[Ethereum] Prompting a transaction to metamask

You're missing the last argument to sendTransaction, the callback function(error, transactionHash) { ... }

The MetaMask FAQ states:

All Async - Think of MetaMask as a light client

The user does not have the full blockchain on their machine and so data lookups can be a little slow. For this reason, we are unable to support most synchronous methods. The exception to this is:

eth_accounts (web3.eth.accounts)

eth_coinbase (web3.eth.coinbase)

Usually a method call can be made async by simply adding a callback as the last argument to a synchronous method.

See ethereum wiki on "using callbacks"

Not only is this a technical limitation, it's also a user experience issue. When you use synchronous calls, you block the user's interface, and so it's a generally bad practice anyways. Think of this API restriction as a gift to your users.

So all calls to web3.js should have a callback as their last argument: the only calls where that's not needed are for web3.eth.accounts and web3.eth.coinbase. The generic form of the callback is function(error, result).

I was looking for a solution for this too. I am using truffle and looked up their documentation. http://truffleframework.com/docs/getting_started/contracts

Contract.deployed().then(function(contractInstance){ 
  contractInstance.function({from: web3.eth.accounts[0]}).then(function(result){             
      alert('transaction success')}).catch(function(e){ 
      console.log('error')                                          
})