Web3 JS

For Dapp call a peer/node. There are libraries for JS, Java, .Net & Python, etc.

https://github.com/ethereum/wiki/wiki/JavaScript-API#web3netlistening

Web3JS exposes objects to group functions:

• eth - Ethereum blockchain related
• net - Node's network status
• personal - accountbut even BigNumber cannot handle more than 20 floating points
• db - et/put for local LevelDB
• shh - p2p messaging with Whisper

Asynchronous calls : Error-First Callback pattern

• callback function receive two results: error, result
• callback function first check if error is defined, otherwise process result according to API
• callback is always the last parameter of call (convention)

Big Numbers :

• JS cannot handle big numbers correctly
• web3JS uses the BigNumber library
• but even BigNumber cannot handle more than 20 floating points
• Solution: manage balance in Wei not in ETH
• Using
  • bigNumberValue.plus .minus .times .div ...
  • however there's no isEqualTo, only "equals"!!! (why? I don't know)

Notes

Will not repeat the document (which is changing anyway), but what's more:

Start up and connect:

• web3 may already be injected by MetaMask - just check
  • if (typeof web3 !== 'undefined')
  • the injected web3 object may be different....

Contract

• Transaction Hash is created IMMEDIATELY after transaction is created locally (not mined)
• Contract Instance Address is created after transaction is mined
• MyContractObj.new(.....) the callback will be called twice
• in Remix, the compiled contract's detail contain copy/paste ready web3 deploy code for the contract.
• call contract methods
  • TheCodedContractMethod.call(...)
    • executed locally on the node
    • return method value
    • no state change of contract
    • 0 cost
  • TheCodedContractMethod.sendTransaction(...)
    • executed by mining
    • return transaction hash
    • state changes
    • gas
• can also use sendTransaction API to deploy or call contract method
  • use "getData" to get data for the transaction
  • then use "call" or "sendTransaction" to call
  • use " web3.eth.getTransactionReceipt() " to get deployed contract address

Events and log

• "watch" - listening for incoming events
• "get" - access the log data
• to ways to watch & get
  • Filter API
    • allows watching events and reading logs from multiple contract instances
      • create filter object with selection criteria
        • "latest" - block hash of latest block
        • "pending" - transaction hash of latest transaction
        • options_object
          • block range: "fromBlock" -> "toBlock" (optional, default "latest")
          • contract instance : address : [addr1, addr2...]
          • event data
            • data in the log fields topics : ['0x.....event-signature','data1','data2','data3']
            • fields must be declared INDEXED used in topics (in event definition)
            • maximum 3 indexed fields & order is important
      • watch() / stopWatching()
        • call back function
        • result: array of events
      • get()
  • Contract instance
    • watch and read for specific contract instance

Quick Ref