topoSort.js

const courseMap = {
    C: ['A', 'B', 'F', 'H', 'D', 'G'],
    B: ['K', 'L'],
    H: ['E', 'J'],
    D: ['A', 'B']
};


const learnCourse = (course) => {
  const coursesChecked = {};
  const fullList = [];
  (function learnCourseRecursive(course) {
    const courseDep = courseMap[course];
    if (!coursesChecked[course]) {
      coursesChecked[course] = true;
    }
    
    if (courseDep === undefined) {
        fullList.push(course);
    } 
  
    else {
      for (let i=0, length=courseDep.length; i<length; i++) {
          if (!coursesChecked[courseDep[i]]) {
            learnCourseRecursive(courseDep[i]);
          }  
      }
      fullList.push(course);
    }
  })(course);    
  
    return fullList;
};

console.log(learnCourse('C'));
console.log(learnCourse('A'));
console.log(learnCourse('H'));
console.log(learnCourse('Z'));