Get return value from solidity function

Question

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!!! :(

Answer 1

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.

Answer 2

Let's say the abi of my contract is 'abi' and the variable I am using to interact with my contract is 'agreem'. The address of deployed contract is contract_address.

let agreem =  new web3.eth.Contract(abi, contract_address);

Now, I will say my function's name is retrieve and it returns two values. My function takes two parameters as well. To call my function-

const bal=await agreem.methods.retrieve(uname,ps).call({from:accounts[0]});

The variable bal will store both the return values returned by my function.To access them you can use bal[0] and bal[1] .

Hope it helps.

Answer 3

You need your ABI and your address saved as a string

You create the contract instance like this:

var contract = web3.eth.contract(JSON.parse(abi)).at(address);

You can save the contract call like this:

contract.contractInstance.call();

Do you use truffle to develop? Truffle calls are different from web3js calls.

EDIT: This solution works with plain web3.js not truffle

Answer 4

Here is an example:

contract("MyContractTest", () => {
    let myContract;

    before(async () => {
        myContract = await artifacts.require("MyContract.sol").new(construction arguments);
    });

    describe(`test:`, async () => {
        it(`getIdentifier:`, async () => {
            let identifier = await myContract.getIdentifier();
            console.log(identifier);
        });
    });
});

If the function is not pure/view/constant, then it's a little more complicated (you need to add an event in the Solidity contract, and extract it from the returned value in the Javascript test).

Answer 5

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

Though this will work only in async function. In synchronous code yuu will need to use callback:

contractInstance.methods.getIdentifier().call().then (x => {/* use x here */});