Open-Closed Principle

Program entities (classes, modules, functions) must be open for extension, but not for modification.

Let's continue with TodoList class, check code below and try to refactor it according open-closed principle:

enum EStatus {
    pending,
    progress,
    completed,
}

interface ITodo {
    status: EStatus;
    // ...other todo fields
}

class TodoList {
    items: ITodo[];

    processTodos(status: EStatus) {
        for (const todo of this.items) {
            if (todo.status === EStatus.pending) {
                // ...logic1
            }
            if (todo.status === EStatus.progress) {
                // ...logic2
            }
            if (todo.status === EStatus.completed) {
                // ...logic3
            }
        }
    }
}

Example Solution

class TodoList {
    processTodos(status, cb) {
        for (const todo of this.items) {
            if (todo.status === status) {
                cb();
            }
        }
    }
}