How to sort an array with parents and children objects with JavaScript?

Question

I have an array with object, which I need to sort in a such way that first the parent object should appear, and then its children objects, and so on. However, when I try to find index of a parent object in array in order to push the children object after it, the findIndex() method returns -1. Can somebody point to the root of this problem, as I cannot clearly see why it does that. The code and data array that I'm using is written below.

 const data = [ { "_id": "0", "parent": null, "title": "All" }, { "_id": "61c0a9cb8f67e811d55abb2d", "parent": null, "title": "Electronics" }, { "_id": "61c0a9cb8f67e811d55abb2e", "parent": { "_id": "61c0a9cb8f67e811d55abb2d" }, "title": "Phones" }, { "_id": "61c0a9cb8f67e811d55abb2f", "parent": { "_id": "61c0a9cb8f67e811d55abb2d" }, "title": "Laptops" }, { "_id": "61c0a9cb8f67e811d55abb30", "parent": { "_id": "61c0a9cb8f67e811d55abb2d" }, "title": "TVs" }, { "_id": "61c0a9cb8f67e811d55abb31", "parent": null, "title": "Literature" }, { "_id": "61c0a9cb8f67e811d55abb32", "parent": { "_id": "61c0a9cb8f67e811d55abb31" }, "title": "Study Literature" }, { "_id": "61c0a9cb8f67e811d55abb33", "parent": { "_id": "61c0a9cb8f67e811d55abb31" }, "title": "Fictional Literature" }, { "_id": "61c0a9cb8f67e811d55abb34", "parent": { "_id": "61c0a9cb8f67e811d55abb31" }, "title": "Comic books" }, { "_id": "61c0a9cb8f67e811d55abb35", "parent": { "_id": "61c0a9cb8f67e811d55abb2e" }, "title": "Smartphones" }, { "_id": "61c0a9cb8f67e811d55abb36", "parent": { "_id": "61c0a9cb8f67e811d55abb35" }, "title": "Accessories" } ]; let parents= []; data.forEach(element => { if(element.parent == null ) { parents.push(element); } else { let parentId = element.parent._id; let index = parents.findIndex(item => { return item._id == parentId; }); parents.splice(index+1, 0, element); }; });

Answer 1

Using item => {parentId == item._id;} does require a 'return' to be used: item => {return parentId == item._id;} without the return the functions is basically item => null ; which is than seen as false by .findIndex() resulting in a -1

If you use the arrow function without the curly braces a return is implied (But limits you to single line expressions as a trade-off): item => parentId == item._id

Source: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions