React - Functions are not valid as a React child - reactjs

Question

I am new to react this is my first application.
I am calling one component inside to another component then those function a moved to app.js
//app.js
class App extends React.Component {
state = {
todos:[
{id:1, title:'get haircut',completed: false},
{id:2, title:'learn react',completed: false},
{id:3, title:'chaaa',completed: false},
]
}
markComplete=(id) =>{
this.setState({
todos: this.state.todos.map((myTodo)=>{
if(myTodo.id === id ){
myTodo.completed = !myTodo.completed;
}
return myTodo
})
})
};
deleteTodo =(id) =>{
this.setState({
todos: [...this.state.todos.filter((myTodo) =>{
return myTodo !==id
})]
})
}
render(){
return (
<div className="App">
<Header/>
<RetrivedTodos todos={this.state.todos}
markComplete={this.markComplete}
deleteTodo={this.deleteTodo}
/>
</div>
);
}
}
//RetrivedTodos.js
class RetrivedTodos extends Component {
render () {
return this.props.todos.map((retrivedTodos) =>(
<TodosItems key={retrivedTodos.id} todos={retrivedTodos}
markComplete={this.props.markComplete}
deleteTodo={this.props.deleteTodo}
/>
))
}
}
//TodoItems.js
class TodosItems extends Component {
getStrikeMark = () => {
return {
textDecoration:this.props.todos.Completed ? 'line-through': 'none'
}
}
render () {
const { id , title } = this.props.todos
return (
<div className='main-todos-div' style={this.getStrikeMark()}>
<div className='todo-div'>
<input type="checkbox" className='checkbox-round'
onChange={this.props.markComplete.bind(this,id)}/>
<span>{title}</span>
</div>
<div className='btn-div'>
<button onClick={this.props.deleteTodo.bind(this,id)}>
<i className="fas fa-trash"></i>
</button>
</div>
</div>
)
}
}
//header
class Header extends Component {
render () {
const date= new Date();
const todayDate = date.getDate();
const month = date.toLocaleString('default',{month:'long'});
const year = date.getFullYear;
const day = date.toLocaleDateString('default',{weekday:'long'});
return(
<div className='main-header-div'>
<div className='background-div'> </div>
<div className='date-month-div'> </div>
<span>{todayDate}</span>
<span>{month}</span>
<span>{year}</span>
<span>{day}</span>
</div>
)
}
}
What is the problem here? It shows this error
Warning: Functions are not valid as a React child. This may happen if
you return a Component instead of from render. Or maybe
you meant to call this function rather than return it.
thanks in advance

Answer 1

Check the sandbox link:
https://codesandbox.io/s/affectionate-goodall-mh0t7?file=/src/Header.js
The problem is with Header componentnt, it should be :
const year = date.getFullYear();
instead of
const year = date.getFullYear;
getFullYear is a function, that's the reason you were getting the error.

Answer 2

RetrivedTodos seems invalid to me. You are returning a map function instead of a React component. This map function should be executed inside the return value instead of being the return value itself.
Here is how it should look like:
class RetrivedTodos extends Component {
render () {
return (
<div>
{this.props.todos.map((retrivedTodos) => (
<TodosItems key={retrivedTodos.id} todos={retrivedTodos}
markComplete={this.props.markComplete}
deleteTodo={this.props.deleteTodo}
/>
))
}
</div>
)
}
EDIT: Inside Header you are returning a function instead of it's value:
const year = date.getFullYear;
Should be:
const year = date.getFullYear();