JavaScript: Create a new object from the combination of two or more objects

Last update on February 26 2025 11:10:44 (UTC/GMT +8 hours)

JavaScript fundamental (ES6 Syntax): Exercise-178 with Solution

Merge Objects into New Object

Write a JavaScript program to create a new object from the combination of two or more objects.

Use Array.prototype.reduce() combined with Object.keys() to iterate over all objects and keys.
Use Object.prototype.hasOwnProperty() and Array.prototype.concat() to append values for keys existing in multiple objects.

Sample Solution:

JavaScript Code:

// Define a function 'merge' that takes any number of objects as arguments using the rest parameter syntax '...objs'
const merge = (...objs) =>
  // Use the spread operator to create a shallow copy of the 'objs' array, then use 'reduce' to merge the objects
  [...objs].reduce(
    // Callback function for 'reduce' with accumulator 'acc' and current object 'obj'
    (acc, obj) =>
      // Reduce the keys of the current object 'obj' into the accumulator 'acc' using another 'reduce' function
      Object.keys(obj).reduce(
        // Callback function for inner 'reduce' with accumulator 'a' and current key 'k'
        (a, k) => {
          // Check if the accumulator 'acc' already has the key 'k'
          acc[k] = acc.hasOwnProperty(k) ? // If yes, concatenate the values of 'acc[k]' and 'obj[k]' arrays or add the value 'obj[k]' to 'acc[k]'
            [].concat(acc[k]).concat(obj[k]) : obj[k];
          return acc; // Return the updated accumulator 'acc'
        },
        {} // Initial value of the inner 'reduce' accumulator is an empty object
      ),
    {} // Initial value of the outer 'reduce' accumulator is an empty object
  );

// Define two objects 'object' and 'other' with properties 'a', 'b', and 'c'
const object = {
  a: [{ x: 2 }, { y: 4 }],
  b: 1
};
const other = {
  a: { z: 3 },
  b: [2, 3],
  c: 'foo'
};
// Merge the two objects using the 'merge' function and log the result
console.log(merge(object, other));

Output:

{"a":[{"x":2},{"y":4},{"z":3}],"b":[1,2,3],"c":"foo"}

Flowchart:

Live Demo:

For more Practice: Solve these Related Problems:

Write a JavaScript program that merges two or more objects into a new object without mutating the originals.
Write a JavaScript function that uses the spread operator to combine multiple objects into one.
Write a JavaScript program that deep merges objects, resolving conflicts by taking the value from the last object.
Write a JavaScript function that iterates through an array of objects and constructs a single aggregated object.

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