[Ethereum] Get return value from solidity function

solidityweb3js

This may be javascript question, but I am totally out of ideas.

I have a function in solidity code returning a variable:

  function getIdentifier() public view returns (string) {
    return identifier;
  }

I want to simply call this function with web3.js and store result as a variable. But all I can do is console.log the result (I know, I know =/).

Following code console.logs the variable:

contractInstance.methods.getIdentifier().call().then(console.log);

Works fine, but I simply want to store the return of getIdentifier() in a variable x.

None of the following naive code work:

x = contractInstance.methods.getIdentifier();
x = contractInstance.methods.getIdentifier().call();

I am sorry for a dumb question, but I have tried tons of things and it still does not work.

EDIT:
Thanks smarx your code works, but I still would like to keep the variable for later use. Strangely something like this does not work:

var y = "";
contractInstance.methods.getIdentifier().call()
.then(function (x) {
  y = x;
});
console.log(y) // still empty!!! :(

Best Answer

This is just a JavaScript question. :-)

contractInstance.methods.getIdentifier()
.then(function (x) {
  // Use x in here.
});

// This code runs BEFORE the callback, and x won't be defined out here.

Related Solutions

[Ethereum] Web3 Function Calls are very slow !

Is there a better way to pull all of this data and store it in a dictionary or is this the best way?

It would almost certainly be faster to parse the data directly from the .ldb files in the chaindata directory, and then cache it. To do that would require you to parse it, which itself requires knowledge of how the data is stored.

If you're using Node.js, you could take a look at "Exploring Ethereum's state trie with Node.js". (Node.js is probably not the best language to use for this.) Note that the code in the tutorial is slightly out of date. I've included a tweaked working version below.

var levelup = require('levelup');
var rlp = require('rlp');
var trie = require('merkle-patricia-tree');
var db = levelup('chaindata');

// The genesis state root.
var stateRoot = 
  'd7f8974fb5ac78d9ac099b9ad5018bedc2ce0a72dad1827a1709da30580f0544';

// ERROR: AssertionError: Invalid root length. Roots are 32 bytes
var stateTrie = new trie(db, stateRoot);

// Note: We're doing everything using binary encoding.
db.get(new Buffer(stateRoot, 'hex'), {
  encoding: 'binary'
}, function (err, value) {
  console.log("Printing the entry for the State Root")
  var decoded = rlp.decode(value);
  console.log(decoded);
});

// Gav's address.
var gav = new Buffer('8a40bfaa73256b60764c1bf40675a99083efb075', 'hex');

trie.get(gav, function (err, val) {
  console.log("Printing the contents of Gav's address")
  var decoded = rlp.decode(val);
  console.log(decoded);
});

Smart Contract Error – ‘Returned Values Aren’t Valid’ and Out of Gas Issues

I fixed the similar error by deleting .json files from build/contracts folder and then running truffle migrate in the terminal.

However, the error occurred in Visual Studio Code, not Remix IDE. It appeared after running truffle migrate --reset.

I'd like to add to this comment, after you delete .json files from build/contracts folder the problem persisted until I deleted files from the trash🗑 😅 So make sure trash is empty

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