JavaScript Console

Felix Lange edited this page Dec 21, 2017 · 88 revisions

Ethereum implements a javascript runtime environment (JSRE) that can be used in either interactive (console) or non-interactive (script) mode.

Ethereum's Javascript console exposes the full web3 JavaScript Dapp API and the admin API.

Interactive use: the JSRE REPL Console

The ethereum CLI executable geth has a JavaScript console (a Read, Evaluate & Print Loop = REPL exposing the JSRE), which can be started with the console or attach subcommand. The console subcommands starts the geth node and then opens the console. The attach subcommand will not start the geth node but instead tries to open the console on a running geth instance.

$ geth console
$ geth attach

The attach node 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 ipc:/some/custom/path
$ geth attach
$ geth attach ws://

Note that by default the geth node doesn't start the http and weboscket service and not all functionality is provided over these interfaces due to 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 --verbosity 5 console 2>> /tmp/eth.log

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

$ geth console 2>> /dev/null


$ geth --verbosity 0 console

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

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

Non-interactive use: JSRE 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 --exec "eth.blockNumber" attach

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 --exec 'loadScript("/tmp/checkbalances.js")' attach
$ geth --jspath "/tmp" --exec 'loadScript("checkbalances.js")' attach

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

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


The go-ethereum JSRE 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.

Note that the other known limitation of Otto (namely the lack of timers) is taken care of. Ethereum JSRE implements both setTimeout and setInterval. In addition to this, the console provides admin.sleep(seconds) as well as a "blocktime sleep" method admin.sleepBlocks(number).

Since web3.js uses the bignumber.js library (MIT Expat Licence), it is also autoloded.


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".

Management APIs

Beside the official DApp API interface the go ethereum node has support for additional management API's. These API's are offered using JSON-RPC and follow the same conventions as used in the DApp API. The go ethereum package comes with a console client which has support for all additional API's.

The management API has its own wiki page.

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