Implement a filter() for Objects in JavaScript

In JavaScript, objects do not have a built-in filter() method like arrays. However, we can implement similar functionality by filtering an object's key-value pairs based on a specific condition.

• Use Object.entries() to convert an object into key-value pairs.
• Use Array.filter() to select the required properties.
• Use Object.fromEntries() to convert the filtered pairs back into an object.
• Use reduce() to implement a custom object filtering function.

Approach 1: Using Object.entries() and Object.fromEntries()

The idea is to convert the object into an array of key-value pairs using Object.entries(). We then use filter() to select the properties that satisfy the given condition and finally use Object.fromEntries() to create a new object from the filtered pairs.

Example: Filters the properties whose values are greater than 20.

let employees = {
    Tony: 25,
    Peter: 18,
    Bruce: 30,
    Clark: 20
};

// Filter properties based on their values
let filtered = Object.fromEntries(
    Object.entries(employees)
        .filter(([key, value]) => value > 20)
);

console.log(filtered);

{ Tony: 25, Bruce: 30 }

Approach 2: Using reduce()

The idea is to use reduce() to iterate through the object's key-value pairs and add only those properties that satisfy the given condition to a new object.

Example: Filters employees belonging to the "IT" department.

let employees = {
    Tony: "IT",
    Peter: "HR",
    Bruce: "IT",
    Clark: "Editing"
};

// Function to filter object properties
function objectFilter(obj, predicate) {
    return Object.entries(obj).reduce(
        (result, [key, value]) => {
            if (predicate(value, key)) {
                result[key] = value;
            }
            return result;
        }, {});
}

// Filter properties with value "IT"
let filtered = objectFilter(
    employees,
    value => value === "IT"
);

console.log(filtered);

{ Tony: 'IT', Bruce: 'IT' }

Approach 3: Using a Custom filterObject() Function

The idea is to create a reusable function that accepts an object and a condition. The function converts the object into key-value pairs, filters them according to the condition, and returns the matching properties as a new object.

Example: Filters properties whose values are greater than 50.

let marks = {
    Math: 85,
    English: 45,
    Science: 72,
    History: 40
};

// Function to filter object properties
function filterObject(obj, predicate) {
    return Object.fromEntries(
        Object.entries(obj).filter(
            ([key, value]) => predicate(value, key)
        )
    );
}

// Filter values greater than 50
let result = filterObject(
    marks,
    value => value > 50
);

console.log(result);

{ Math: 85, Science: 72 }

Note: The filter() method is directly available for arrays, not objects. For an object, Object.entries() can be combined with filter() and Object.fromEntries() to achieve similar functionality.