Big O of "in" operator inside for loop

Question

Consider this snippet:

const obj1 = {1:1, 2:1, 3:1}
const obj2 = {1:1, 4:1, 9:1}

for(let key in obj1){
    if(!(key ** 2 in obj2)) return false
  }

Can the Big O of this algorithm considered O(n) or it should be O(n^2) because of:

if(!(key ** 2 in obj2))

Is considered to loop through all items (search) of obj2

** Note**: assume the length of obj1 and obj2 are equal

It depends on an actual implementation of the property lookup algorithm. It's a good be that it's better than linear on any modern JavaScript runtime. — Pointy, May 21 '20 at 20:09
Why would the big O of `in` operator change when it's in a for loop? — Barmar, May 21 '20 at 20:11
@Pointy Am I the only one who thought that his concern was that `key ** 2` caused it to be quadratic? — Barmar, May 21 '20 at 20:15
@Barmar It has nothing to do with key ** 2, my concern was that "in" search is looping through all items, and that will occur inside the outer loop, hence results in O(n^2), but it turns out that "in" search is O(1) as mates mentioned here. — Tarreq, May 21 '20 at 20:19
I figured that out, but I thought it was suspicious that your question was specifically about `key**2`. — Barmar, May 21 '20 at 20:20
And Googling could have answered your question about the complexity of `in`. — Barmar, May 21 '20 at 20:21
@Barmar Just wanted to mention, that post was closed as "duplicate" referring to a "python" question, not a Javascript question? It might be the same yes, but also it might happen that complexity and implementation differs from one language to another! right? — Tarreq, May 21 '20 at 20:24
https://stackoverflow.com/questions/7700987/performance-of-key-lookup-in-javascript-object — Barmar, May 21 '20 at 20:26

score 3 · Accepted Answer · answered May 21 '20 at 20:11

3

A JavaScript object is effectively a hash table, so looking up a key is O(1).

The total algorithm therefore is O(n).

answered May 21 '20 at 20:11

Buh Buh

score 1 · Answer 2 · answered May 21 '20 at 20:10

1

I would suspect that the in is not looping through all the items, but works like a hashtable. So I would say its O(n).

answered May 21 '20 at 20:10

2 Answers2