Use reduce to summarize values of a specific key in javascript object

Question

Goal: to return an array listing the unique values for one specific key within an object.

const people = [
    {id: 1, name: 'Barb', gender: 'female'},
    {id: 2, name: 'Carl', gender: 'male'},
    {id: 3, name: 'John', gender: 'male'},
    {id: 4, name: 'Jane', gender: 'female'},
    {id: 5, name: 'Pete', gender: 'male'},
    {id: 6, name: 'Annie', gender: 'female'},
    {id: 7, name: 'Katie', gender: 'female'},
    {id: 8, name: 'Sally', gender: 'female'},
    {id: 9, name: 'Steve', gender: 'male'},
]

Desired output:

['male','female']

mr hr · Answer 1 · 2020-12-21T19:10:41.470

You could use Set Object to return an array listing the unique values for one specific key within an object.

Create new Set.
Traverse the array using forEach() method.
Check the specified property and add the value to the Set
At last, return the unique values array.

const people = [
  { id: 1, name: 'Barb', gender: 'female' },
  { id: 2, name: 'Carl', gender: 'male' },
  { id: 3, name: 'John', gender: 'male' },
  { id: 4, name: 'Jane', gender: 'female' },
  { id: 5, name: 'Pete', gender: 'male' },
  { id: 6, name: 'Annie', gender: 'female' },
  { id: 7, name: 'Katie', gender: 'female' },
  { id: 8, name: 'Sally', gender: 'female' },
  { id: 9, name: 'Steve', gender: 'male' },
];
const unique = (key, array) => {
  const set = new Set();
  array.forEach(
    (x) => Object.prototype.hasOwnProperty.call(x, key) && set.add(x[key])
  );
  return [...set];
};

console.log(unique('gender', people));

score 0 · Answer 2 · edited Jan 03 '21 at 10:22

The category key can be reduced to unique values using reduce() and Set, like this:

Returns: ['female', 'male']

const people = [
    {id: 1, name: 'Barb', gender: 'female'},
    {id: 2, name: 'Carl', gender: 'male'},
    {id: 3, name: 'John', gender: 'male'},
    {id: 4, name: 'Jane', gender: 'female'},
    {id: 5, name: 'Pete', gender: 'male'},
    {id: 6, name: 'Annie', gender: 'female'},
    {id: 7, name: 'Katie', gender: 'female'},
    {id: 8, name: 'Sally', gender: 'female'},
    {id: 9, name: 'Steve', gender: 'male'},
]

const genders = people.reduce((aSex, currItem) => {
   aSex.push(currItem.gender);
   return aSex
}, [])

let unique = [...new Set(genders)];

console.log(unique)

Similarly, the number of instances of each unique value within the key "category" can be returned like this:

Returns: {"female": 3, "male": 4}

const people = [
    {id: 1, name: 'Barb', gender: 'female'},
    {id: 2, name: 'Carl', gender: 'male'},
    {id: 3, name: 'John', gender: 'male'},
    {id: 4, name: 'Jane', gender: 'female'},
    {id: 5, name: 'Pete', gender: 'male'},
    {id: 6, name: 'Annie', gender: 'female'},
    {id: 7, name: 'Katie', gender: 'female'},
    {id: 8, name: 'Sally', gender: 'female'},
    {id: 9, name: 'Steve', gender: 'male'},
]

const genderCount = people.reduce((oSex, currItem) => {
   oSex[currItem.gender] = (oSex[currItem.gender] || 0) + 1
   return oSex
}, {})

console.log(genderCount)

References:

Nina Scholz' one-line solution to the first example

https://www.freecodecamp.org/news/reduce-f47a7da511a9/#creating-a-tally-with-the-reduce-method-in-javascript-

https://wsvincent.com/javascript-remove-duplicates-array/

Why not simply `const uniqueGenders = [...new Set(people.map(({gender}) => gender))]` for the first scenario? — Yevhen Horbunkov, Dec 21 '20 at 18:22
@NinaScholz Since you asked that question I've been trying to work out how to do that.... What would that syntax look like? *If it's easier from your perspective, please feel free to edit the answer.* — cssyphus, Dec 21 '20 at 18:48
I guess, the point was to do something, like `const uniqeGenders = [...people.reduce((acc,{gender}) => acc.add(gender), new Set)]`, though `.map()`-based method (see my previous comment) is much more straightforward. — Yevhen Horbunkov, Dec 21 '20 at 18:51

Use reduce to summarize values of a specific key in javascript object

2 Answers2

References: