodejs runtime. At the moment, for simplicity reasons, I just implemented getting gas for a simple transaction (no data, so 21000 units of gas), but I made it easy to add custom gas numbers if you want.
To run it yourself you need to create a .env file in the ./backend_firebase directory, this is a sample:
INFURA_URL="https://rinkeby.infura.io/v3/YOUR_KEY"
YOUR_ADDRESS="0x3a4e6eD8B0F02BFBfaA3C6506Af2DB939eA5798c"
TOKEN_ADDRESS="0xA0b86991c6218b36c1d19D4a2e9Eb0cE3606eB48" # USDC token
PRIVATE_KEY="0x0"And in the file App.js you need to change variables YOUR_ADDRESS and TOKEN_ADDRESS, and to let the user connect with a wallet, add an Infura JSON-RPC Url in the getProvider.js file at the line 13.
Caution: This is still a prototype and it needs more safe-guards for production, so use it with caution!
The owner, in this case the person running the server and exposing this service to users, can benefit from arbitrage opportunities, for example buying eth for cheaper and then "selling" it as gas with an added premium.
Metamask, which is currently the biggest wallet i think, is not supported, because they disabled the ability to sign a transaction without sending it. The one wallet that I found was working is the Coinbase Wallet, but it only lets you sign a transaction if you have enough gas to pay for it (wtf?). Perhaps there are other wallets that have better support, you can create an Issue if you found one.
If you like my work, please consider buying me a coffee at mhaas.eth or 0x3a4e6eD8B0F02BFBfaA3C6506Af2DB939eA5798c. Thanks!