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When we call the method on a value that is not an array map(), we get the error “TypeError: map is not a function”. To fix this error, we need to console.logkeep track of the value we are calling the map() method on and make sure we only call map on valid arrays.

Below is sample code that produces the error.

const App = () => {
  const obj = {};

  // ⛔️ Uncaught TypeError: map is not a function

  return (
    <div>
      {obj.map(element => {
        return <h2>{element}</h2>;
      })}
    </div>
  );
};

export default App;

We called Array.map()a method on an object and an error occurred.

To fix this error, we need to use console.logto log the value we call the map method on and make sure it is a valid array.

export default function App() {
  const arr = ['one', 'two', 'three'];

  return (
    <div>
      {arr.map((element, index) => {
        return (
          <div key={index}>
            <h2>{element}</h2>
          </div>
        );
      })}
    </div>
  );
}

We can use Array.isArraythe method to conditionally check if a value is an array.

const App = () => {
  const obj = {};

  return (
    <div>
      {Array.isArray(obj)
        ? obj.map(element => {
            return <h2>{element}</h2>;
          })
        : null}
    </div>
  );
};

export default App;

We used a ternary operator, which is very similar to an if/else statement.

If the value is an array, we return the result of calling its map method, otherwise we return null. This way, we don’t get an error message even if the value is not an array.

If the value is being fetched from a remote server, make sure it is the type we expect by logging it to the console, and make sure it is parsed as a native JavaScript array before calling the map method on it.

If we have an array-like object and trying to convert it into an array before calling the map method, use Array.from()the method.

const App = () => {
const set = new Set(['one', 'two', 'three']);

return (
  <div>
    {Array.from(set).map(element => {
      return (
        <div key={element}>
          <h2>{element}</h2>
        </div>
      );
    })}
  </div>
);
};

export default App;

Before calling the map method, we convert the value to an array. This also works for array-like objects, such as getElementsByClassNamea NodeList returned by calling the map method.

If we try to iterate over an object, use Object.keys()the method to get an array of the object's keys, on which we can call the map() method.

export default function App() {
  const employee = {
    id: 1,
    name: 'Alice',
    salary: 100,
  };

  return (
    <div>
      {/* 👇️ 迭代对象的 KEYS */}
      {Object.keys(employee).map((key) => {
        return (
          <div key={key}>
            <h2>
              {key}: {employee[key]}
            </h2>

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

      <br />
      <br />
      <br />

      {/* 👇️ 迭代对象的 VALUES */}
      {Object.values(employee).map((value, index) => {
        return (
          <div key={index}>
            <h2>{value}</h2>

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

We use Object.keysthe method to get an array of the object's keys.

const employee = {
  id: 1,
  name: 'Alice',
  salary: 100,
};

// 👇️ ['id', 'name', 'salary']
console.log(Object.keys(employee));

// 👇️ [1, 'Alice', 100]
console.log(Object.values(employee));

We can only call the map() method on arrays, so we need to get either an array of object keys or an object's values.