The mnemonic I use is "for index", as in for keys you can use to index things. One gotcha to remember here is that officially all property names are strings [0], and that includes array indexes. To exemplify, `for (let x in { a: 40, b: 41, c: 42 }) { console.log(x); }` would give you "a", "b", "c" as that's what you can use as an index to key into that object to get the values. And `for (let x in [40, 41, 42]) { console.log(x); }` would give you "0", "1", "2" as that what's you can use as an index to key into that object. I always remember `for..of` as just the other one.

[0] https://stackoverflow.com/q/27537677/1470607

The trick is to remember that “for in” is the obvious one, meaning it’s the older one, meaning it’s the one that’s next to useless and should almost never be used.

A great way to remember: Objects have keys -> foreign keys -> `for .. in`