Reason 1: You're Rendering an Iterator, Not an Array

This is the absolute core of the issue. When you call myMap.entries(), it does not return an array. It returns a MapIterator object.

Let's look at what that means. Consider this simple map:

const userRoles = new Map([
  ['admin', 'Administrator'],
  ['editor', 'Content Editor'],
  ['viewer', 'Read-Only User']
]);

console.log(userRoles.entries());
// Output: MapIterator {["admin", "Administrator"], ["editor", "Content Editor"], ["viewer", "Read-Only User"]}
    

React is incredibly smart about rendering lists from arrays. When it sees an array of strings, numbers, or JSX elements, it knows exactly what to do: render them in order. However, when you pass it a MapIterator object, React doesn't know how to render it. It sees an object, not a list of renderable items, and effectively renders nothing.

Your React component might look like this, which is the source of the problem:

// The WRONG way
function RoleList({ roles }) {
  return (
    

      
User Roles
      

        {/* This returns an iterator, which React can't render! */}
        {roles.entries()}
      
    
  );
}
    

The result? A blank <ul> tag. The browser gets the iterator object but has no instructions on how to display it as list items.

Reason 2: You Forgot to Convert the Iterator

Now that we know the problem is the iterator object, the solution becomes clear: we need to convert that iterator into an array that React understands. Luckily, modern JavaScript gives us a couple of elegant ways to do this.

Solution A: The Spread Syntax (...)

The spread syntax (...) is the most common and arguably the most readable way to convert an iterable (like a MapIterator) into an array. You simply wrap the expression in square brackets and prepend it with three dots.

// The CORRECT way with Spread Syntax
function RoleList({ roles }) {
  // 1. Convert the iterator to an array of [key, value] pairs
  const roleArray = [...roles.entries()];

  return (
    

      
User Roles
Advertisement

      

        {/* 2. Now map over the newly created array */}
        {roleArray.map(([key, value]) => (
          
{key}: {value}
        ))}
      
    
  );
}
    

Solution B: Array.from()

Another excellent method is the Array.from() static method. It creates a new, shallow-copied Array instance from an array-like or iterable object. The functionality is identical to the spread syntax in this context, and it's sometimes preferred for its explicit nature.

// The CORRECT way with Array.from()
function RoleList({ roles }) {
  // Convert using Array.from()
  const roleArray = Array.from(roles.entries());

  return (
    

      
User Roles
      

        {roleArray.map(([key, value]) => (
          
{key}: {value}
        ))}
      
    
  );
}
    

Both methods achieve the same goal. They transform the unusable MapIterator into a render-friendly Array.

Reason 3: Incorrect Destructuring in Your Loop

So, you've successfully converted your iterator to an array. Great! But you might still see strange output if you don't handle the array's structure correctly. Remember, an array created from map.entries() is an array of arrays, where each inner array is a [key, value] pair.

A common mistake is to forget to destructure this pair in the .map() callback.

The Incorrect Way (Without Destructuring)

// This will try to render the inner array object
[...roles.entries()].map(item => (
  // React can't render an array directly inside a 

  // You might see an error or weird string output.
  
{item}
 
))
    

React will throw an error here because you're trying to render an array (e.g., ['admin', 'Administrator']) as a child of a React element. You must access the elements inside that array.

The Correct Way (With Destructuring)

By using array destructuring right in the arguments of the map callback, you can assign the key and value to their own variables in one clean step. This makes your code more readable and directly gives you the values you need.

// Destructuring [key, value] directly in the map callback
[...roles.entries()].map(([key, value]) => (
  // Now you have direct access to key and value!
  
{value}
))
    

Crucially, notice the key={key} prop. React requires a unique key prop for each item in a list to efficiently track changes. Since keys in a Map are guaranteed to be unique, they are the perfect candidate for React's key prop!

Reason 4: The Sanity Check - Your Map is Actually Empty

Sometimes the simplest explanation is the right one. Before you tear your hair out debugging iterators and destructuring, take a moment for a quick sanity check: is there anything in your Map to begin with?

This often happens when data is fetched asynchronously or depends on user input. The component might render for the first time before the Map has been populated. Your iteration logic could be perfect, but you can't render data that doesn't exist yet.

Here’s how to quickly debug this:

• Log the size: Before your return statement, add a console.log(myMap.size). If it prints 0, you've found your culprit.
• Conditional Rendering: A robust solution is to only render the list if the map has entries. This prevents errors and provides a better user experience, allowing you to show a loading or empty state.

function UserList({ usersMap }) {
  // Sanity check!
  if (usersMap.size === 0) {
    return 
No users found.
;
  }

  return (
    

      {[...usersMap.entries()].map(([id, user]) => (
        
{user.name}
      ))}
    
  );
}
    

Reason 5: The Silent Killer - State Mutability

This is a more advanced React-specific issue. If your Map is stored in component state, you might be updating it incorrectly. React's re-rendering process relies heavily on detecting changes in state and props. For objects and arrays, it does this with a shallow comparison.

If you mutate the Map object directly, React won't see a change because the object's reference in memory is still the same. Therefore, it won't trigger a re-render, and your UI will remain stuck on the old version, which might have been blank.

Incorrect: Mutating State Directly

// This will NOT trigger a re-render in React
const [myMap, setMyMap] = useState(new Map());

function addUser() {
  // DANGER: Mutating the existing map object
  myMap.set('newUser', { name: 'Alice' });
  // The reference to myMap hasn't changed, so React sees no update!
  setMyMap(myMap); 
}
    

Correct: Creating a New Map for Updates

The solution is to treat state as immutable. Whenever you want to update the Map, you must create a new Map instance with the updated data. This gives React a new object reference, which it will detect as a change, triggering a re-render.

// This WILL trigger a re-render
const [myMap, setMyMap] = useState(new Map());

function addUser() {
  // Create a new Map, copying the old one's values
  const newMap = new Map(myMap);
  // Update the new map
  newMap.set('newUser', { name: 'Alice' });
  // Set the new map in state, giving React a new reference
  setMyMap(newMap);
}
    

This immutable pattern is fundamental to writing predictable and bug-free React applications.