call-then-sendtx pattern for Ethereum Dapps

call-then-sendtx-pattern.js

/*
In Ethereum, a contract can be written so that it returns a value for eth_call.
A Dapp can then check for success or error value of eth_call, before calling eth_sendTransaction,
to take advantage of eth_call effectively being a "preview" of the code flow that the transaction
will take.  In traditional client-server, clients can't ask servers beforehand what's going to
happen when the client makes a call; with Dapps contracts can be written so that clients can ask
for a "preview" of what is going to happen, before any funds/ethers are actually utilized
(eth_call does not cost any ethers).

Note: it is possible that in between eth_call and when eth_sendTransaction is actually mined,
there could be state changes which means eth_sendTransaction will not behave according to the
"preview" behavior seen by eth_call.  Like most patterns, some discretion is needed about use.

Here's an example of the pattern:
*/


  var callResult = contractInstance.functionName.call(arg1, arg2, ...);
  if (callResult.toNumber() === <success code>) {
    console.log('eth_call succeeds so now sendTx...')
  }
  else {
    return;
  }
  contractInstance.functionName.sendTransaction(arg1, arg2, ...);