Hyperledger Composer is an application development framework as well as a set of powerful tools that make it easier to create blockchain applications and writing smart contracts that can eventually be executed in Hyperledger Fabric blockchain.
Composer makes deployment and testing of prototype business networks easier. It has its own runtime, that is a generic chaincode and it hosts the business network deployed - so your smart contract doesnt need chaincode in Go, only Javascript.
This architecture link should help you with further understanding:
https://www.slideshare.net/SimonStone8/hyperledger-composer-architecture