Blockchain Smart Contracts with Solidity · Lesson

ERC-721 Non-Fungible Tokens (NFTs)

Explore the ERC-721 standard for unique digital assets and learn how to mint and manage NFTs.

ERC-721 Non-Fungible Tokens (NFTs) is a free Blockchain Smart Contracts with Solidity lesson on CoddyKit — lesson 3 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Blockchain Smart Contracts with Solidity learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Intro to Unique Digital Assets

Welcome to the world of Non-Fungible Tokens (NFTs)! Unlike regular money or cryptocurrencies, NFTs are unique digital items.

Think of them like rare collectibles or one-of-a-kind art pieces, but entirely digital. Each NFT has distinct properties and cannot be replaced by another.

The ERC-721 Standard

On Ethereum, NFTs are defined by a standard called ERC-721. This standard is a set of rules that smart contracts must follow to create, manage, and transfer unique tokens.

It ensures that all compliant NFTs work similarly across different platforms, making them interoperable.

Key Concepts: Token ID & Ownership

Every ERC-721 token has a unique Token ID (a large number). This ID distinguishes it from all other NFTs, even those from the same collection.

The standard also defines functions to track ownership:

ownerOf(uint256 tokenId): Returns the address that owns a specific NFT.
balanceOf(address owner): Returns the number of NFTs an address owns from that contract.

Building with OpenZeppelin

Creating an ERC-721 contract from scratch can be complex. Luckily, OpenZeppelin provides battle-tested, secure implementations.

We can simply import their contracts and build upon them. This saves time and reduces the risk of vulnerabilities.

Your First ERC-721 Contract

Here's a basic structure for an ERC-721 contract using OpenZeppelin. It defines a name and symbol for our NFT collection in the constructor.

We'll add a minting function next.

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

import "@openzeppelin/contracts/token/ERC721/ERC721.sol";

contract SimpleNFT is ERC721 {
    constructor() ERC721("MySimpleNFT", "MSN") {
        // Constructor for ERC721 takes name and symbol
    }
}

Creating New NFTs: Minting

To bring new NFTs into existence, we use a process called minting. The _safeMint function (provided by OpenZeppelin's ERC721) creates a new token and assigns it to an owner.

Let's add a public mint function to our contract to create new NFTs.

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

import "@openzeppelin/contracts/token/ERC721/ERC721.sol";

contract SimpleNFT is ERC721 {
    uint256 private _tokenIdCounter;

    constructor() ERC721("MySimpleNFT", "MSN") {
        _tokenIdCounter = 0;
    }

    function mint(address to) public returns (uint256) {
        uint256 newItemId = _tokenIdCounter;
        _tokenIdCounter++;
        _safeMint(to, newItemId); // Mints token and assigns to 'to' address
        return newItemId;
    }
}

NFT Metadata with Token URI

An NFT's value often comes from what it represents (an image, a song, a game item). This information, called metadata, isn't stored directly on the blockchain due to cost.

Instead, the ERC-721 standard includes a tokenURI(uint256 tokenId) function. This function returns a URL pointing to a JSON file that describes the NFT's metadata (e.g., name, description, image URL).

Transferring Ownership

Once an NFT is minted, it can be transferred between owners. The ERC-721 standard includes functions for this:

transferFrom(address from, address to, uint256 tokenId): Transfers ownership of tokenId from from to to.
safeTransferFrom(...): A safer version that checks if the recipient contract can receive NFTs.

These functions ensure only the current owner or an approved address can initiate a transfer.

Delegated Transfers: Approvals

Sometimes, you might want to allow another address (like a marketplace) to transfer your NFT on your behalf. This is done using approvals.

The approve(address to, uint256 tokenId) function lets you grant permission to another address to to manage a specific tokenId. This is crucial for decentralized exchanges and other dApps.

ERC-721 Knowledge Check

Let's test your understanding of ERC-721 NFTs. Select all statements that are true.

Recap: ERC-721 NFTs

You've explored the fascinating world of ERC-721 Non-Fungible Tokens!

NFTs are unique digital assets, unlike fungible cryptocurrencies.
The ERC-721 standard defines how these unique tokens are created and managed on Ethereum.
Key features include unique Token IDs, ownership tracking, minting, and linking to off-chain metadata via tokenURI.
OpenZeppelin contracts simplify ERC-721 development.

NFTs are a cornerstone of digital ownership and are used in art, gaming, and many other applications!

Frequently asked questions

Is the “ERC-721 Non-Fungible Tokens (NFTs)” lesson free?

Yes — the full text of “ERC-721 Non-Fungible Tokens (NFTs)” is free to read here on the web, and the Blockchain Smart Contracts with Solidity course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Blockchain Smart Contracts with Solidity course, upgrade to CoddyKit PRO.

What will I learn in “ERC-721 Non-Fungible Tokens (NFTs)”?

Explore the ERC-721 standard for unique digital assets and learn how to mint and manage NFTs. You practise Blockchain Smart Contracts with Solidity with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Blockchain Smart Contracts with Solidity?

No prior experience is required. Blockchain Smart Contracts with Solidity on CoddyKit is structured for beginners through advanced learners; this is — lesson 3 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “ERC-721 Non-Fungible Tokens (NFTs)” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Blockchain Smart Contracts with Solidity lesson?

Yes. Every Blockchain Smart Contracts with Solidity lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

All lessons in this course

← Back to Blockchain Smart Contracts with Solidity