JavaScript Console

The Geth JavaScript console exposes the full web3 JavaScript Dapp API and further administrative APIs.

Interactive Use: The Console

The geth JavaScript console is started with the console or attach geth sub-commands. The console subcommands starts the geth node and then opens the console. The attach subcommand attaches to the console to an already-running geth instance.

geth console
geth attach

Attach mode accepts an endpoint in case the geth node is running with a non default ipc endpoint or you would like to connect over the rpc interface.

geth attach /some/custom/path.ipc
eth attach http://191.168.1.1:8545
geth attach ws://191.168.1.1:8546

Note that by default the geth node doesn’t start the HTTP and WebSocket servers and not all functionality is provided over these interfaces for security reasons. These defaults can be overridden when the --rpcapi and --wsapi arguments when the geth node is started, or with admin.startRPC and admin.startWS.

If you need log information, start with:

geth console --verbosity 5 2>> /tmp/eth.log

Otherwise mute your logs, so that it does not pollute your console:

geth console 2> /dev/null

Geth has support to load custom JavaScript files into the console through the --preload option. This can be used to load often used functions, or to setup web3 contract objects.

geth console --preload "/my/scripts/folder/utils.js,/my/scripts/folder/contracts.js"

Non-interactive Use: Script Mode

It’s also possible to execute files to the JavaScript interpreter. The console and attach subcommand accept the --exec argument which is a javascript statement.

geth attach --exec "eth.blockNumber"

This prints the current block number of a running geth instance.

Or execute a local script with more complex statements on a remote node over http:

geth attach http://geth.example.org:8545 --exec 'loadScript("/tmp/checkbalances.js")'
geth attach http://geth.example.org:8545 --jspath "/tmp" --exec 'loadScript("checkbalances.js")'

Use the --jspath <path/to/my/js/root> to set a library directory for your js scripts. Parameters to loadScript() with no absolute path will be understood relative to this directory.

You can exit the console by typing exit or simply with CTRL-C.

Caveats

go-ethereum uses the Otto JS VM which has some limitations:

  • "use strict" will parse, but does nothing.
  • The regular expression engine (re2/regexp) is not fully compatible with the ECMA5 specification.

web3.js uses the bignumber.js library. This library is auto-loaded into the console.

Timers

In addition to the full functionality of JS (as per ECMA5), the ethereum JSRE is augmented with various timers. It implements setInterval, clearInterval, setTimeout, clearTimeout you may be used to using in browser windows. It also provides implementation for admin.sleep(seconds) and a block based timer, admin.sleepBlocks(n) which sleeps till the number of new blocks added is equal to or greater than n, think “wait for n confirmations”.