Let's learn how to deploy a new contract, get a reference to a deployed contract using its address, send ether to a contract, send a transaction to invoke a contract method, and estimate the gas of a method call.

To deploy a new contract or to get a reference to an already deployed contract, you need to first create a contract object using the web3.eth.contract() method. It takes the contract ABI as an argument and returns the contract object.

Here is the code to create a contract object:

var proofContract = web3.eth.contract([{"constant":false,"inputs":[{"name":"fileHash","type":"string"}],"name":"get","outputs":[{"name":"timestamp","type":"uint256"},{"name":"owner","type":"string"}],"payable":false,"type":"function"},{"constant":false,"inputs":[{"name":"owner","type":"string"},{"name":"fileHash","type":"string"}],"name":"set","outputs":[],"payable":false,"type":"function"},{"anonymous":false,"inputs":[{"indexed":false,"name":"status","type":"bool"},{"indexed":false,"name":"timestamp","type":"uint256"},{"indexed":false,"name":"owner","type":"string"},{"indexed":false,"name":"fileHash","type":"string"}],"name":"logFileAddedStatus","type":"event"}]); 

Once you have the contract, you can deploy it using the new method of the contract object or get a reference to an already deployed contract that matches the ABI using the at method.

Let's take a look at an example of how to deploy a new contract:

var proof = proofContract.new({ 
     from: web3.eth.accounts[0],  
     data: "0x606060405261068...",  
     gas: "4700000" 
    },  
    function (e, contract){ 
    if(e) 
    { 
    console.log("Error " + e);     
}     
else if(contract.address != undefined) 
  {     
    console.log("Contract Address: " + contract.address);     
   }     
else      
  {     
    console.log("Txn Hash: " + contract.transactionHash)     
  } 
}) 

Here, the new method is called asynchronously, so the callback is fired twice if the transaction was created and broadcasted successfully. The first time, it's called after the transaction is broadcasted, and the second time, it's called after the transaction is mined. If you don't provide a callback, then the proof variable will have the address property set to undefined. Once the contract is mined, the address property will be set.

In the proof contract, there is no constructor, but if there is a constructor, then the arguments for the constructor should be placed at the beginning of the new method. The object we passed contains the from address, the byte code of the contract, and the maximum gas to use. These three properties must be present; otherwise, the transaction won't be created. This object can have the properties that are present in the object passed to the sendTransaction() method, but here, data is the contract byte code and the to property is ignored.

You can use the at method to get a reference to an already deployed contract. Here is the code to demonstrate this:

var proof = proofContract.at("0xd45e541ca2622386cd820d1d3be74a86531c14a1"); 

Now let's look at how to send a transaction to invoke a method of a contract. Here is an example to demonstrate this:

proof.set.sendTransaction("Owner Name", "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855", { 

from: web3.eth.accounts[0], 
}, function(error, transactionHash){ 

if (!err) 

console.log(transactionHash); 
}) 

Here, we call the sendTransaction method of the object of the method namesake. The object passed to this sendTransaction method has the same properties as web3.eth.sendTransaction(), except that the data and to properties are ignored.

If you want to invoke a method on the node itself instead of creating a transaction and broadcasting it, then you can use call instead of sendTransaction. Here is an example to demonstrate this:

var returnValue = proof.get.call("e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855"); 

Sometimes, it is necessary to find out the gas that would be required to invoke a method so that you can decide whether to invoke it or not. web3.eth.estimateGas can be used for this purpose. However, using web3.eth.estimateGas() directly requires you to generate the data of the transaction; therefore, we can use the estimateGas() method of the object of the method namesake. Here is an example to demonstrate this:

var estimatedGas = proof.get.estimateGas("e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855");