Building a Todo List App
Put your JavaScript skills together to build a functional, persistent todo application.
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📋 Copy State-Driven Todo Store Architecture Snippet
// State-Driven Todo Store Pattern
class TodoStore {
constructor() {
this.todos = JSON.parse(localStorage.getItem("todos") || "[]");
}
add(text) {
this.todos.push({ id: Date.now(), text, done: false });
this.save();
}
toggle(id) {
const t = this.todos.find(x => x.id === id);
if (t) { t.done = !t.done; this.save(); }
}
save() {
localStorage.setItem("todos", JSON.stringify(this.todos));
}
}⚠️ 5 Fatal Traps & JavaScript Pitfalls
Trap #1: Storing State in the DOM
Reading state directly from HTML classes and text leads to desynchronization. Maintain an in-memory JavaScript state array and render the DOM as a pure projection of that data.
Trap #2: Direct innerHTML Injection of Todo Text (XSS)
Rendering
item.innerHTML = todo.text allows malicious users to inject script tags. Always use textContent or sanitized DOM creation.Trap #3: Duplicate IDs from Date.now() in Automated Tests
Generating IDs with
Date.now() creates duplicate keys when multiple items are added within the same millisecond. Use crypto.randomUUID().Trap #4: Full DOM Re-Renders on Single Item Mutations
Wiping and regenerating the entire list on every checkbox click destroys user input focus and degrades performance. Update the specific item element.
Trap #5: Untrimmed Empty String Submissions
Failing to validate
input.value.trim() allows users to add blank todos that clutter storage.💬 Frequently Asked Questions
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