TypeError: app.getGreeting is not a function

Question

I'm trying to connect a NodeJs server to a solidity contract thats been migrated to a local blockchain and then call the methods on this class via javascript. However, the code returns an error stating that getGreeting() is not defined.

I'm very new Solidity and JavaScript (background in C and Java) so I feel like I'm overlooking something really simple?

My question is how to get it to work and find the method and return "Hello, World!" to the terminal?

My development environment is:

Truffle v5.1.10 (core: 5.1.10)
Solidity - 0.4.25 (solc-js)
Node v12.14.1
Web3.js v1.2.1

Below is the code I'm working on:

// import our compiled JSON
const contractJSON = require("./build/contracts/Hello.json");
const contract = require("@truffle/contract");
const Web3 = require('web3');

// create instance of ganache-cli provider
const web3 = new Web3("http://localhost:9545");

var Hello = contract(contractJSON);

Hello.setProvider(web3);

// if contract is deployed return instance of it and store in app variable
let app = Hello.deployed().then((instance) =>{
    return instance;

}).catch(function(error) {
    return error;
});

// call a method on our contract via javascript 
app.getGreeting().then(() => {
    console.log(app);  
});

For context the Solidity contract is as follows:

pragma solidity >=0.4.0 <0.7.0;

contract Hello {

    string greeting;

    constructor() public {
        greeting = "Hello, World!";
    }

    function getGreeting() public view returns (string memory) {
        return greeting;
    }

    function setGreeting(string memory _greeting) public {
        greeting = _greeting;
    }
}

app is a Promise, and Promises don't have a getGreeting method. This should work: app.then(instance => instance.getGreeting()).then(...); — blex
@blex Thanks for your input, I've tried something similar app.then(instance => instance.getGreeting()).then(console.log); However, i still get the same error...UnhandledPromiseRejectionWarning: TypeError: instance.getGreeting is not a function — user5770672
Then is it possible that instance.getGreeting returns a String, not a function? Try logging instance.getGreeting without () — blex
@blex Ok, I changed the code to not call a function like suggested but it returns undefined in the terminal. Any suggestions? I appreciate the help. — user5770672
Weird, this seems impossible. If it were really undefined, you would get the error Cannot read property 'getGreeting' of undefined, and not instance.getGreeting is not a function. What happens if you log the instance like this? — blex

Unknown Unknown · Accepted Answer · 2020-01-28T19:43:31

For those of you who view this in the future the issue was that I imported the provider incorrectly according to the @truffle/contract documentation here. Also, forgot to fulfill the second promise. Therefore the answer to this solution as of the time of writing is as follows:

const provider = new Web3.providers.HttpProvider("http://localhost:9545");

let Hello = contract(contractJSON);
Hello.setProvider(provider);

Hello.deployed().then((instance) => {
    deployed = instance;
    return instance.getGreeting();
}).then((result) => {
    console.log(result);
}).catch((error) => {
    console.log(error);
});

Many thanks to @blex for the assistance.

TypeError: app.getGreeting is not a function

2 Answers