Reactjs map works but forEach doesn't

Question

I'm struggling to understand the difference between forEach and map. In the following render function if the 'forEach' is replaced with 'map' it works. I don't understand why it doesn't work with the 'forEach'. Both {item.id} and {item.text} are present with both methods. So, why are the props for 'TodoItem' not being set when using 'forEach' ?

render() {    
  return(

     <ul>
        {this.props.items.forEach(function(item) {

           return (
              <TodoItem id={item.id} text={item.text} />)
        })} 
     </ul>
  );
}

So if 'forEach' doesn't return anything how come this doesn't work either:

render() {    
  return(

     <ul>
        {this.props.items.forEach(function(item) {               

              <TodoItem id={item.id} text={item.text} />
        })} 
     </ul>
  );
}

Answer 1

The map function returns an array of items and forEach just loop over them. To make this code work use :

render() {    
  const items = [];
  this.props.items
    .forEach(item => items.push(
                       <li>
                          <TodoItem id={item.id} key={item.id} text={item.text} />
                       </li>
                     ))

  return(
     <ul>{items}</ul>
  );
}

Answer 2

Try this simple example for understand why forEach doesn't work in this context:

[1,2,3].forEach((n)=> n); => returns undefined

[1,2,3].map((n)=> n); => returns [1,2,3]

Answer 3

As @Nenad Vracar mentioned map will actually return stuff. If you wanted to do somethings to another array, object or piece of code you could use forEach. But since you want to return something that ends up being shown on the DOM. Use map.

Also, don't forget to return whatever you're mapping. It's a common mistake because you don't need to use the return for forEach.

Answer 4

Basically map returns an array while forEach returns nothing,

in jsx/react context you need to return a list of components/node-tags that the parser will transform in nodes both in the real and virtual dom;

working by side-effect like forEach does you won't have anything to parse.