Composite Pattern

Real world example

Every organization is composed of employees. Each of the employees has same features i.e. has a salary, has some responsibilities, may or may not report to someone, may or may not have some subordinates etc.

In plain words

Composite pattern lets clients to treat the individual objects in a uniform manner.

Wikipedia says

In software engineering, the composite pattern is a partitioning design pattern. The composite pattern describes that a group of objects is to be treated in the same way as a single instance of an object. The intent of a composite is to "compose" objects into tree structures to represent part-whole hierarchies. Implementing the composite pattern lets clients treat individual objects and compositions uniformly.

Programmatic Example

Taking our employees example from above. Here we have different employee types

/*
Employee interface :

constructor(name, salary)
getName()
setSalary()
getSalary()
getRoles()
*/

class Developer {

    constructor(name, salary) {
        this.name = name
        this.salary = salary
    }

    getName() {
        return this.name
    }

    setSalary(salary) {
        this.salary = salary
    }

    getSalary() {
        return this.salary
    }

    getRoles() {
        return this.roles
    }

    develop() {
        /* */
    }
}

class Designer {

    constructor(name, salary) {
        this.name = name
        this.salary = salary
    }

    getName() {
        return this.name
    }

    setSalary(salary) {
        this.salary = salary
    }

    getSalary() {
        return this.salary
    }

    getRoles() {
        return this.roles
    }

    design() {
        /* */
    }
}

Then we have an organization which consists of several different types of employees

class Organization {
    constructor(){
        this.employees = []
    }

    addEmployee(employee) {
        this.employees.push(employee)
    }

    getNetSalaries() {
        let netSalary = 0

        this.employees.forEach(employee => {
            netSalary += employee.getSalary()
        })

        return netSalary
    }
}

And then it can be used as

// Prepare the employees
const john = new Developer('John Doe', 12000)
const jane = new Designer('Jane', 10000)

// Add them to organization
const organization = new Organization()
organization.addEmployee(john)
organization.addEmployee(jane)

console.log("Net salaries: " , organization.getNetSalaries()) // Net Salaries: 22000