map

1. const courses = [

2.     { name: "HTML", levels: ["L1", "L2"] },

3.     { name: "CSS", levels: ["L1", "L2"] },

4.     { name: "JS", levels: ["L2", "L3", "M1"] }

5. ]

6. 
   

7. const levels = new Set(courses.reduce((acc, { levels: levels }) => ([...acc, ...levels]), []));

8. 
   

9. const programme = courses.reduce((a, { levels: levels, name }) => {

10.     for (let level of levels) {

11.         if (!a[level]) a[level] = [];

12.         a[level].push(name);

13.     }

14.     return a;

15. }, {});

16. 
    
17. 
    

18. const menu = Object.entries(programme).map(([level, courses]) => {

19.     return {

20.         title: level,

21.         subnav: courses.map((course) => {

22.             return { title: course }

23.         })

24.     }

25. })

26. 
    

27. console.dir(JSON.stringify(menu));

Affiche.

[

{"title":"L1","subnav":[{"title":"HTML"},{"title":"CSS"}]}

,{"title":"L2","subnav":[{"title":"HTML"},{"title":"CSS"},{"title":"JS"}]}

,{"title":"L3","subnav":[{"title":"JS"}]}

,{"title":"M1","subnav":[{"title":"JS"}]}

]