I’ve recently been doing Udacity’s Blockchain Developer Nanodegree, and the gotcha that has caused me the most wasted hours is the difference between calling a method in my smart contract using the call() vs send() web3.js methods.

Using web3.js to call smart contract functions

Web3.js is a library that allows you to do a number of things related to developing for the ethereum ecosystem, including interacting with a smart contract from a frontend application.

Let’s say we have a simple smart contract that allows you to do just two things - get a string variable name , and set that name variable to a new string.

pragma solidity >=0.4.24; contract NameContract { string private name = "Ire"; function getName() public view returns (string) { return name; } function setName(string newName) public { name = newName; } }

In order to interact with this smart contract via a frontend application, we would first initialise web3.js with the smart contract ABI and address.

import Web3 from 'web3'; const web3 = new Web3(window.ethereum); await window.ethereum.enable(); const NameContract = web3.eth.Contract(contract_abi, contract_address);

Once initialised, we can call any of the methods of our smart contract using either the call() or send() methods.

NameContract.methods.getName().call(); NameContract.methods.setName("bitsofcode").send();

What’s the difference between call() and send() ?

The difference between the call() and send() methods has to do with the type of function they are calling and their effect. Solidity functions can be divided into two categories:

Functions that alter the state of the contract

the state of the contract Functions that do not alter the state of the contract

To make it clear which category a function belongs to, we can specify the function type.

contract MyContract { function myFunction () [visibility] [type] { // do something } }

There are 4 function type keywords we can use, including leaving the function type unspecified.

Description Alter's State? pure Doesn't read or write data 🚫 view Reads, but doesn't write data 🚫 payable Expecting payment of ether ✅ (unspecified) Probably writes data ✅

Going back to my example contract, we can see that the function getName() has the type view which means it does not change the state of the contract. When calling this function via web3.js, we should use the call() method.

NameContract.methods.getName().call();

On the contrary, the setName() function doesn’t have a specified type and does change the state of the contract, in this case the value of the name variable. For these types of funtions, we should use the send() method.

NameContract.methods.setName("bitsofcode").send();

What’s the effect of call() vs send() ?

The reason we have two separate methods for calling functions that do or do not alter the state of a smart contract is because the former requires a transaction on the blockchain and the expense of ether in order to be effective.

The ether required to execute a smart contract function is called “gas” and it is a transaction that the user of the application will have to accept. If the user is using an ethereum wallet such as the Metamask plugin in their browser, they will have to accept the expense of the transaction via a popup window.

With functions called via the call() method on the other hand, they can happen silently in the background and do not require user input to complete.

Creates Transaction? Alter's State? call() 🚫 🚫 send() ✅ ✅

What happens if you use the wrong method?

The reason this difference is such a big gotcha is because there is no explicit warning if you are using the wrong method to call your smart contract function.

For example, if I were to use the call() method to execute the setName() function, I would get no warnings.

NameContract.methods.setName("bitsofcode").call();

Will not work

Ostensibly, the method would have successfully completed with no errors. However, if we were to query the name variable that is supposed to have changed, we will see that it was not changed.

NameContract.methods.getName().call(); // => "Ire" NameContract.methods.setName("bitsofcode").call(); NameContract.methods.getName().call(); // => "Ire"

This is why I spent hours trying to debug why my function wasn't working, when in fact the problem was I was using the wrong method to call it!