Top 5 uses of reduce method in javascript

Arvind Dhakar

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2 min read

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Apr 7, 2023

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1. To sum numbers

const numbers = [1, 2, 3, 4, 5];

const sum = numbers.reduce((currentSum, currentNum) => currentSum + currentNum);

/*
output sum = 15
*
/

2. Getting object by reducing an array

const fruits = ['apple', 'banana', 'orange', 'banana', 'apple'];

const fruitsMap = fruits.reduce((accumulator, fruit) => {
   if(accumulator[fruit]) {
        accumulator[fruit]++;
     } else {
        accumulator[fruit] = 1;
    }
    
    return accumulator;

}, {});

/*
output:- 

fruitsMap = {
 apple: 2,
 banana: 2,
 orange: 1
}
*/

3. Flattening an array

const nestedArray = [[1, 2, 3], [4, 4, 5]];

const flattenArray = nestedArray.reduce((acc, currentArray) => [...acc, ...currentArray])


/*

output:- 

flattenArray = [1, 2, 3, 4, 4, 5]
*/

4. Running Promise in sequence

const users = [
    'https://jsonplaceholder.typicode.com/users/1',
    'https://jsonplaceholder.typicode.com/users/2',
  ];


const fetchSequentially = users.reduce((accumulator, user) => {
  return accumulator.then(results => {
      return fetch(url)
              .then(response => response.json())
              .then(data => results.concat(data));
  });
}, Promise.resolve([]))

/*
output:

 fetchUsersSequentially = [
{
    "id": 1,
    "name": "Leanne Graham",
    "username": "Bret",
    "email": "Sincere@april.biz",
    "address": {
      "street": "Kulas Light",
      "suite": "Apt. 556",
      "city": "Gwenborough",
      "zipcode": "92998-3874",
      "geo": {
        "lat": "-37.3159",
        "lng": "81.1496"
      }
    },
    "phone": "1-770-736-8031 x56442",
    "website": "hildegard.org",
    "company": {
      "name": "Romaguera-Crona",
      "catchPhrase": "Multi-layered client-server neural-net",
      "bs": "harness real-time e-markets"
    }
  },
  {
    "id": 2,
    "name": "Ervin Howell",
    "username": "Antonette",
    "email": "Shanna@melissa.tv",
    "address": {
      "street": "Victor Plains",
      "suite": "Suite 879",
      "city": "Wisokyburgh",
      "zipcode": "90566-7771",
      "geo": {
        "lat": "-43.9509",
        "lng": "-34.4618"
      }
    },
    "phone": "010-692-6593 x09125",
    "website": "anastasia.net",
    "company": {
      "name": "Deckow-Crist",
      "catchPhrase": "Proactive didactic contingency",
      "bs": "synergize scalable supply-chains"
    }
  },
]
*/

5. Grouping elements

const people = [ 
  {
   name: "Diya",
   age : 21,
   gender: "female"
  },
  {
   name: "Sanjay",
   age : 22,
   gender: "male"
  },
  {
   name: "Nisha",
   age : 20,
   gender: "female"
  }
]

const peopleByGender = people.reduce((accumulator, person) => {
    if(accumulator[person.gender]) {
        accumulator[person.gender].push(person)
    } else {
       accumulator[person.gender] = [];
    }

    return accumulator;
}, {})

/*
output:- 

peopleByGender = {
 female: [
            {
               name: "Diya",
               age : 21,
               gender: "female"
            },
            {
             name: "Nisha",
             age : 20,
             gender: "female"
            }
        ],

male: [
         {
           name: "Sanjay",
           age : 22,
           gender: "male"
         }
      ]
}
*/