Encapsulation

Encapsulation is the bundling of data and the methods that act on that data such that access to that data is restricted from outside the bundle, or as Alan Kay describes it, “local retention and protection and hiding of state-process.” In OOP, that means that an object stores its state privately, and only the object’s methods have access to change it.

// Only works for arrays
const doubleAllImperative = data => {
  const doubled = [];
  for (let i = 0, length = data.length; i < length; i++) {
    doubled[i] = data[i] * 2;
  }
  return doubled;
};
// Same as above, but works for anything with the
// map operation.
const doubleAllInterface = data => data.map(x => x * 2);

Encapsulation can be a powerful tool capable of helping you prevent bugs resulting from shared mutable state, and also for reducing tight coupling between components and the data structures they rely on. Another example might be the following:

class Student {
    constructor() {
        var name;
        var marks;
    }
    getName() {
        return this.name;
    }
    setName(name) {
        this.name = name;
    }

    getMarks() {
        return this.marks;
    }
    setMarks(marks) {
        this.marks = marks;
    }

}
var stud = new Student();
stud.setName("John");
stud.setMarks(80);
console.log(stud.getName() + " " + stud.getMarks());