I am running this code on NodeJS application:

  let destinationBalanceWei = web3.eth.getBalance(process.env.DESTINATION_WALLET_ADDRESS).then(result => result.toString())
  let destinationBalance = web3.utils.fromWei(destinationBalanceWei, 'ether')

but I get following error in the console:

(node:79279) UnhandledPromiseRejectionWarning: Error: Please pass numbers as strings or BigNumber objects to avoid precision errors.
    at Object.fromWei (/Users/sahelanthropus/progetti/nodejs/node_modules/web3-utils/dist/web3-utils.cjs.js:440:11)
    at proveTransaction (/Users/sahelanthropus/progetti/nodejs/routes/index.js:49:39)
    at /Users/sahelanthropus/progetti/nodejs/routes/index.js:105:7
    at IncomingForm.<anonymous> (/Users/sahelanthropus/progetti/nodejs/node_modules/formidable/lib/incoming_form.js:107:9)
    at IncomingForm.emit (events.js:197:13)
    at IncomingForm._maybeEnd (/Users/sahelanthropus/progetti/nodejs/node_modules/formidable/lib/incoming_form.js:557:8)
    at /Users/sahelanthropus/progetti/nodejs/node_modules/formidable/lib/incoming_form.js:238:12
    at WriteStream.<anonymous> (/Users/sahelanthropus/progetti/nodejs/node_modules/formidable/lib/file.js:79:5)
    at Object.onceWrapper (events.js:285:13)
    at WriteStream.emit (events.js:197:13)
    at finishMaybe (_stream_writable.js:646:14)
    at stream._final (_stream_writable.js:624:5)
    at WriteStream._final (internal/fs/streams.js:270:3)
    at callFinal (_stream_writable.js:617:10)
    at processTicksAndRejections (internal/process/next_tick.js:76:17)
(node:79279) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 1)
(node:79279) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.

What I can do to solve it?

2 Answers 2


This is incorrect handling of asynchronous function call:

let destinationBalanceWei = web3.eth.getBalance(...).then(result => result.toString());
let destinationBalance = web3.utils.fromWei(destinationBalanceWei, 'ether');

The value of destinationBalanceWei is probably undefined when you pass it to fromWei.

You can do it like this:

let destinationBalance;
web3.eth.getBalance(...).then(function(destinationBalanceWei) {
    destinationBalance = web3.utils.fromWei(destinationBalanceWei, 'ether');

Where you will have to find a way to be sure that destinationBalance has been set.

Or you can do it like this:

async function func() {
    let destinationBalanceWei = await web3.eth.getBalance(...);
    let destinationBalance = web3.utils.fromWei(destinationBalanceWei, 'ether');
    return destinationBalance;

I personally find the latter more convenient, unless of course you have a good reason for which you must run getBalance without waiting for it complete (performance-related reason).

  • Can you update your solution to work in order to print destinationBalanceWei with console.log(...) please?
    – kitsune
    Feb 14, 2019 at 10:41
  • @kitsune: Done. Feb 14, 2019 at 10:53

The issue is that web3.eth.getBalance returns a Promise and you have to "wait" until it gets executed.

You can use ES2018 and encapsulate your code in an async function:

async function myFunc() {
  let destinationBalanceWei = await web3.eth.getBalance(process.env.DESTINATION_WALLET_ADDRESS);
  let destinationBalance = web3.utils.fromWei(destinationBalanceWei, 'ether');
  console.log("destinationBalance", destinationBalance);

Also, note that the web3.utils.fromWei method expects a string or a BigNumber object passed as the first parameter. destinationBalanceWei is already a string so we don't have to mind that in the scenario above.

  • No no no. The problem is that the promise is handled asynchronously, while the (meaningless) return value destinationBalanceWei is passed to web3.utils.fromWei. Feb 14, 2019 at 9:49
  • Whoops, I jumped straight to the error message. I updated my solution, thanks @goodvibration! Feb 14, 2019 at 9:54
  • Not working... cannot see the console.log(...) output.
    – kitsune
    Feb 14, 2019 at 10:40
  • 1
    is already a string - true in web3.js v.1.x, not true in web3.js v.0.x (where getBalance returns a BigNumber instance). Feb 14, 2019 at 11:01

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