A few quick notes about some of the less obvious aspects of interacting with Ethereum in JavaScript.

Why can’t I just use numbers?

The first problem many encounter when dealing with Ethereum is the concept of numbers. Most common currencies are broken down with very little granularity. For example, there are only 100 cents in a single dollar. However, there are 1018 wei in a single ether.

JavaScript uses IEEE 754 double-precision binary floating point numbers to represent numeric values. As a result, there are holes in the integer set after 9,007,199,254,740,991; which is problematic for Ethereum because that is only around 0.009 ether (in wei).

To demonstrate how this may be an issue in your code, consider:

> (Number.MAX_SAFE_INTEGER + 4 - 5) == (Number.MAX_SAFE_INTEGER - 1)

To remedy this, all numbers (which can be large) are stored and manipulated as Big Numbers.

The functions parseEther( etherString ) and formatEther( wei ) can be used to convert between string representations, which are displayed to or entered by the user and Big Number representations which can have mathematical operations handled safely.


A Promise in JavaScript is an object which simplifies many aspects of dealing with asynchronous functions.

It allows a pending result to be treated in many ways as if it has already been resolved.

The most useful operations you will need are:

Promise . all ( promises )
Returns a new promise that resolves once all the promises have resolved.
prototype . then ( onResolve, onReject )

Returns another Promise, which once the Promise was resolved, the onResolve function will be executed and if an error occurs, onReject will be called.

If onResolve returns a Promise, it will be inserted into the chain of the returned promise. If onResolve throws an Error, the returned Promise will reject.

Cleaning out an account
var ethers = require('ethers');
var targetAddress = "0x02F024e0882B310c6734703AB9066EdD3a10C6e0";

var privateKey = "0x0123456789012345678901234567890123456789012345678901234567890123";
var wallet = new ethers.Wallet(privateKey);

// Promises we are interested in
var provider = ethers.getDefaultProvider('ropsten');
var balancePromise = provider.getBalance(wallet.address);
var gasPricePromise = provider.getGasPrice();
var transactionCountPromise = provider.getTransactionCount(wallet.address);

var allPromises = Promise.all([

var sendPromise = allPromises.then(function(results) {
     // This function is ONLY called once ALL promises are fulfilled

     var gasPrice = results[0];
     var balance = results[1];
     var transactionCount = results[2];

     // Sending a transaction to an externally owned account (EOA) is 21000 gas)
     var txFeeInWei = gasPrice.mul(21000);

     // This will send the maximum amount (our balance minus the fee)
     var value = balance.sub(txFeeInWei);

     var transaction = {
         to: targetAddress,
         gasPrice: gasPrice,
         gasLimit: 21000,
         nonce: transactionCount,

         // The amount to send
         value: value,

         // Prevent replay attacks across networks
         chainId: provider.chainId,

     var signedTransaction = wallet.sign(transaction);

     // By returning a Promise, the sendPromise will resolve once the
     // transaction is sent
     return provider.sendTransaction(signedTransaction);

var minedPromise = sendPromise.then(function(transaction) {
    // This will be called once the transaction is sent

    // This promise will be resolve once the transaction has been mined.
    return provider.waitForTransaction(transaction);

minedPromise.then(function(transaction) {
    console.log("The transaction was mined: Block " + transaction.blockNumber);

// Promises can be re-used for their value; it will not make the external
// call again, and will provide the exact same result every time.
balancePromise.then(function(balance) {
    // This *may* return before the above allPromises, since it only
    // required one external call. Keep in mind asynchronous calls can
    // be called out of order.

Checksum Address

A checksum address uses mixed case hexadecimal strings to encode the checksum information in the capitalization of the alphabetic characters, while remaining backwards compatible with non-checksum addresses.


// Valid; checksum (mixed case)

// Valid; NO checksum (no mixed case)

// INVALID; (mixed case, but case differs from first example)

To convert to a checksum addresses, see getAddress().

ICAP Address

The original method of adding a checksum to an Ethereum address was by using the a format compatible with IBAN addresses, using the country code XE. However, only addresses which have 0 as the first byte (i.e. the address begins with 0x00) are truly compatible with IBAN, so ICAP extends the definition to allow for 31 alphanumeric characters (instead of the standard 30).

An ICAP address has the following format:

XE [2 digit checksum] [up to 31 alphanumeric characters]

To convert to an ICAP addresses, see getIcapAddress().

Supported Platforms

The ethers.js library aims to be as inclusive as possible. People often ask, “why don’t you use feature X or syntax Y”, to which the response is usually that it begins to heavily restricts the potential user-base.

The current target for ethers.js is to support an environment which is close to ES3, with the addition of Object.defineProperty, which is a bit more advanced than an old ES3 environment, but which adds considerable safety and security to the library.

The phantomjs test harness (npm run test-phantomjs) has a handful of shims included in the tests/test.html, but serves as a good benchmark for what minimum features as supported.

Currently the Test Suite runs against:

  • node 6
  • node 8
  • node 10
  • phantomjs

Another supported aspect is the use of paths in require. A small part of the library may be included, for example, keccak256, by using:

var keccak256 = require('ethers/utils/keccak256').keccak256;

Which means renaming files is a breaking change, and may only be done between major version releases. This is useful for people using older, pre-ES6 tree-shaking, to keep their package sizes small.

Now that the library also supports TypeScript, another question that often comes up is (for example) “why are you doing runtime checks that a value is a number, the TypeScript compiler checks that for you”. It is important to keep in mind that TypeScript, while a useful tool, is not the tool that everyone uses, and so for anyone using JavaScript sans TypeScript, the library should guarantee safety and correctness for them too and fail early and fail loud if anything is out of the ordinary.


I fully welcome anyone to contribute to the project, and appreciate all the help I can get. That said, if you have ideas for a PR, please discuss them as an issue on GitHub first.

A few notes on contributing.

  • Please read the above section on Supported Platforms.
  • An important feature of ethers.js is that it is small, which means uncommon features or large features need a great deal of discussion.
  • Since ethers.js is designed to be extensible, there are often ways to add optional packages; for example, look at the BIP39 mnemonic wordlists, which are not bundled into the browser version, but are designed to be seamlessly loaded into the browser if their functionality is required.
  • Dependencies; part A) in line with the above, “keep things small”, adding a dependency is a big deal, as they often bring many other packages with them. A great deal of effort has been used to tune the build process and dependency list to keep things tight
  • Dependencies; part B) adding additional third party libraries, adds a huge attack vector fun malicious code or unexpected consequences, so adding a dependency is certainly something that needs to be very convincingly argued.
  • Dependencies; part C) part B applies to dev dependencies too. A devDependency can inject or otherwise do strange things and increases the attack vector for bugs and malicious code
  • Changing filenames or breaking backwards compatibility is a no-go for minor version changes
  • Major version changes do not happen often. We place @TODO in the source code for things that will be updated at the next version change.
  • Please use the GitHub issue system to make requests, or discuss changes you would like to make.
  • Testing is a must. It should generally take you longer to write test cases than it does the actual code.
  • All test cases must pass on all platforms supported on Travis CI.


A lot of people store a lot of value in Ethereum and the code that runs it. As such, security is important.

The GitHub and NPM Package

The keys used to sign code on GitHub are well protected, but anyones computer can be compromised.

All services involved have two-factor authentication set up, but please keep in mind that bleeding-edge technology should probably not be used in production environments.

Keep in mind, however, that at the end of the day, if NPM were hacked, anything in the system could be replaced.

By using a version that is perhaps a few weeks old, providing there are no advisories otherwise, there has been adequate time for any compromise to have been broadcast.

Also, one of the test cases verifies the deterministic build on Travis CI. Never install a version which has failed the Continuous Integration tests on Travis CI.

Long story short, be careful.

In the event of any significant issue, it will be posted on the file, have an issue posted, with ALL CAPS in the title and will be broadcast on the @ethersproject twitter.

Memory Hard Brute-Force Encrpyting

A topic that often comes up is the poor performance of decrypting Wallet.

While it may not be immediately obvious, this is intentional for security purposes.

If it takes the legitimate user, who knows the password 5 seconds or so to unlock their account, that means that an attacker must spend 5 seconds per password attempt, so to guess a million passwords, requires 5 million seconds. Client software can streamline the process by using Secure Enclaves or other secure local places to store the decrypted wallet to improve the customer experience past the first decryption.

Responsible Disclosure

If you find a critical bug or security issue with ethers.js, please contact so that we can address it before you make it public. You will receive credit for the discovery after it is fixed and announced. :)