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Learn to build a full-featured todo list app in JavaScript with add, complete, delete, filter, and localStorage persistence. Perfect beginner JS project.
What You'll Learn
- The core state → render pattern used in every JavaScript app
- How to add, complete, and delete items in an array
- How to filter a list (All / Active / Completed)
- How to use event delegation for dynamically created elements
- How to persist data with
localStorageacross reloads
Project Overview
A todo app is the "Hello World" of JavaScript projects. It demonstrates every fundamental pattern: maintaining an array of objects as state, rendering that array to the DOM, and responding to user actions that mutate the array.
Component Structure
How It Works — Flow Diagram
Step-by-Step Build
Step 1 — HTML Structure
html example
<div class="todo-app">
<h1 class="todo-title">My Tasks</h1>
<form class="todo-form" id="todoForm">
<input type="text" id="todoInput" placeholder="Add a new task..." autocomplete="off">
<button type="submit" class="btn-add">+ Add</button>
</form>
<div class="filter-bar">
<button class="filter-btn active" data-filter="all">All</button>
<button class="filter-btn" data-filter="active">Active</button>
<button class="filter-btn" data-filter="completed">Completed</button>
</div>
<ul class="todo-list" id="todoList"></ul>
<footer class="todo-footer">
<span id="itemsLeft"></span>
<button id="clearCompleted">Clear Completed</button>
</footer>
</div>Step 2 — State
Example
let todos = JSON.parse(localStorage.getItem("todos")) || [];
let currentFilter = "all";
Step 3 — Add a Todo
Example
document.getElementById("todoForm").addEventListener("submit", (e) => {
e.preventDefault();
const input = document.getElementById("todoInput");
const text = input.value.trim();
if (!text) return;
addTodo(text);
input.value = "";
});
function addTodo(text) {
todos.push({
id: Date.now(), // unique-enough ID
text,
completed: false
});
saveTodos();
render();
}
Step 4 — Toggle & Delete
Example
function toggleTodo(id) {
const todo = todos.find(t => t.id === id);
if (todo) todo.completed = !todo.completed;
saveTodos();
render();
}
function deleteTodo(id) {
todos = todos.filter(t => t.id !== id);
saveTodos();
render();
}
Step 5 — Render Function
Example
function render() {
const list = document.getElementById("todoList");
list.innerHTML = "";
const visible = getFilteredTodos();
if (visible.length === 0) {
list.innerHTML = `<li class="empty-state">No tasks to show</li>`;
}
visible.forEach(todo => {
const li = document.createElement("li");
li.className = `todo-item ${todo.completed ? "completed" : ""}`;
li.dataset.id = todo.id;
li.innerHTML = `
<input type="checkbox" class="todo-check" ${todo.completed ? "checked" : ""}>
<span class="todo-text">${escapeHtml(todo.text)}</span>
<button class="todo-delete" aria-label="Delete task">✕</button>
`;
list.appendChild(li);
});
updateFooter();
}
Step 6 — Event Delegation
Example
document.getElementById("todoList").addEventListener("click", (e) => {
const li = e.target.closest(".todo-item");
if (!li) return;
const id = Number(li.dataset.id);
if (e.target.classList.contains("todo-check")) toggleTodo(id);
if (e.target.classList.contains("todo-delete")) deleteTodo(id);
});
Step 7 — Filter Tabs
Example
document.querySelector(".filter-bar").addEventListener("click", (e) => {
if (!e.target.classList.contains("filter-btn")) return;
currentFilter = e.target.dataset.filter;
document.querySelectorAll(".filter-btn").forEach(b => b.classList.remove("active"));
e.target.classList.add("active");
render();
});
function getFilteredTodos() {
if (currentFilter === "active") return todos.filter(t => !t.completed);
if (currentFilter === "completed") return todos.filter(t => t.completed);
return todos;
}
Step 8 — Footer & Persistence
Example
function updateFooter() {
const active = todos.filter(t => !t.completed).length;
document.getElementById("itemsLeft").textContent =
`${active} item${active !== 1 ? "s" : ""} left`;
}
function saveTodos() {
localStorage.setItem("todos", JSON.stringify(todos));
}
function escapeHtml(str) {
return str.replace(/&/g,"&").replace(/</g,"<").replace(/>/g,">");
}
document.getElementById("clearCompleted").addEventListener("click", () => {
todos = todos.filter(t => !t.completed);
saveTodos();
render();
});
Step 9 — Initialize
js example
render(); // render on page load to show persisted todosConsole Output
Output
// Adding "Buy groceries"
addTodo("Buy groceries")
todos = [{ id: 1749686400000, text: "Buy groceries", completed: false }]
render() → 1 <li> item in list, "1 item left"
// Checking it off
toggleTodo(1749686400000)
todos[0].completed = true
render() → <li class="todo-item completed">, "0 items left"
// Switching to "Active" filter
currentFilter = "active"
render() → getFilteredTodos() returns [] → shows "No tasks to show"
// Adding another task
addTodo("Write code review")
todos.length = 2, 1 active
render() → "Write code review" visible in "Active" filterEdge Cases to Handle
| Situation | Expected Behavior |
|---|---|
| Empty input | trim() check — don't add blank tasks |
| Duplicate text | Allowed (same text, different IDs) |
| XSS in task text | Escape HTML before injecting |
| All completed | "0 items left" shown |
| Clear Completed when none | Button is a no-op, no errors |
Challenge Yourself
- Drag-to-reorder — implement drag-and-drop sorting of tasks
- Due dates — add a date picker to each task and highlight overdue items
- Priority levels — add Low / Medium / High priority with color coding
- Edit task — double-click a task text to edit it in-place
- Search bar — add a search input that filters tasks by keyword
Best Practices
- Always call
render()once at startup to hydrate fromlocalStorage - Use event delegation on the list — never attach listeners to each
<li> - Sanitize user input before inserting it into
innerHTML - Keep
saveTodos()andrender()as the final two lines of every mutation function - Use
Date.now()for IDs in simple projects,crypto.randomUUID()in production
Continue the lesson path
Study next in this module
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