为什么我得到 Uncaught (in promise) TypeError: Cannot read property 'type' of undefined？

Question

Here is my initial code I wrote to console log the data I want

const fetchPokemon2 = () =>
  {
  for (let i = 1; i <= 151; i++) 
    {
    const url = `https://pokeapi.co/api/v2/pokemon/${i}`;
    fetch(url)
      .then( res => res.json())
      .then( data => 
        {
        console.log(data);
        const pokemon = {}

        if (data.types['1'])
          {
          pokemon['name']   = data.name;
          pokemon['id']     = data.id;
          pokemon['sprite'] = data.sprites['front_default'];
          pokemon['type1']  = data.types["0"].type.name;
          pokemon['type2']  = data.types["1"].type.name;   
          } 
        else 
          {
          pokemon['name']   = data.name;
          pokemon['id']     = data.id;
          pokemon['sprite'] = data.sprites['front_default'];
          pokemon['type1']  = data.types["0"].type.name;
          }
        console.log(pokemon);
        })
    }
  }
fetchPokemon2();

It worked fine but this isnt the recommended way so I'm trying to do this

const fetchPokemon = () =>
  {
  const promises = [];
  for (let i = 1; i <= 151 ; i++) 
    {
    const url = `https://pokeapi.co/api/v2/pokemon/${i}`
    promises.push(fetch(url).then(resp => resp.json()));
    };
  Promise.all(promises).then( results => 
    {
    const pokemon = results.map(data => 
      ( { name   : data.name
        , id     : data.id
        , sprite : data.sprites["front_default"]
        , type   : data.types["0"].type.name
        , type2  : data.types["1"].type.name
        }
      ));
    console.log(pokemon);
    });
  };
fetchPokemon()

However this line of code

type2: data.types["1"].type.name
is giving the error Uncaught (in promise)
TypeError: Cannot read property 'type' of undefined

which is confusing me a lot because this type: data.types["0"].type.name is perfectly fine could anyone help me out with this?

Answer 1

This happens when there is no value for the specified array or object index. That means data.types[1] is probably undefined, hence the error Cannot read property 'type' of undefined for data.types[1].type...

In the second code sample, you aren't checking if data.types[1] exists. You can always access the value safely by doing:

...
type2: data.types[1] && data.types[1].type && data.types[1].type.name
...

Answer 2

In the API response, some of the values have only 1 element in the types array. That could be the issue. You need to check if data exists or not before assigning them.

type2: data.types[1] && data.types[1].type && data.types[1].type.name