JSON Data Transformation Patterns: Map, Filter, and Reshape

jsonjavascriptdata-transformationpatterns

Transforming JSON data is a daily task for developers. This guide covers the most common transformation patterns.

Mapping JSON Arrays

Convert each item in an array:

const users = [


  { firstName: "Alice", lastName: "Smith", age: 30 },


  { firstName: "Bob", lastName: "Jones", age: 25 }


];
const names = users.map(u => ({


  fullName: ${u.firstName} ${u.lastName},


  isAdult: u.age >= 18


}));

Filtering Data

Remove unwanted items:

const adults = users.filter(u => u.age >= 18);


const activeUsers = users.filter(u => u.status !== 'deleted');

Grouping by Property

Group data by a common field:

const groupBy = (arr, key) =>


  arr.reduce((acc, item) => {


    const group = item[key];


    acc[group] = acc[group] || [];


    acc[group].push(item);


    return acc;


  }, {});
// Group users by role


const byRole = groupBy(users, 'role');

Flattening Nested JSON

Convert nested structures to flat arrays:

const data = {


  users: [


    { name: "Alice", posts: [{id: 1}, {id: 2}] },


    { name: "Bob", posts: [{id: 3}] }


  ]


};
const allPosts = data.users.flatMap(u =>


  u.posts.map(p => ({ ...p, author: u.name }))


);

Picking and Omitting Fields

// Pick specific fields


const pick = (obj, keys) =>


  Object.fromEntries(keys.map(k => [k, obj[k]]));
// Omit specific fields


const omit = (obj, keys) =>


  Object.fromEntries(Object.entries(obj).filter(([k]) => !keys.includes(k)));
const publicUser = omit(user, ['password', 'token']);

Pivot Tables from JSON

Transform row data into summary tables:

const sales = [


  {product: "A", month: "Jan", amount: 100},


  {product: "B", month: "Jan", amount: 200},


  {product: "A", month: "Feb", amount: 150}


];
const pivot = {};


sales.forEach(s => {


  if (!pivot[s.product]) pivot[s.product] = {};


  pivot[s.product][s.month] = s.amount;


});

Use our JSON to CSV converter when you need to export transformed data, or JSON Viewer to inspect results.

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