Array or object?

An array is for things in order, reached by position; an object or Map is for things with a name, reached by key.

Hone is a place to practise programming. This is one of its lessons, written out in full and free to read without an account.

What it is for

Searching an array for a user by id on every render is the front-end's most common slow path. Keying by id makes lookups instant.

How to think about it

How will I get things OUT: by position or by id? Ask how you get things OUT: 'the next one', 'in order': array. 'The one with this id': object or Map. Often you keep both: an array of ids for order and a map of id to record.

Worked example

const order = ["u2", "u1"];

Display order.

const byId = { u1: {name: "Ada"}, u2: {name: "Bo"} };

Lookup by id.

order.map(id => byId[id].name)

["Bo", "Ada"]: order from the array, data from the object.

Your turn

Index records by id.

const byId = {};
for (const r of records) byId[r.] = r;

The trap

Using an array's index as an id. Delete one item and every id after it changes.

Practise array or Map? on HoneA question on it now, a coding challenge where there is one, and it is remembered for review. Free, no email needed.