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I'm building a dapp and would like to interact with an already existing contract.

Previously working with Truffle I was writing my own contract, compiling it and deploying it. This process generated a Contract Artifact that I could use with the truffle-contract library and make calls using web3.

How would I go about making calls to a pre existing contract that I never compiled and therefor don't have a Contract Artifact for?

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You would require ABI and the address at which the contract has been deployed.

var abi = <ABI of contract>;                                // Set contract ABI
var newContract = web3.eth.contract(abi);                   // Contract object
var contractInstance = newContract.at(<Contract Address>);  // instance of the contract

contractInstance.functionName.call();                       // Invoke any function of the contract
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In that case, you will have to deploy the contract using web3js and then you can call contract functions without the help of truffle.

Sometime back I also had the similar need of deploying the contract using web3, so I have a sample code for contract deployment using web3 in this gist, you may find worth looking at.

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