What is the meaning and reasoning behind the Open/Closed Principle?

Question

The Open/Closed Principle states that software entities (classes, modules, etc.) should be open for extension, but closed for modification. What does this mean, and why is i

Answer 1

Software entities should be open for extension but closed for modification

That means any class or module should be written in a way that it can be used as is, can be extended, but neve modified

Bad Example in Javascript

var juiceTypes = ['Mango','Apple','Lemon'];
function juiceMaker(type){
    if(juiceTypes.indexOf(type)!=-1)
        console.log('Here is your juice, Have a nice day');
    else
        console.log('sorry, Error happned');
}

exports.makeJuice = juiceMaker;

Now if you want to add Another Juice type, you have to edit the module itself, By this way, we are breaking OCP .

Good Example in Javascript

var juiceTypes = [];
function juiceMaker(type){
    if(juiceTypes.indexOf(type)!=-1)
        console.log('Here is your juice, Have a nice day');
    else
        console.log('sorry, Error happned');
}
function addType(typeName){
    if(juiceTypes.indexOf(typeName)==-1)
        juiceTypes.push(typeName);
}
function removeType(typeName){
  let index = juiceTypes.indexOf(typeName)
    if(index!==-1)
        juiceTypes.splice(index,1);
}

exports.makeJuice = juiceMaker;
exports.addType = addType;
exports.removeType = removeType;

Now, you can add new juice types from outside the module without editing the same module.