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 granulairty. 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.providers.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 teh 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 hexidecimal 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 between ICAP and 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 truely 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 between ICAP and checksum addresses, see getAddress().