How to create an array of partial objects from another array in JavaScript ?

A partial object array contains some selected key-value pairs of the object array.  It Lets us understand how to create an array of partial objects from another object’s array. here are some common methods 

• map() method
• reduce() method

Approach 1: Using the map() method: By using the map() method we can create an array of partial objects from another object’s array. Map() method takes the keys of the object array as the parameters which are required to be in the desired object array and returns the key-value pairs of that particular key. In the below example, we obtain the key-value pairs of the name and age as the array of partial objects of which the parameters are given in the map method.

Syntax:

partialObjectArray = ObjectArray.map((
    {key 1, key 2, ...key n}) => ({key 1, key 2, ...key n}));

Note: key 1, key 2, …key n are the fields that are needed in the partial object array

Example 1: This is the illustration to create an array of partial objects from another array using the map() method:

Javascript

let = studentDetails = [
    {
        name: 'dinesh',
        age: 20,
        marks: 30,
        Grade: 'F'
    },
    {
        name: 'divi',
        age: 20,
        marks: 60,
        Grade: 'B'
    },
    {
        name: 'vignesh',
        age: 30,
        marks: 80,
        Grade: 'A'
    }]
let partialStudentDetails = studentDetails.map((
    { name, age, Grade }) => ({ name, age, Grade }));
console.log(partialStudentDetails);

Output:

{name: 'dinesh', age: 20, Grade: 'F'}
{name: 'divi', age: 20, Grade: 'B'}
{name: 'vignesh', age: 30, Grade: 'A'}

Rather than giving all the required fields of the object array in the map() method which is a difficult task if there are multiple fields in the object. we can use the map method in the below way to create a partial object array. The first parameters must be the fields that are not needed in the resultant object array. 

Syntax:

partialObjectArray = ObjectArray.map(({ key 1, key 2, ...rest }) => rest);

Note: key 1, and key 2 are the fields that are not required in the resultant object array.

Example 2: This is another illustration to create an array of partial objects from another array using the map() method:

Javascript

let = studentDetails = [
    {
        name: 'dinesh',
        age: 20,
        marks: 30,
        Grade: 'F'
    },
    {
        name: 'divi',
        age: 20,
        marks: 60,
        Grade: 'B'
    },
    {
        name: 'vignesh',
        age: 30,
        marks: 80,
        Grade: 'A'
    }]

// The parameter marks is not required
// in the resultant object array

let partialStudentDetails = studentDetails.map((
    { marks, ...rest }) => rest);
console.log(partialStudentDetails);

Output:

{name: 'dinesh', age: 20, Grade: 'F'}
{name: 'divi', age: 20, Grade: 'B'}
{name: 'vignesh', age: 30, Grade: 'A'}

In the above example, we get name, age, and marks key-value pairs in a partial object array.

Approach 2: Using reduce method(): reduce method also use to iterate over an array and create a single value and object,

Example: In this example, we are using reduce method to create a partial object from another array.

Javascript

let studentDetails = [
    {
        name: 'dinesh',
        age: 20,
        marks: 30,
    },
    {
        name: 'divi',
        age: 20,
        marks: 60,
    },
    {
        name: 'vignesh',
        age: 30,
        marks: 80,
    }]

const partialStudentDetails = studentDetails.reduce((res, item) => {
    res.push({ name: item.name, age: item.age, marks: item.marks });
    return res;
}, []);
console.log(partialStudentDetails);

Output:

{name: 'dinesh', age: 20, marks: 30}
{name: 'divi', age: 20, marks: 60}

{name: 'vignesh', age: 30, marks: 80}