245
votes
Accepted
What is the difference between a transaction and a call?
Call
A call is a local invocation of a contract function that does not broadcast or publish anything on the blockchain.
It is a read-only operation and will not consume any Ether. It simulates what ...
eth♦
- 85k
86
votes
Accepted
How to get contract internal transactions
There's not currently any way to do this using the web3 API. Internal transactions, despite the name (which isn't part of the yellowpaper; it's a convention people have settled on) aren't actual ...
- 8,094
73
votes
Accepted
What are Solidity events and how they are related to topics and logs?
Topics are indexed parameters to an event.
topic[0] always refers to the hash of the hash of the event itself, and can have up to 3 indexed arguments, which will each be reflected in the topics.
EVM ...
- 21.7k
65
votes
Accepted
What are the limitations to estimateGas and when would its estimate be considerably wrong?
estimateGas works by pretending the transaction was actually being included in the blockchain, and then returning the exact gas amount that would have been charged if that pretend operation was real. ...
- 22k
57
votes
Accepted
How to detect if an address is a contract?
you can use getcode function.
if the address is representing an External Owned Account you will get 0x as response otherwise you will get the contract's bytecodeode.
for example :
web3.eth.getCode("...
- 18.7k
56
votes
Accepted
ecrecover from Geth and web3.eth.sign
I was stuck on this issue as well for a very long time.
So the solution is: Add this prefix string to your Solidity smart contract.
function verify(bytes32 hash, uint8 v, bytes32 r, bytes32 s) ...
- 676
53
votes
Accepted
Returning a struct and reading via Web3
Summary
Return the fields of the struct as separate return variables.
Edit: As of 2021, it's possible to return the struct directly. See this answer.
Example
I'm running this code in my local dev ...
- 40.1k
53
votes
Accepted
error: replacement transaction underpriced
I have been running into this issue as well. The network thinks you are trying to replace an existing unmined transaction. There are two ways to avoid this:
Adding a gas price %10 higher than the ...
- 670
51
votes
What is the difference between a transaction and a call?
The difference between in a call and a transaction is the following:
transactions are created by your client, signed and broadcasted to the network. They will eventually alter the state of the ...
- 32.8k
48
votes
How to get return values when a non view function is called?
It is not currently possible to return values from functions which modify the blockchain. To receive a return value, you can mark functions as "pure" or "view".
For state-changing functions, the only ...
- 9,560
46
votes
Accepted
How to detect if on mainnet or testnet?
Prior to Web 1.0 you can use web3.version.getNetwork as following:
web3.version.getNetwork((err, netId) => {
switch (netId) {
case "1":
console.log('This is mainnet')
break
...
- 18.7k
46
votes
Accepted
How to convert string to bytes32 in web3js?
For web3 0.x:
You need to use web3.fromAscii(val) to write correct bytes32 input to contract.
And you are right, anyone can read it with web3.toAscii(val).
Full web3.js code in terms of your ...
- 2,017
43
votes
Accepted
How to call my contract's function using sendTransaction
Posting solution to my own question for anyone who needs it.
To call a contract function from geth:
const contractAbi = eth.contract(AbiOfContract);
const myContract = contractAbi.at(contractAddress);
...
- 8,016
42
votes
How do I parse the transaction receipt log with web3.js?
Do this: You'll need to pull code from web3, and it works best if your frontend is bundled using something like webpack or browserify:
var SolidityCoder = require("web3/lib/solidity/coder.js");
var ...
- 1,049
38
votes
Accepted
How to find the date of an Ethereum transaction while parsing it with web3?
web3.eth.getTransaction({txhash}) will contain a blockNumber.
Use web3.eth.getBlock to retrieve the block details and you will find the following field:
timestamp: Number - the unix timestamp for ...
- 40.1k
36
votes
Accepted
How to access the event log by knowing the contract address (web3)?
Take a look at web3.eth.filter and watch.
Something like this:
const filter = web3.eth.filter({
fromBlock: 0,
toBlock: 'latest',
address: contractAddress,
topics: [web3.sha3('newtest(string,...
eth♦
- 85k
36
votes
How to get contract internal transactions
In the Ethereum protocol there's only transactions and message calls. A transaction is a type of message call.
A transaction may perform other message calls, but these are not transactions (even ...
eth♦
- 85k
36
votes
Accepted
How can I detect change in account in Metamask?
As suggested by Metamask FAQs this might be an option:
var account = web3.eth.accounts[0];
var accountInterval = setInterval(function() {
if (web3.eth.accounts[0] !== account) {
account = web3....
- 4,347
34
votes
web3.eth.contract is not a function when making contract
Solution:
MyContract = new web3.eth.Contract(abi)
Docs https://web3js.readthedocs.io/en/1.0/web3-eth-contract.html#eth-contract
When using version 1.0.x you need to pass in the Application Binary ...
- 459
34
votes
How can I detect change in account in Metamask?
From the MetaMask docs:
window.ethereum.on('accountsChanged', function (accounts) {
// Time to reload your interface with accounts[0]!
})
window.ethereum.on('networkChanged', function (networkId) {
...
- 4,327
33
votes
Accepted
Is there a way to find an account's current transaction nonce?
Your comment is correct: use web3.eth.getTransactionCount(accountAddress)
Note that the client (Geth) should be in sync with the blockchain.
If you don't have a node and want to trust a service, you ...
eth♦
- 85k
33
votes
Accepted
Getting ABI data for ERC20 tokens programatically with Web3
Thank you K.Fichter for the help, I just want to fully clarify the answer for anyone else who may run into this issue.
Let's say you want to get the balance of a token at an Ethereum Address. Using ...
- 7,928
32
votes
How to get contract internal transactions
Fortunately, Geth EVM has new tools to get this done. It's possible to use debug_traceTransaction with RPC API.
In NodeJS:
var web3 = require('web3').web3;
web3.currentProvider.sendAsync({
...
- 2,713
30
votes
Accepted
Can a contract listen to events of another contract?
A contract cannot listen to events of another contract. From Solidity docs:
Log and event data is not accessible from within contracts (not even
from the contract that created a log).
web3.js ...
eth♦
- 85k
30
votes
What is msg.data?
I had to dig a little deeper into the contents of msg.data for this feature.
I thought it would be worth noting that variable length parameters like arrays, bytes and strings produce a more complex ...
- 401
30
votes
Accepted
What is the difference between web3.js and ethers.js?
For me, it is less about features (however there are a couple cool features/design choices in ethers.js).
Ethers.js is:
smaller
well tested
well documented
well maintained
less buggy
If you want to ...
- 533
29
votes
Accepted
Is it a good practice to log an event every time I throw in Solidity?
With an exception, as with throw, all effects (including events) of a transaction are reverted, except for the payment to the miner. So there would be no benefit in firing an event before a throw.
...
eth♦
- 85k
29
votes
web3: How do I get past events of myContract.myEvent?
I finally figured out how to properly use .get() mentioned in the answer by Roland:
myContract.myEvent({}, { fromBlock: 0, toBlock: 'latest' }).get((error, eventResult) => {
if (error)
...
- 8,704
29
votes
web3.js execution - server vs browser
You raise a topic that can be confusing for newcomers to the space.
In summary, Ethereum can be accessed by a browser, or by a server; anything that can speak the protocol. The Mist browser is more ...
- 54.7k
29
votes
'authentication needed: password or unlock' Error when trying to call smart contract method via web3
First, make sure you have an account.
web3.personal.listAccounts
If you get [] ...
web3.personal.newAccount()
Try again.
Then ...
web3.personal.unlockAccount(web3.personal.listAccounts[0],"&...
- 54.7k
Only top scored, non community-wiki answers of a minimum length are eligible