Understanding how to call a smart contract is a big part of developing this revolutionary type of program and a great way to learn about them.
What you will learn
Calling a smart contract is a way of accessing the coding of the smart contract and its data in a read-only manner.
Developers use this technique frequently to test and debug a smart contract before it is deployed.
Although smart contract calls can be helpful, it can be dangerous to call untrusted smart contracts because it can execute malicious code.
Smart contracts are programs on the blockchain that run when certain conditions are met. These contracts have conditions and rules, and can store, update and provide data. However, once a smart contract is deployed on the blockchain, those conditions and rules can't be changed. When developing or testing a new contract, it’s often best to call the contract rather than initiate a transaction. Understanding how to call a smart contract is a great way to learn about smart contracts.
Calling is an excellent option for debugging during smart contract development. It's a way of accessing the coding of the smart contract and its data in a read-only manner. The process of calling functions on a smart contract does not publish anything on the blockchain. It simulates what would occur in a transaction without changing the contract instance. And because it’s a read-only operation, it doesn’t cost anything.
Alternatively, transactions are broadcast to the network, processed by miners, and potentially published on the blockchain. Transactions affect other accounts, change the state of the blockchain, and require gas to be paid. (It is possible, however, that a transaction won’t be included in a block if miners don’t accept it.)
Smart contract calls can be used to examine a contract’s indexed parameters, such as token transfers sent or received by a given address. For these reasons and many others, calling a contract before sending a transaction is typically recommended.
A smart contract call can, to give another example, identify whether an out of gas exception will occur. (For that reason, it is best to specify the gas limit for the call, even though it won’t cost anything.)
Although smart contract calls can be helpful, it can be dangerous to call untrusted smart contracts. External calls to untrusted contracts can execute malicious code that may hijack the control flow, leading to reentrancy vulnerabilities. These reentrancy vulnerabilities often involve repeatedly calling functions before the first invocation is finished. Because their balance isn’t set to zero until the end of the function, attackers can use this bug to withdraw a balance multiple times.
If a call is made to an untrusted smart contract, it’s best to avoid state changes after the call.
Knowing how to interact with smart contracts directly is advantageous in web3. For example, anyone with the right technical knowledge can call an in-contract function to read nearly all the code for a project’s smart contract, providing valuable insight into the project’s popularity and security.
Dependency injection is another way to call contracts from a smart contract. Dependency injection allows the caller to instantiate the contract, receiving the return values.
Before a smart contract is deployed on its blockchain, the programming language must be compiled, or compressed, into an unreadable format called bytecode. That means that an Application Binary Interface is needed to call the unreadable functions from the contract address. In computer science, ABIs are interfaces between two program modules. Data must be converted back to the ordered list of return values to interpret the unreadable information. A smart contract ABI documents and translates these names between human-intended method calls and smart-contract operations.
ABIs are typically stored along with the source code or the tool used for compiling. If the source code is published, tools like Truffle and Remix can be used to generate the ABI.
Generally speaking, anyone can call a smart contract. Smart contracts can even call other smart contracts. This functionality is useful when minting new tokens. Still, it’s possible to limit who can execute functions on a deployed contract.
Creating an access control list allows smart contract creators to manage who can do what with their contracts. The contract’s ACL governs who can mint tokens, vote on proposals, and more. For example, the “Owned” ACL paradigm means the creator of a contract has access to all functions while others have access to only the functions that change their own data.
A contract can own another contract, meaning developers can use multi-sig wallets, an ERC725/uPort identity contract, or their own custom contract to add additional layers of access complexity. As a smart contract developer, you must understand how to implement an ACL
Smart contract functions can be called from JavaScript. Using a relatively simple JavaScript code, developers can execute a contract’s smart contract method in the EVM without paying gas. For example, the “jsonInterface - Object” parameter specifies the json interface to instantiate. The “address - String” parameter, which specifies the smart contract address to call, is a second parameter worth noting.
Numerous tools make it easier to interact with smart contracts. Truffle, for example, is a development environment and testing framework for blockchains that use the EVM. With Truffle, developers get a personal blockchain for Ethereum development that can be used for contract deployment, application development, and running tests. Truffle is available as a desktop application or a command-line tool. Remix is another open-source tool that allows you to write Solidity contracts straight from the browser.
A wallet is the most common way to interact with a deployed smart contract by signing transactions. For example, when buying an NFT on a popular marketplace, the decentralized app (dApp) uses an on-chain smart contract to release the NFT from escrow to the buyer’s wallet, pay the seller, apply the gas price, and collect fees simultaneously. In some cases, NFTs are minted at the time of the transaction, utilizing smart contracts.
NFTs are sometimes embedded with smart contracts that alter their digital content. For example, NFT games may include the ability to level up after specific conditions are met. Once a user has met the NFTs conditions, such as collecting other NFTs or staking their NFT, a smart contract will trigger the asset’s level to increase.
Services like HashScan allow users to read essential data from smart contracts and transactions. Although these services don’t allow people to interact directly with deployed smart contracts, important contract data such as the latest transactions and the contract’s source code can be seen. These sites present transaction hash data, block number data, and more in an easy-to-digest format. Although smart contract data is publicly available, you won’t be able to see sensitive information such as the project team’s private key.
Knowing how to call smart contracts offers significant advantages for those interested in web3. Hedera’s smart contracts 2.0 linked the Hedera project to the Ethereum Virtual Machine, which means contracts can be called on Hedera the same as on the Ethereum blockchain. Still, there are numerous benefits to interacting with smart contracts on Hedera. Due to Hedera’s underlying hashgraph consensus, developers and users get low gas fees, carbon-negative operations, and improved EVM processing speeds. Hedera can accomplish in 3 - 5 seconds what Ethereum takes over 13 minutes to perform.