`Synchronous requests are not supported.` in truffle 4.0

Question

The following code in truffle (v4.0-beta) is failing:

.then(() => web3.eth.sendTransaction(
        {from: accounts[0], to: swt.address, value: 1e10}))

Error: Synchronous requests are not supported.

Is there a way to send ETH using asynchronous transaction?

Answer 1

This is not a truffle issue. This is related to web3. web3 doesn't support synchronous request. You can use sendTransaction function implementing callback.

web3.eth.sendTransaction({from: accounts[0], to: swt.address, value: 1e10},function(err,txHash){
    if(err){
        console.log("Could not send transaction");
    }
    else{
        console.log("Transaction send successfully. Transaction Hash:",txHash );
    }
})

Answer 2

Truffle 4 in it's test framework requires that all functions are Promise. web3.eth functions are synchronous. So, to use them in the Truffle framework we need to wrap them:

// wraps web3 sync/callback function into a Promise
function web3AsynWrapper (web3Fun) {
  return function (arg) {
    return new Promise((resolve, reject) => {
      web3Fun(arg, (e, data) => e ? reject(e) : resolve(data))
    })
  }
}

exports.sendTransaction = web3AsynWrapper(web3.eth.sendTransaction)
exports.getBalance = web3AsynWrapper(web3.eth.getBalance)

The same issue exists when you are connecting using the IPC provider: Synchronous requests are not supported by the IPC provider