JIYIK CN >

map()Rendering a nested array using :

1. Use map()the method to iterate over the outer array.
2. On each iteration, the nested array's map()method is called.
3. Renders the elements of a nested array.

export default function App() {
  const people = [
    {id: 1, name: 'Alice', pets: ['dog', 'cat']},
    {id: 2, name: 'Bob', pets: ['turtle', 'rabbit']},
    {id: 3, name: 'Carl', pets: ['hamster', 'parrot']},
  ];

  return (
    <div>
      {people.map((person, index) => {
        return (
          <div key={index}>
            <h2>Name: {person.name}</h2>

            {person.pets.map((pet, index) => {
              return (
                <div key={index}>
                  <h2>Pet: {pet}</h2>
                </div>
              );
            })}

            <hr />
          </div>
        );
      })}
    </div>
  );
}

The function we pass to Array.mapthe method will be called with each element in the array and the index of the current iteration.

In each iteration, we render the name property of the person object and use nested map()to iterate over the pets array for each person.

注意, when calling a method in JSX code map(), we must use curly braces {}to wrap map()the call to .

This is only needed in our JSX code and signals to React that we are writing an expression that must be evaluated.

We map()have used arrow functions with explicit return statements in both calls to the method. If we only need to render some JSX elements and don’t use conditionals, declare variables, etc., we can use implicit returns, which will make our code more readable.

export default function App() {
  const people = [
    {id: 1, name: 'Alice', pets: ['dog', 'cat']},
    {id: 2, name: 'Bob', pets: ['turtle', 'rabbit']},
    {id: 3, name: 'Carl', pets: ['hamster', 'parrot']},
  ];

  return (
    <div>
      {people.map((person, index) => (
        <div key={index}>
          <h2>Name: {person.name}</h2>

          {person.pets.map((pet, index) => (
            <div key={index}>
              <h2>Pet: {pet}</h2>
            </div>
          ))}

          <hr />
        </div>
      ))}
    </div>
  );
}

map()We used implicit returns on both arrow functions passed to the method.

const arr = ['a', 'b', 'c'];

// 👇️ explicit return
const result1 = arr.map(element => {
  return element;
});

// 👇️ implicit return
const result2 = arr.map(element => element);

map()We can only use implicit returns when we don't have to use conditionals or define variables in the function passed to .

We used the index of the key prop in the example, but if you have a stable unique identifier, it is better to use it. We can use the id property on each object.

React uses the key prop internally for performance reasons. It helps the library ensure that only array elements that have changed are re-rendered.

That being said, unless we are dealing with thousands of array elements we won't see any noticeable difference.