signERC2612Permit.js

/** 
* @file signERC2612Permit
*/
const MAX_INT = '0xffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff';

async function signERC2612Permit(web3, token, owner, spender, value, deadline, nonce) {
    const message = {
        owner,
        spender,
        value,
        nonce: nonce || await web3.contract('pair', token).methods.nonces(owner).call(),
        deadline: deadline || MAX_INT
    }
    
    const typedData = {
        types: {
            EIP712Domain: [
                { name: "name", type: "string" },
                { name: "version", type: "string" },
                { name: "chainId", type: "uint256" },
                { name: "verifyingContract", type: "address" },
            ],
            Permit: [
                { name: "owner", type: "address" },
                { name: "spender", type: "address" },
                { name: "value", type: "uint256" },
                { name: "nonce", type: "uint256" },
                { name: "deadline", type: "uint256" },
            ],
        },
        primaryType: "Permit",
        domain: {
            name: await web3.contract('erc20', token).methods.name().call(),
            version: '1',
            chainId: 1,
            verifyingContract: token
        },
        message: message
    };    

    return new Promise((resolutionFunc, rejectionFunc) => {
        web3.currentProvider.sendAsync({ method: "eth_signTypedData_v4", params: [owner, JSON.stringify(typedData)], from: owner }, function (error, result) {
            if (!error) {
                const signature = result.result.substring(2);
                const r = "0x" + signature.substring(0, 64);
                const s = "0x" + signature.substring(64, 128);
                const v = parseInt(signature.substring(128, 130), 16);
                resolutionFunc({ r, s, v, deadline: message.deadline });
            }
        });
    });
};