为什么我们在承诺链中使用 return ，即使我们已经在函数中返回了？

Question

I'm new to JavaScript. In the following code, may I know why I still have to use return getRecipe(IDs[2]) instead of just call getRecipe(IDs[2]) in the .then method? Even getRecipe() already have return new Promise inside it? I find that I'll get an undefined error if I don't use return in the .then method. Is the return actually return the promise we get to the next then? But why and how? Thank you so much!

const getIDs = new Promise((resolve, reject) => {
  setTimeout(() => {
    resolve([523, 883, 432, 974]);
  }, 1500);
});

const getRecipe = recID => {
  return new Promise((resolve, reject) => {
    setTimeout(
      ID => {
        const recipe = { title: 'Fresh tomato pasta', publisher: 'Jonas' };
        resolve(`${ID} : ${recipe.title}`);
      },
      1500,
      recID
    );
  });
};

getIDs
  .then(IDs => {
    console.log(IDs);
    return getRecipe(IDs[2]);
  })
  .then(recipe => {
    console.log(recipe);
  })
  .catch(error => {
    console.log('Error!!');
  });

Answer 1

In a chain of .then statements, when you return something from a .then, it goes to the next .then, if there is one. In this case, we are using .then statements to do multiple tasks, and the first task is to get a recipe according to some ID. Once this recipe is received (as a result of the getRecipe function) we return it into the next .then, which has the task of console.log'ing the recipe. If we did not return the getRecipe(ID[2]) result, we would have no 'recipe' parameter of the next .then statement