Todo App with Signals

Build a fully functional Todo App using Angular Signals, standalone components, and modern Angular best practices. This project covers CRUD operations, local storage, filtering, and more!

🎯 What You’ll Build

A modern Todo application with:

• ✅ Add, edit, delete, and toggle todos
• ✅ Filter by status (All, Active, Completed)
• ✅ Mark all as complete
• ✅ Clear completed todos
• ✅ Persist data in localStorage
• ✅ Responsive design
• ✅ TypeScript strict mode
• ✅ Signal-based state management

📋 Prerequisites

Before starting this project, make sure you have:

• Node.js (v18 or higher) installed
• Angular CLI installed globally
• Basic understanding of TypeScript
• Familiarity with Angular components and signals

For project setup instructions, please refer to:

• Prerequisites & Setup Guide
• Creating Your First App

🚀 Project Setup

Step 1: Create New Angular Project

ng new todo-app --routing=false --style=css --strict
cd todo-app

Options explained:

• --routing=false - We don’t need routing for this simple app
• --style=css - Use CSS for styling
• --strict - Enable strict TypeScript checking

Step 2: Project Structure

Create the following folder structure:

src/app/
├── components/
│   ├── todo-list/
│   │   └── todo-list.component.ts
│   ├── todo-item/
│   │   └── todo-item.component.ts
│   ├── todo-form/
│   │   └── todo-form.component.ts
│   └── todo-filters/
│       └── todo-filters.component.ts
├── models/
│   └── todo.model.ts
├── services/
│   └── todo.service.ts
└── app.component.ts

📝 Step-by-Step Implementation

Step 1: Create the Todo Model

Create src/app/models/todo.model.ts:

export interface Todo {
  id: string;
  title: string;
  completed: boolean;
  createdAt: Date;
}

export type FilterType = 'all' | 'active' | 'completed';

Step 2: Create the Todo Service

Create src/app/services/todo.service.ts:

import { Injectable, signal, computed } from '@angular/core';
import { Todo, FilterType } from '../models/todo.model';

@Injectable({
  providedIn: 'root'
})
export class TodoService {
  // State
  private todos = signal<Todo[]>(this.loadFromStorage());
  private filter = signal<FilterType>('all');

  // Public read-only signals
  allTodos = this.todos.asReadonly();
  currentFilter = this.filter.asReadonly();

  // Computed values
  filteredTodos = computed(() => {
    const todos = this.todos();
    const filterType = this.filter();

    switch (filterType) {
      case 'active':
        return todos.filter(todo => !todo.completed);
      case 'completed':
        return todos.filter(todo => todo.completed);
      default:
        return todos;
    }
  });

  activeCount = computed(() =>
    this.todos().filter(todo => !todo.completed).length
  );

  completedCount = computed(() =>
    this.todos().filter(todo => todo.completed).length
  );

  // Actions
  addTodo(title: string): void {
    if (!title.trim()) return;

    const newTodo: Todo = {
      id: crypto.randomUUID(),
      title: title.trim(),
      completed: false,
      createdAt: new Date()
    };

    this.todos.update(todos => [...todos, newTodo]);
    this.saveToStorage();
  }

  toggleTodo(id: string): void {
    this.todos.update(todos =>
      todos.map(todo =>
        todo.id === id ? { ...todo, completed: !todo.completed } : todo
      )
    );
    this.saveToStorage();
  }

  deleteTodo(id: string): void {
    this.todos.update(todos => todos.filter(todo => todo.id !== id));
    this.saveToStorage();
  }

  updateTodo(id: string, title: string): void {
    if (!title.trim()) return;

    this.todos.update(todos =>
      todos.map(todo =>
        todo.id === id ? { ...todo, title: title.trim() } : todo
      )
    );
    this.saveToStorage();
  }

  toggleAll(): void {
    const allCompleted = this.todos().every(todo => todo.completed);

    this.todos.update(todos =>
      todos.map(todo => ({ ...todo, completed: !allCompleted }))
    );
    this.saveToStorage();
  }

  clearCompleted(): void {
    this.todos.update(todos => todos.filter(todo => !todo.completed));
    this.saveToStorage();
  }

  setFilter(filter: FilterType): void {
    this.filter.set(filter);
  }

  // LocalStorage
  private saveToStorage(): void {
    localStorage.setItem('todos', JSON.stringify(this.todos()));
  }

  private loadFromStorage(): Todo[] {
    const stored = localStorage.getItem('todos');
    if (!stored) return [];

    try {
      const parsed = JSON.parse(stored);
      return parsed.map((todo: any) => ({
        ...todo,
        createdAt: new Date(todo.createdAt)
      }));
    } catch {
      return [];
    }
  }
}

Step 3: Create Todo Form Component

Create src/app/components/todo-form/todo-form.component.ts:

import { Component, signal, inject } from '@angular/core';
import { FormsModule } from '@angular/forms';
import { TodoService } from '../../services/todo.service';

@Component({
  selector: 'app-todo-form',
  imports: [FormsModule],
  template: `
    <form (submit)="onSubmit($event)" class="todo-form">
      <input
        type="text"
        [(ngModel)]="newTodoTitle"
        placeholder="What needs to be done?"
        class="todo-input"
        [disabled]="isSubmitting()"
      >
    </form>
  `,
  styles: [`
    .todo-form {
      margin-bottom: 20px;
    }

    .todo-input {
      width: 100%;
      padding: 16px;
      font-size: 24px;
      border: none;
      border-bottom: 1px solid #ededed;
      outline: none;
      box-sizing: border-box;
    }

    .todo-input::placeholder {
      font-style: italic;
      color: #e6e6e6;
    }

    .todo-input:disabled {
      opacity: 0.6;
    }
  `]
})
export class TodoFormComponent {
  private todoService = inject(TodoService);

  newTodoTitle = '';
  isSubmitting = signal(false);

  onSubmit(event: Event): void {
    event.preventDefault();

    if (!this.newTodoTitle.trim()) return;

    this.isSubmitting.set(true);
    this.todoService.addTodo(this.newTodoTitle);
    this.newTodoTitle = '';
    this.isSubmitting.set(false);
  }
}

Step 4: Create Todo Item Component

Create src/app/components/todo-item/todo-item.component.ts:

import { Component, input, output, signal, inject } from '@angular/core';
import { FormsModule } from '@angular/forms';
import { Todo } from '../../models/todo.model';
import { TodoService } from '../../services/todo.service';

@Component({
  selector: 'app-todo-item',
  imports: [FormsModule],
  template: `
    <li [class.completed]="todo().completed" [class.editing]="isEditing()">
      <div class="view">
        <input
          type="checkbox"
          class="toggle"
          [checked]="todo().completed"
          (change)="onToggle()"
        >
        <label (dblclick)="startEdit()">{{ todo().title }}</label>
        <button class="destroy" (click)="onDelete()"></button>
      </div>

      @if (isEditing()) {
        <input
          #editInput
          type="text"
          class="edit"
          [(ngModel)]="editTitle"
          (blur)="finishEdit()"
          (keyup.enter)="finishEdit()"
          (keyup.escape)="cancelEdit()"
        >
      }
    </li>
  `,
  styles: [`
    li {
      position: relative;
      font-size: 24px;
      border-bottom: 1px solid #ededed;
      list-style: none;
    }

    li:last-child {
      border-bottom: none;
    }

    li.editing {
      border-bottom: none;
      padding: 0;
    }

    li.editing .view {
      display: none;
    }

    li.completed label {
      color: #d9d9d9;
      text-decoration: line-through;
    }

    .view {
      display: flex;
      align-items: center;
      padding: 15px;
    }

    .toggle {
      width: 40px;
      height: 40px;
      margin-right: 15px;
      cursor: pointer;
    }

    label {
      flex: 1;
      cursor: pointer;
      word-break: break-all;
    }

    .destroy {
      width: 40px;
      height: 40px;
      margin-left: auto;
      border: none;
      background: none;
      font-size: 30px;
      color: #cc9a9a;
      cursor: pointer;
      opacity: 0;
      transition: opacity 0.2s;
    }

    .destroy:after {
      content: '×';
    }

    li:hover .destroy {
      opacity: 1;
    }

    .edit {
      width: 100%;
      padding: 12px 16px;
      font-size: 24px;
      border: 1px solid #999;
      box-shadow: inset 0 -1px 5px 0 rgba(0, 0, 0, 0.2);
      box-sizing: border-box;
    }
  `]
})
export class TodoItemComponent {
  private todoService = inject(TodoService);

  todo = input.required<Todo>();

  isEditing = signal(false);
  editTitle = '';

  onToggle(): void {
    this.todoService.toggleTodo(this.todo().id);
  }

  onDelete(): void {
    this.todoService.deleteTodo(this.todo().id);
  }

  startEdit(): void {
    this.editTitle = this.todo().title;
    this.isEditing.set(true);
  }

  finishEdit(): void {
    if (this.isEditing()) {
      this.todoService.updateTodo(this.todo().id, this.editTitle);
      this.isEditing.set(false);
    }
  }

  cancelEdit(): void {
    this.isEditing.set(false);
  }
}

Step 5: Create Todo Filters Component

Create src/app/components/todo-filters/todo-filters.component.ts:

import { Component, inject } from '@angular/core';
import { TodoService } from '../../services/todo.service';
import { FilterType } from '../../models/todo.model';

@Component({
  selector: 'app-todo-filters',
  template: `
    <footer class="footer">
      <span class="todo-count">
        <strong>{{ todoService.activeCount() }}</strong>
        {{ todoService.activeCount() === 1 ? 'item' : 'items' }} left
      </span>

      <ul class="filters">
        <li>
          <a
            [class.selected]="todoService.currentFilter() === 'all'"
            (click)="setFilter('all')"
          >
            All
          </a>
        </li>
        <li>
          <a
            [class.selected]="todoService.currentFilter() === 'active'"
            (click)="setFilter('active')"
          >
            Active
          </a>
        </li>
        <li>
          <a
            [class.selected]="todoService.currentFilter() === 'completed'"
            (click)="setFilter('completed')"
          >
            Completed
          </a>
        </li>
      </ul>

      @if (todoService.completedCount() > 0) {
        <button class="clear-completed" (click)="clearCompleted()">
          Clear completed
        </button>
      }
    </footer>
  `,
  styles: [`
    .footer {
      display: flex;
      justify-content: space-between;
      align-items: center;
      padding: 10px 15px;
      color: #777;
      border-top: 1px solid #e6e6e6;
    }

    .todo-count {
      text-align: left;
    }

    .filters {
      display: flex;
      gap: 10px;
      margin: 0;
      padding: 0;
      list-style: none;
    }

    .filters a {
      padding: 3px 7px;
      border: 1px solid transparent;
      border-radius: 3px;
      cursor: pointer;
      text-decoration: none;
      color: inherit;
    }

    .filters a:hover {
      border-color: rgba(175, 47, 47, 0.1);
    }

    .filters a.selected {
      border-color: rgba(175, 47, 47, 0.2);
    }

    .clear-completed {
      padding: 0;
      border: none;
      background: none;
      cursor: pointer;
      color: inherit;
    }

    .clear-completed:hover {
      text-decoration: underline;
    }
  `]
})
export class TodoFiltersComponent {
  todoService = inject(TodoService);

  setFilter(filter: FilterType): void {
    this.todoService.setFilter(filter);
  }

  clearCompleted(): void {
    this.todoService.clearCompleted();
  }
}

Step 6: Create Todo List Component

Create src/app/components/todo-list/todo-list.component.ts:

import { Component, inject } from '@angular/core';
import { TodoService } from '../../services/todo.service';
import { TodoItemComponent } from '../todo-item/todo-item.component';

@Component({
  selector: 'app-todo-list',
  imports: [TodoItemComponent],
  template: `
    <section class="main">
      @if (todoService.allTodos().length > 0) {
        <input
          id="toggle-all"
          class="toggle-all"
          type="checkbox"
          [checked]="allCompleted()"
          (change)="toggleAll()"
        >
        <label for="toggle-all">Mark all as complete</label>
      }

      <ul class="todo-list">
        @for (todo of todoService.filteredTodos(); track todo.id) {
          <app-todo-item [todo]="todo" />
        }
      </ul>
    </section>
  `,
  styles: [`
    .main {
      position: relative;
      border-top: 1px solid #e6e6e6;
    }

    .toggle-all {
      width: 1px;
      height: 1px;
      border: none;
      opacity: 0;
      position: absolute;
    }

    .toggle-all + label {
      width: 60px;
      height: 34px;
      font-size: 0;
      position: absolute;
      top: -52px;
      left: -13px;
      transform: rotate(90deg);
    }

    .toggle-all + label:before {
      content: '❯';
      font-size: 22px;
      color: #e6e6e6;
      padding: 10px 27px;
    }

    .toggle-all:checked + label:before {
      color: #737373;
    }

    .todo-list {
      margin: 0;
      padding: 0;
      list-style: none;
    }
  `]
})
export class TodoListComponent {
  todoService = inject(TodoService);

  allCompleted(): boolean {
    const todos = this.todoService.allTodos();
    return todos.length > 0 && todos.every(todo => todo.completed);
  }

  toggleAll(): void {
    this.todoService.toggleAll();
  }
}

Step 7: Update Main App Component

Update src/app/app.component.ts:

import { Component, inject } from '@angular/core';
import { TodoFormComponent } from './components/todo-form/todo-form.component';
import { TodoListComponent } from './components/todo-list/todo-list.component';
import { TodoFiltersComponent } from './components/todo-filters/todo-filters.component';
import { TodoService } from './services/todo.service';

@Component({
  selector: 'app-root',
  imports: [
    TodoFormComponent,
    TodoListComponent,
    TodoFiltersComponent
  ],
  template: `
    <div class="todoapp">
      <header class="header">
        <h1>todos</h1>
        <app-todo-form />
      </header>

      @if (todoService.allTodos().length > 0) {
        <app-todo-list />
        <app-todo-filters />
      }
    </div>

    <footer class="info">
      <p>Double-click to edit a todo</p>
      <p>Created with Angular Signals</p>
    </footer>
  `,
  styles: [`
    :host {
      display: block;
      font-family: 'Helvetica Neue', Helvetica, Arial, sans-serif;
      font-size: 14px;
      color: #4d4d4d;
      background: #f5f5f5;
      min-height: 100vh;
    }

    .todoapp {
      background: #fff;
      margin: 130px auto 40px;
      max-width: 550px;
      box-shadow: 0 2px 4px 0 rgba(0, 0, 0, 0.2),
                  0 25px 50px 0 rgba(0, 0, 0, 0.1);
    }

    .header h1 {
      font-size: 100px;
      font-weight: 100;
      text-align: center;
      color: rgba(175, 47, 47, 0.15);
      margin: 0;
      padding: 20px 0;
    }

    .info {
      margin: 65px auto 0;
      color: #bfbfbf;
      font-size: 10px;
      text-align: center;
    }

    .info p {
      line-height: 1;
    }
  `]
})
export class AppComponent {
  todoService = inject(TodoService);
}

Step 8: Update Global Styles

Update src/styles.css:

* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}

body {
  margin: 0;
  font-family: 'Helvetica Neue', Helvetica, Arial, sans-serif;
  font-size: 14px;
  line-height: 1.4em;
  background: #f5f5f5;
  color: #4d4d4d;
  font-weight: 300;
}

button {
  margin: 0;
  padding: 0;
  border: 0;
  background: none;
  font-size: 100%;
  vertical-align: baseline;
  font-family: inherit;
  font-weight: inherit;
  color: inherit;
}

button:focus {
  outline: none;
}

🚀 Running the Application

# Start development server
ng serve

# Open browser to
http://localhost:4200

✅ Testing Your App

Try these features:

1. Add todos - Type and press Enter
2. Toggle completion - Click checkbox
3. Edit todo - Double-click on todo text
4. Delete todo - Click × button
5. Filter todos - Click All/Active/Completed
6. Toggle all - Click the down arrow
7. Clear completed - Click “Clear completed” button
8. Refresh page - Data persists in localStorage!

🎯 Key Concepts Demonstrated

✅ Angular Best Practices

• Standalone components - No NgModules needed
• Signals for state - Reactive state management
• Computed values - Derived state automatically updates
• inject() function - Modern dependency injection
• input() and output() - Modern component API
• Native control flow - @if, @for instead of directives

✅ TypeScript Best Practices

• Strict mode - Type safety everywhere
• Interfaces - Clear data structures
• Type inference - Let TypeScript infer types
• Readonly signals - Expose read-only state

✅ Architecture Patterns

• Service layer - Business logic separated
• Component composition - Small, focused components
• Unidirectional data flow - Predictable state changes
• LocalStorage persistence - Data survives refresh

🚀 Next Steps

Enhance your Todo app with:

1. Add categories/tags to todos
2. Add due dates with date picker
3. Add priority levels (high, medium, low)
4. Add search functionality
5. Add animations with Angular animations
6. Add dark mode toggle
7. Add backend API integration
8. Add user authentication
9. Add unit tests with Vitest (the default test runner since Angular 19)
10. Deploy to production (Netlify, Vercel, etc.)

📚 Related Topics

• Signals Guide
• Reactive Forms
• Services & DI
• Component Communication
• Testing

---

Congratulations! 🎉 You’ve built a complete Todo app using modern Angular best practices! This project demonstrates signals, standalone components, and clean architecture patterns.