Why is the accumulator.push not valid? accumulator is an empty array!? If i use the predefined variable flarArray, things work just fine!! I know that if no initial value is included in the reduce callback, then it will use the first item in the array (which in my case will be an object) to be the accumulator, but if I tell it to start off as an array as the accumulator, what is the problem with that?

const profiles = [
  {
    id: 1,
    userID: '1',
    favoriteMovieID: '1',
  },
  {
    id: 2,
    userID: '2',
    favoriteMovieID: '1',
  }
];

const users = {
  1: {
    id: 1,
    name: 'Jane Cruz',
    userName: 'coder',
  },
  2: {
    id: 2,
    name: 'Matthew Johnson',
    userName: 'mpage',
  }
};

const movies = {
  1: {
    id: 1,
    name: 'Planet Earth 1',
  },
  2: {
    id: 2,
    name: 'Selma',
  }
};
let flatArray = [];
const flattenedProfiles = profiles.reduce(function(accumulator, currentProfile) {  
  return accumulator.push({id: currentProfile.id, name: users[currentProfile.id].name, favoriteMovie: movies[currentProfile.favoriteMovieID].name})
},[]);

Why is the accumulator.push not valid? accumulator is an empty array!? If i use the predefined variable flarArray, things work just fine!! I know that if no initial value is included in the reduce callback, then it will use the first item in the array (which in my case will be an object) to be the accumulator, but if I tell it to start off as an array as the accumulator, what is the problem with that?

const profiles = [
  {
    id: 1,
    userID: '1',
    favoriteMovieID: '1',
  },
  {
    id: 2,
    userID: '2',
    favoriteMovieID: '1',
  }
];

const users = {
  1: {
    id: 1,
    name: 'Jane Cruz',
    userName: 'coder',
  },
  2: {
    id: 2,
    name: 'Matthew Johnson',
    userName: 'mpage',
  }
};

const movies = {
  1: {
    id: 1,
    name: 'Planet Earth 1',
  },
  2: {
    id: 2,
    name: 'Selma',
  }
};
let flatArray = [];
const flattenedProfiles = profiles.reduce(function(accumulator, currentProfile) {  
  return accumulator.push({id: currentProfile.id, name: users[currentProfile.id].name, favoriteMovie: movies[currentProfile.favoriteMovieID].name})
},[]);

• javascript
• arrays

Share Improve this question Follow asked May 28, 2020 at 13:41 AJSwiftAJSwift 72944 gold badges1414 silver badges2626 bronze badges 1

• 3 The push() method returns the new length of the array, so in your callback you're returning an integer instead of the accumulator. You need to change return accumulator.push({ ... }) into accumulator.push({ ... }); return accumulator; – Lennholm Commented May 28, 2020 at 13:45

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3 Answers 3

Sorted by: Reset to default 4

You need to return accumulator, instead you are returning the return value of push function which is the size of the array after adding the new item. In next iteration, you are calling push function on an integer

const flattenedProfiles = profiles.reduce((acc, curr) => {  
  acc.push({
     id: curr.id,
     name: users[curr.id].name,
     favoriteMovie: movies[curr.favoriteMovieID].name
  });

  return acc;
},[]);

Array.push returns the new length of the array when inside the return of callback of reduce you are returning the new length on which you can't do next push operation. Use Array.concat instead or after Array.push also return the array.

const flattenedProfilesPush = profiles.reduce((acc, curr) => {  
    acc.push({
        id: curr.id,
    })
    return acc
},[])


const flattenedProfilesConcat = profiles.reduce((acc, curr) =>  
    acc.concat({
        id: curr.id,
    }),[]
)

The problem is you return accumulator.push(). This returns the length of the array, so your next accumulator is gonna be a number instead of an array.

I would better return an array and use spread operator on the accumulator.

const profiles = [
  {
    id: 1,
    userID: '1',
    favoriteMovieID: '1',
  },
  {
    id: 2,
    userID: '2',
    favoriteMovieID: '1',
  }
];

const users = {
  1: {
    id: 1,
    name: 'Jane Cruz',
    userName: 'coder',
  },
  2: {
    id: 2,
    name: 'Matthew Johnson',
    userName: 'mpage',
  }
};

const movies = {
  1: {
    id: 1,
    name: 'Planet Earth 1',
  },
  2: {
    id: 2,
    name: 'Selma',
  }
};
let flatArray = [];
const flattenedProfiles = profiles.reduce(function(accumulator, currentProfile) {  
  return [...accumulator, {id: currentProfile.id, name: users[currentProfile.id].name, favoriteMovie: movies[currentProfile.favoriteMovieID].name}]
},[]);

console.log(flattenedProfiles);