Transform javascript array of objects

Question

Could you please suggest any shorter code to solve following problem. I have array of objects:

const arr1=[
  { '1': { grade: 1.3, counter: 2 } },
  { '1': { grade: 2.8, counter: 2 } },
  { '2': { grade: 4.5, counter: 1 } },
  { '2': { grade: 2.4, counter: 1 } }
]

the output should look like:

const obj1={ 
             '1': {grade:4.1,counter:4}
             '2': {grade:6.9,counter:2}
}

here is the code i have tried :

arr1.reduce((acc,e)=> {
  let element = Object.keys(e)
  if(!acc.hasOwnProperty(element)){
    acc[element]={grade:0,counter:0}

  }
  acc[element].grade+= e[element].grade
  acc[element].counter+= e[element].counter
  return acc
}, {})

Thank you

You could remove the empty line (or even all white spaces) to create a shorter code. — user16540390
– user16540390, Commented Apr 30, 2022 at 12:45

RenaudC5 · Accepted Answer · 2022-04-30 12:57:12Z

Your problem here is that Object.keys(e) return an array of keys and not a single key.

If your object will always be the same (with one key) the you can get the key with : let element = Object.keys(e)[0]

I've decomposed the code with 2 main parts.

Getting the key of the current item (with const key = Object.keys(current)[0])
Checking if the newObject has the key
If yes : updating the grade and counter value with the current object
If no : adding the current object to the key

const arr1 = [{
    '1': {
      grade: 1.3,
      counter: 2
    }
  },
  {
    '1': {
      grade: 2.8,
      counter: 2
    }
  },
  {
    '2': {
      grade: 4.5,
      counter: 1
    }
  },
  {
    '2': {
      grade: 2.4,
      counter: 1
    }
  }
]

const newObject = arr1.reduce((newObject, current) => {
  const key = Object.keys(current)[0]
  const associatedObject = newObject[key]
  if (associatedObject) {
    associatedObject.grade += current[key].grade
    associatedObject.counter += current[key].counter
  } else {
    newObject[key] = current[key]
  }
  
  return newObject
}, {})

console.log(newObject)

Thomas Junk · Accepted Answer · 2022-04-30 13:22:02Z

1

I would go for:

arr1.reduce((o,n)=>{
  Object.keys(n).forEach((key)=>{
    if(!o[key]) o[key]={grade:0, counter:0};
    o[key]["grade"] += n[key]["grade"];
    o[key]["counter"] += n[key]["counter"]; 
  });
 return o;
},{});

Step1: reducing over the array, acc = {}

Step2: iterating over every key of the object at the current index

Step3: summing up the according properties

You forgot to iterate over the keys.

Comment: As a pons asinorum I use for array.reduce() the variable names:

o meaning old value (=acc=_accumulator)
n meaning new value.

edited Apr 30, 2022 at 13:22

answered Apr 30, 2022 at 13:12

Thomas Junk

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Comments

ruleboy21 · Accepted Answer · 2022-04-30 14:08:55Z

1

Try this

let arr1=[{'1': {grade: 1.3, counter: 2}},{'1': {grade: 2.8, counter: 2}},{'2': {grade: 4.5, counter: 1}},{'2': {grade: 2.4, counter: 1}}];

arr1 = arr1.reduce((acc, obj) => {
    let [k, v] = Object.entries(obj)[0];
    if(!acc.hasOwnProperty(k)) acc[k] = {grade:0,counter:0};
    acc[k].grade += v.grade;
    acc[k].counter += v.counter;
    return acc
}, {});

console.log(arr1)

edited Apr 30, 2022 at 14:08

answered Apr 30, 2022 at 13:06

ruleboy21

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Comments

k.tten · Accepted Answer · 2022-04-30 12:58:18Z

Object.keys returns an array of strings, so to check here you would probably want the first one:

arr1.reduce((acc, e) => {
  const element = Object.keys(e)[0]; // `[0]` accesses the first key
  if(!acc.hasOwnProperty(element)){
    acc[element]={grade:0,counter:0}
  }
  acc[element].grade+= e[element].grade
  acc[element].counter+= e[element].counter
  return acc
}, {});

Then instead of the if statement you can use the new logical nullish assignment operator:

arr1.reduce((acc, e) => {
  const element = Object.keys(e)[0];
  
  acc[element] ??= { grade: 0, counter: 0 };

  acc[element].grade += e[element].grade;
  acc[element].counter += e[element].counter;
  return acc
}, {});

This is probably as short as I'd go before it starts getting pointless.

Even further; using nullish coalescing and optional chaining:

arr1.reduce((acc, e) => {
  const element = Object.keys(e)[0];
  
  acc[element] = {
    grade: (acc[element]?.grade ?? 0) + e[element].grade,
    counter: (acc[element?.counter ?? 0) + e[element].counter,
  };

  return acc
}, {});

With Object.assign to do it all in one line:

arr1.reduce((acc, e) => {
  const element = Object.keys(e)[0];

  return Object.assign(acc, {
    [element]: {
      grade: (acc[element]?.grade ?? 0) + e[element].grade,
      counter: (acc[element?.counter ?? 0) + e[element].counter,
    },
  });
}, {});

Finally, just for kicks, let's inline the element variable altogether to get this amazing line:

arr1.reduce((acc, e) => Object.assign(acc, {
    [Object.keys(e)[0]]: {
      grade: (acc[Object.keys(e)[0]]?.grade ?? 0) + e[Object.keys(e)[0]].grade,
      counter: (acc[Object.keys(e)[0]]?.counter ?? 0) + e[Object.keys(e)[0]].counter,
    },
}), {});

Rodrigo Rennó · Accepted Answer · 2022-04-30 13:55:05Z

0

Another solution is:

const obj1 = arr1.reduce((acc, value) => {
    if(acc[Object.keys(value)[0]]) {
     acc[Object.keys(value)[0]].grade = acc[Object.keys(value)[0]].grade + value[Object.keys(value)[0]].grade;
     acc[Object.keys(value)[0]].counter = acc[Object.keys(value)[0]].counter + value[Object.keys(value)[0]].counter;
    } else {
     acc[Object.keys(value)[0]] = value[Object.keys(value)[0]];
    }
     return acc;    
})

answered Apr 30, 2022 at 13:55

Rodrigo Rennó

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