Web3 & DApp Development Fundamentals · Lesson

Implementing a Token

transfer and approve.

Implementing a Token is a free Web3 & DApp Development Fundamentals lesson on CoddyKit — lesson 2 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 Web3 & DApp Development Fundamentals learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Building a Token

Now you will implement the core of an ERC-20 token from scratch. Understanding the internals helps you debug and audit real tokens, even when you later use libraries.

We will focus on the state, transfer, and approve.

Core State Variables

A token needs to track balances and total supply. Balances are stored in a mapping from address to amount.

<code>contract Token {
    string public name = 'MyToken';
    string public symbol = 'MTK';
    uint8 public decimals = 18;
    uint256 public totalSupply;
    mapping(address => uint256) public balanceOf;
}</code>

The Constructor

The constructor mints an initial supply to the deployer. We increase totalSupply and credit the creator's balance.

<code>constructor(uint256 initialSupply) {
    totalSupply = initialSupply;
    balanceOf[msg.sender] = initialSupply;
}</code>

The Events

Declare the two required events. We will emit Transfer on every movement and Approval when an allowance is set.

<code>event Transfer(address indexed from, address indexed to, uint256 value);
event Approval(address indexed owner, address indexed spender, uint256 value);</code>

Implementing transfer

The transfer function moves tokens from the caller to a recipient. It must check the caller has enough balance and emit the Transfer event.

<code>function transfer(address to, uint256 amount) public returns (bool) {
    require(balanceOf[msg.sender] >= amount, 'insufficient balance');
    balanceOf[msg.sender] -= amount;
    balanceOf[to] += amount;
    emit Transfer(msg.sender, to, amount);
    return true;
}</code>

Guarding Against Zero Address

Sending tokens to address(0) effectively destroys them by accident. Many implementations reject it explicitly.

<code>function transfer(address to, uint256 amount) public returns (bool) {
    require(to != address(0), 'transfer to zero');
    require(balanceOf[msg.sender] >= amount, 'insufficient');
    balanceOf[msg.sender] -= amount;
    balanceOf[to] += amount;
    emit Transfer(msg.sender, to, amount);
    return true;
}</code>

The allowance Mapping

To support delegated transfers we add a nested mapping: owner to spender to approved amount.

<code>mapping(address => mapping(address => uint256)) public allowance;</code>

Implementing approve

The approve function lets the caller authorize a spender to move up to a certain amount of their tokens. It records the allowance and emits Approval.

<code>function approve(address spender, uint256 amount) public returns (bool) {
    allowance[msg.sender][spender] = amount;
    emit Approval(msg.sender, spender, amount);
    return true;
}</code>

Overflow Safety

Since Solidity 0.8, arithmetic reverts on overflow and underflow automatically. The subtraction in transfer is safe: if the balance is too low it reverts, though the explicit require gives a clearer error.

Why Return bool

The standard specifies a bool return for compatibility. We return true on success. On failure we revert, which is the modern convention and safer than silently returning false.

Putting It Together

With state, events, transfer, and approve in place you have a functioning fungible token. The next lesson adds transferFrom to complete the allowance flow.

<code>// state + constructor + events + transfer + approve
// = a minimal working ERC-20</code>

Quick Check

Test your understanding of implementing a token.

Recap

You implemented the core of an ERC-20 token:

State: name, symbol, decimals, totalSupply, balanceOf mapping
Constructor mints the initial supply to the deployer
transfer checks balance, updates state, emits Transfer
approve records an allowance and emits Approval
Guard against the zero address and rely on 0.8 overflow safety

Frequently asked questions

Is the “Implementing a Token” lesson free?

Yes — the full text of “Implementing a Token” is free to read here on the web, and the Web3 & DApp Development Fundamentals 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 Web3 & DApp Development Fundamentals course, upgrade to CoddyKit PRO.

What will I learn in “Implementing a Token”?

transfer and approve. You practise Web3 & DApp Development Fundamentals 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 Web3 & DApp Development Fundamentals?

No prior experience is required. Web3 & DApp Development Fundamentals on CoddyKit is structured for beginners through advanced learners; this is — lesson 2 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Implementing a Token” 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 Web3 & DApp Development Fundamentals lesson?

Yes. Every Web3 & DApp Development Fundamentals 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.

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