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Bloc is a small command line tool that helps you build blockchain applications on the Ethereum network with the blockapps api. Bloc makes it effortless to: Compile and deploy smart contracts to the blockchain Automatically wire those contracts to the front-end, so you can bring the blockchain to the world!


Installation is currently done by cloning:

git clone

Enter the bloc directory:

cd bloc

Install bloc as a global package:

npm install -g

Generate a new blockchain app

bloc init

bloc init

bloc init builds a base structure for your blockchain app as well as sets some default parameters values for creating transactions. These can be edited in the config.yaml file in your app directory.

Now in your app directory run

bloc register

bloc register

bloc register registers your app with the Blockapps api. Now generate a new private key and fill it with test-ether

bloc genkey

bloc genkey

Compile your smart contracts

bloc compile -s

bloc compile

Upload a contract and scaffold (-s) your dApp

bloc upload <ContractName> -s

bloc upload



Usage: bloc <command> (options)

  init      start a new project
  compile   compile contracts in contract folder
  upload    upload contracts to blockchain
  genkey    generate a new private key and fill it at the faucet
  register  register your app with BlockApps

  -s, --scaffold  scaffold html / js / css from your contracts when compiling or

Structure of your dApps


Additional Resources

bloc uses blockapps-js, our simple library for interfacing with the blockchain. Smart contracts that are written in javascript-like language called Solidity. A good place to start playing around with Solidity is the online compiler.


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blockapps-js is a library that exposes a number of functions for interacting with the Blockchain via the BlockApps API. Currently it has strong support for compiling Solidity code, creating the resulting contract, and querying its variables or calling its functions through Javascript code.



npm install blockapps-js


Run the script in the source directory created by npm (e.g. "node_modules/blockapps-js/"). It will create a file called blockapps.js in the same directory that can be included in an HTML file as a script. The call require("blockapps-js") is made available by the script to encourage you to write your supporting Javascript files in the same style as any Node module.

BlockApps documentation

Documentation is available at Below is the API for this particular module.


All functionality is included in the blockapps-js module:

var blockapps = require('blockapps-js');
/* blockapps = {
 *   ethbase : { Account, Address, Int, Storage, Transaction, Units },
 *   routes,
 *   query,
 *   polling,
 *   setProfile,
 *   Solidity
 *   MultiTX
 * }

The various submodules of blockapps are described in detail below. Aside from Address and Int, all the public methods return promises (from the bluebird library).

Quick start

See the bulkQuery sample dApp in the examples/ directory for a complete, working example. Here are some snippets illustrating common operations.

Query an account's balance

var Account = require('blockapps-js').ethbase.Account;

// The "0x" prefix is optional for addresses
var address = "16ae8aaf39a18a3035c7bf71f14c507eda83d3e3"

Account(address).balance.then(function(balance) {
  // In here, "balance" is a big-integer you can manipulate directly.

Send ether between accounts

var ethbase = require('blockapps-js').ethbase
var Transaction = ethbase.Transaction;
var Int = ethbase.Int;
var ethValue = ethbase.Units.ethValue;

var addressTo = "16ae8aaf39a18a3035c7bf71f14c507eda83d3e3";
var privkeyFrom = "1dd885a423f4e212740f116afa66d40aafdbb3a381079150371801871d9ea281";

// This statement doesn't actually send a transaction; it just sets it up.
var valueTX = Transaction({"value" : ethValue(1).in("ether")});

valueTX.send(privkeyFrom, addressTo).then(function(txResult) {
  // txResult.message is either "Success!" or an error message
  // For this transaction, the error would be about insufficient balance.

Compile Solidity code

var Solidity = require('blockapps-js').Solidity

var code = "contract C { int x = -2; }"; // For instance

Solidity(code).then(function(solObj) {
  // solObj.vmCode is the compiled code.  You could submit it directly with
  // a Transaction, but there is a better way.

  // solObj.symTab has more information than you could possibly want about the
  // global variables and functions defined in the code.

  // is the name of the contrat, i.e. "C"

  // solObj.code is the code itself.
}).catch(function(err) {
  // err is the compiler error if the code is malformed.

Create a Solidity contract and read its state

var Solidity = require('blockapps-js').Solidity

var code = "contract C { int x = -2; }"; // For instance
var privkey = "1dd885a423f4e212740f116afa66d40aafdbb3a381079150371801871d9ea281";

Solidity(code).newContract(privkey, {"value": 100}).then(function(contract) {
  contract.account.balance.equals(100); // You shouldn't use == with big-integers
  contract.state.x == -2; // If you do use ==, the big-integer is downcast.

Call a Solidity method

var Solidity = require('blockapps-js').Solidity;
var Promise = require('bluebird'); // This is the promise library we use

var code = 'contract C {                        \n\
  uint knocks;                                  \n\
  function knock(uint times) returns (string) { \n\
    knocks += times;                            \n\
    if (times == 0) {                           \n\
      return "I couldn\'t hear that!";          \n\
    }                                           \n\
    else {                                      \n\
      return "Okay, okay!";                     \n\
    }                                           \n\
  }                                             \n\

var privkey = "1dd885a423f4e212740f116afa66d40aafdbb3a381079150371801871d9ea281";

Solidity(code).newContract(privkey).then(function(contract) {
  // This sets up a call to the code's "knock" method;
  // The account owned by this private key pays the execution fees.
  var knock = function(n) {
    return contract.state.knock(n).callFrom(privkey);
  };[0,1,2,3], knock).then(function(replies) {
    replies[0] == "I couldn't hear that!";
    replies[1] == "Okay, okay!";
    // etc.
  }).then(function() {
    contract.state.knocks == 6; 

Call many methods in a single message transaction

var Solidity = require('blockapps-js').Solidity;
var MultiTX = require('blockapps-js').MultiTX;
var Promise = require('bluebird'); // This is the promise library we use

var code = 'contract C {                        \n\
  uint knocks;                                  \n\
  function knock(uint times) returns (string) { \n\
    knocks += times;                            \n\
    if (times == 0) {                           \n\
      return "I couldn\'t hear that!";          \n\
    }                                           \n\
    else {                                      \n\
      return "Okay, okay!";                     \n\
    }                                           \n\
  }                                             \n\

var privkey = "1dd885a423f4e212740f116afa66d40aafdbb3a381079150371801871d9ea281";

Solidity(code).newContract(privkey).then(function(contract) {
  // This time, we don't actually call the Solidity method yet.
  // However, we should take care to specify gas limits individually.
  // If this limit seems never know.  But it should not
  // be so high that paying it in the middle of a VM run would
  // cause an out-of-gas exception.
  function knock(n) {
    return contract.state.knock(n).txParams({gasLimit: 100000});

  // This does all four calls in a single transaction, which saves a
  // lot of gas and time.
  .then(function(replies) {
    replies[0] == "I couldn't hear that!";
    replies[1] == "Okay, okay!";
    // etc.
  }).then(function() {
    contract.state.knocks == 6; 

API details

BlockApps profiles

The blockapps-js library is designed to connect to any BlockApps node. Depending on the node, different default parameters (particularly the polling parameters and gas prices) are appropriate. To handle this, the function blockapps.setProfile is provided with several default profiles. Its usage is:

setProfile(<profile name>, <optional version>)

where <profile name> is any of the keys of setProfile.profiles, currently one of:

  • "hacknet": connects to the sandbox with very permissive defaults.

  • "ethereum": connects to the live network with reasonable defaults given those of the official Ethereum clients.

The <optional version>, if present, must be of the form n.m, for example, 1.0, and indicates which version of the BlockApps routes is requested. Currently 1.0 is the only one.

The ethbase submodule

This component provides Javascript support for the basic concepts of Ethereum, independent of high-level languages or implementation features.

The following names are member functions of blockapps.ethbase:


The constructor for an abstraction of Ethereum's 32-byte words, which are implemented via the big-integer library. The constructor accepts numbers or Ints, 0x(hex) strings, decimal strings, or Buffers, but does not truncate to 32 bytes. Note that arithmetic must be performed with the .plus (etc.) methods rather than the arithmetic operators, which degrade big integers to 8-byte (floating-point) Javascript numbers.


The constructor for Ethereum "addresses" (20-byte words), which are implemented as the Buffer type. Its argument can be a number, an Int, a hex string, or another Buffer, all of which are truncated to 20 bytes.


This constructor accepts an argument convertible to Address and defines an object with three properties.

  • address: the account's constructing address.

  • nonce: the "nonce", or number of successful transactions sent from this account. The value of this property is a Promise resolving to the Int value of the nonce.

  • balance: the balance, in "wei", of the account. The value of this property is a Promise resolving to the Int value of the balance. Note that 1 ether is equal to 1e18 wei.


The constructor for the key-value storage associated with an address. It accepts an argument convertible to address and returns an object with the following methods:

  • getSubKey(key, start, size): fetches the size (number) bytes at storage key key (hex string) starting start (number) bytes in. It returns a Promise resolving to the Buffer of these bytes.

  • getKeyRange(start, itemsNum): fetches itemsNum (number) keys beginning at start (hex string) in a single contiguous Buffer. It returns a Promise resolving to this Buffer.


A constructor accepting hex strings, numbers, or Ints and encoding them into 32-byte Buffers. It throws an exception if the input is too long.


The constructor for Ethereum transactions. blockapps-js abstracts a transaction into two parts:

  • parameters: The argument to Transaction is an object with up to four members: the numbers value, gasPrice, and gasLimit, whose defaults are provided in ethbase.Transaction.defaults as respectively 0, 1, and 3141592; and the hex string or Buffer data. Optionally, this object may contain to as well, a value convertible to Address.

  • participants: A call to ethbase.Transaction returns an object with a method send taking two arguments, respectively a private key (hex string) and Address, denoting the sender and recipient of the transaction. The second argument is optional if to is passed as a parameter to ethbase.Transaction, and overrides it if present. Calling this function sends the transaction and returns a Promise resolving to the transaction result (see the "routes" section).


Provides some simple conversions between denominations of ether currency. The interface imitates the convert-units Node.js package. There are two main functions:

  • ethValue: called as ethValue(x).in(denom), produces a numerical-type result (actually a value from the bignumber.js package) equal to the number of wei in a value of x in denom. For example, ethValue(1).in("ether") is 1e18 wei. This numerical value can be converted to Int and is acceptable as a value parameter in a Transaction.

  • convertEth: this converts between two denominations, and is called like convertEth(x).from(denom1).to(denom2). In particular, ethValue(x).in(denom) is the same as convertEth(x).from(denom).to("wei"). It also accepts two arguments, as convertEth(n,d), which is mathematically equivalent to convertEth(n/d) (except that n and d are integral types).

The routes submodule

This submodule exports Javascript interfaces to the BlockApps web routes for querying the Ethereum "database". All of them return Promises, since they must perform an asychronous request. These requests are made to the BlockApps server and path at:

  • query.apiPrefix: by default, /eth/v1.0.
  • query.serverURI: by default,

Some of the routes (namely, faucet and submitTransaction) poll the server for their results, with the following parameters:

  • polling.pollEveryMS: by default, 500 (milliseconds)
  • polling.pollTimeoutMS: by default, 10000 (milliseconds)

The following "routes" are member functions of blockapps.routes:


Takes Solidity source code and returns a Promise resolving to an object {vmCode, symTab, name}, where vmCode is the compiled Ethereum VM opcodes, name is the name of the Solidity contract (currently, only code defining a single contract is supported), and symTab is an object containing storage layout and type information for all state variables and functions in the source. Normally you will not need to use this object.


Like solc, but returns only the symTab directly.


Takes an argument convertible to Address and supplies it with 1000 ether. This is available only on the BlockApps hacknet, for obvious reasons.


Queries the block database, returning a list of Ethereum blocks as Javascript objects. The following queries are allowed, and may be combined:

  • ntx : number of transactions in the block
  • number : block number; minnumber, maxnumber: range for number
  • gaslim : gas limit for the block; mingaslim, maxgaslim: range for gaslim
  • gasused : total gas used in the block; mingasused, maxgasused: range for gasused
  • diff : block difficulty (the basic mining parameter); mindiff, maxdiff: range for diff
  • txaddress: matches any block containing any transaction either from or to the given address.
  • coinbase: the address of the "coinbase"; i.e. the address mining the block.
  • address: matches any block in which the account at this address is present.
  • hash: the block hash


Returns the last n blocks in the database.


Like routes.block, but queries accounts. Its queries are:

  • balance, minbalance, maxbalance: queries the account balance
  • nonce, minnonce, maxnonce: queries the account nonce
  • address: the account address


A shortcut to routes.account({"address" : address}) returning a single Ethereum account object (not an ethcore.Account) rather than a list.


Like routes.block, but queries transactions. Its queries are:

  • from, to, address: matches transactions from, to, or either a particular address.
  • hash: the transaction hash.
  • gasprice, mingasprice, maxgasprice: the gas price of a transaction.
  • gaslimit, mingaslimit, maxgaslimit: the gas limit of a transaction.
  • value, minvalue, maxvalue: the value sent with the transaction.
  • blocknumber: the block number containing this transaction.


Returns a list of the last n ransactions received by the client operating the database.


This is the low-level interface for the ethcore.Transaction object. It accepts an object containing precisely the following fields, and returns a Promise resolving to "transaction result" object with fields summarizing the VM execution. The Transaction and Solidity objects (below) handle the most useful cases, so when using this route directly, the most important fact about the transaction result is that its presence indicates success.

  • nonce, gasPrice, gasLimit: numbers.
  • value: a number encoded in base 10.
  • codeOrData, from, to: hex strings, the latter two addresses.
  • r, s, v, hash: cryptographic signature of the other parts.


This takes the hash of a transaction and returns an object containing information about its processing. Notably, it contains the fields:

  • message: either "Success!" or an error
  • trace: the course of its EVM run
  • contractsCreated, contractsDeleted: comma-separated lists.


Like routes.block, but queries storage. It accepts the following queries:

  • key, minkey, maxkey: queries storage "keys", i.e. locations in memory. These are base-10 integer strings.
  • keystring, keyhex: alternative formats for key accepting UTF-8 strings or hex strings to denote the key. They do not have corresponding ranges.
  • value, minvalue, maxvalue: base-10 storage values.
  • valuestring: alternative format as a UTF-8 string.
  • address: limits the storage to a particular address. This is virtually required.


Gets all storage from address.

The Solidity submodule

The member blockapps.Solidity is the interface to the Solidity language, allowing source code to be transformed into Ethereum contracts and these contracts' states queried and methods invoked directly from Javascript.

Solidity constructor

Invoked as Solidity(code), it is effectively an interface to routes.solc, returning a Promise of an object with the following prototype:

  • code: the constructing code.
  • name: the Solidity contract name.
  • vmCode: the compiled bytecode.
  • symTab: the storage layout and type "symbol table" of functions and variables
  • newContract(privkey, [txParams]): submits a contract creation transaction for the vmCode with the optional parameters (subject to ethcore.Transaction.defaults) as well as the required parameter privkey. This returns the promise of a "contract object" described next.

Contract object

The contract object has as its prototype the Solidity object that created it, as well as the following properties:

  • account: the ethcore.Account object for its address
  • state: an object containing as properties every state variable and top-level function in the Solidity code. The value of state.varName is a Promise resolving to the value of that variable at the time the query is made, of the types given below. Mappings and functions have special syntax.

Attaching to an existing contract

Finally, it is possible to "attach" some metadata to a Solidity or contract object. This facilitates recording and reloading these objects between sessions without creating new Ethereum contracts or even recompiling.

Solidity.attach({code, name, vmCode, symTab[, address]}), given the metadata in the argument, creates either a Solidity or contract object with this data. More specifically:

  • If address is absent, a Solidity object is returned. This object is equivalent to Solidity(code) with the other properties set to the values in the argument; no check is performed that these values are actually correct. The only way you should use this is by the equivalent of Solidity.attach(JSON.stringify(solObj)), as it avoids recompilation.

  • If address is present, a contract object is returned. This is the same as performing Solidity(code).newContract(???), except that no private key is necessary and the resulting object's account member has address equal to address. No check is performed that this address actually exists or has the Solidity ABI indicated by the other parameters. It simply allows resuming work with a contract object previously created directly by newContract. (Note that JSON.stringify(contractObj) does not have the correct format to submit to Solidity.attach.)

State variables

Every Solidity type is given a corresponding Javascript (or Node.js) type. They are: - address: the ethcore.Address (i.e. Buffer) type, of length 20 bytes. - bool: the boolean type. - bytes and its variants: the Buffer type of any length - int, uint, and their variants: the ethcore.Int (i.e. big-integer) type - string: the string type - arrays: Javascript arrays of the corresponding type. Fixed and dynamic arrays are not distinguished in this representation. - enums: the type itself is represented via the enum library; each name/value is a value of this type. - structs: Javascript objects whose enumerable properties are the names of the fields of the struct, with values equal to the representations of the struct fields.


These are treated specially in two ways. First, naturally, a key must be supplied and the corresponding value returned. Second, a Solidity mapping has no global knowledge of its contents, and thus, the entire mapping cannot be retrieved with a single query. Therefore, a mapping variable accepts keys and returns promises of individual values, not the promise of an associative array (as might be expected from the description). Each state.mappingName is a function having argument and return value:

  • argument: The mapping key, provided as any value that represents (as above) or can be converted to the type of the mapping key (i.e. hex strings for Addresses).
  • return value: a Promise resolving to that value.


A Solidity function fName appears as state.fName in the corresponding contract object. This is actually a member function that takes the arguments of fName, either as:

  • A single object whose enumerable properties are the names of the Solidity function's arguments, and whose values (like those of mapping keys) are apropriate representations of the arguments to be passed. All arguments must be passed at once.

  • Multiple parameters corresponding to the arguments of the Solidity function. This is chiefly useful for functions with anonymous or positionally meaningful parameters. Again, all arguments must be passed at once.

The return value of this function has two methods:

  • txParams(params): takes optional transaction parameters {value, gasPrice, gasLimit}. Returns the same object, now with these parameters remembered.

  • callFrom(privkey): calls the function from the account with the given private key. Its return value is a Promise of the return value of the Solidity function, if any.

Thus, one calls a solidity function as

contractObj.state.fName(args|arg1, arg2, ..)[   .txParams(params)].callFrom(privkey);

The MultiTX submodule

The member function blockapps.MultiTX makes use of a contract (currently only available on that sequentially executes a list of transactions in a single message call.


This function has several advantages over sending transactions individually in series:

  • The overhead for a single Ethereum message call is 21,000 gas before the VM even begins execution. This cost is not incurred for CALL opcodes within an existing execution environment, though, so enormous gas savings are possible if several transactions are rendered as CALLs in a single message-call transaction for which the up-front cost is only paid once.

    These savings are not realized for contract-creation transactions, since the CREATE opcode is even more expensive (32,000 gas). However, the following benefit may compensate even for this.

  • A valid transaction must contain the current nonce of the sender, and if successful, increments that nonce. Thus, each transaction in a sequence must wait for confirmation of the success of the previous one before it can be sent with any hope of acceptance. The MultiTX facility, however, only sends one transaction, and therefore only needs to query the nonce once, so there is no delay in executing the latter members of the sequence.

  • Similarly, if one wishes to make a series of related transactions each depending on the outcome of the earlier ones, then they have to be sent in strict sequence. MultiTX respects the sequencing of its arguments, but compresses the time frame for execution.

Unsent transactions

The MultiTX function takes one argument, a Javascript array of "unsent transactions". An unsent transaction can be either:

  • The result of a call to ethbase.Transaction({data, value, gasLimit, to}); gasPrice, if present, is ignored. Though gasLimit is technically optional, it is recommended to include it in each case, because as the calls are made, these limits are deducted in advance, and one must take care not to run out of gas.

  • The result of a call to contract.state.method(args), possibly with a subsequent txParams call, where contract is any Solidity contract object as described previously.


The complete syntax of a call to MultiTX is:

MultiTX([<unsent-tx-1>, <unsent-tx-2> ..])
.txParams({"gasPrice": <price in wei>, "gasLimit": <limit in gas units>})
.multiSend(<private key of sender>);

Unlike for the Transaction and Solidity modules, in the (optional) txParams subcall, only the parameters gasPrice and gasLimit are respected. Each transaction <unsent-tx-n> is run with the gasLimit given in its construction, and the gas limit provided in txParams (or the default gas limit for a Transaction, if not present) is used for the overall MultiTX call. Since only one gas price can be set for a single VM run, the overall gasPrice applies to all the transactions.

Since a single transaction may have only one originator, it is not possible to send a MultiTX from several accounts.

The value sent with MultiTX is automatically computed from the values given in the construction of the <unsent-tx-n>s; there may be a fee in addition (at the moment, it is 0x400 wei), which is automatically added on. It is still possible for a transaction to fail if the value you gave it is rejected by the contract on-blockchain.

Return value

A call to MultiTX returns the Promise of a list of return values, one for each transaction. If the return type is unknown (as for a bare unsent Transaction) or void (as for a Solidity function with no return value) then the corresponding entry in the list is null; otherwise, it is the same as what would be returned from a single Solidity method call. If a constituent transaction failed for some reason, then its return value is undefined.