[nilkn]

I'm confused. You've never even seen a hash table come up before? What sort of front-end work are you doing?

I just find this extremely unlikely if not borderline impossible. Hash tables are one of the most universal data structures in practical software development. This is just as true in JavaScript for front-end work as it is anywhere else.

[jonahx]

I use js objects, of course, which are hash tables.

Would I hire a front-end developer who didn't know that, or didn't know that they had O(1) lookup time? Very possibly. Because there are so many things that are more important than that knowledge when you are actually building software for a business. Things like experience with usability testing, JS/CSS build tools, unit testing and refactoring, a decent eye for graphic design (even if they won't be doing that work themselves), an open-mind and lack of ego -- just off the top of my head, these are all things I'd consider orders of magnitude more important than knowing technical details about hash tables.

[cageface]

Expecting somebody to know what a hash table is and understand the time complexity of inserting and looking up elements is fair. Expecting them to build one from scratch on a whiteboard is just ritualized hazing.

[nilkn]

I don't see anyone mentioning requiring one to build all these data structures from scratch. I'd agree with you that this is a less useful exercise for interviews, in particular front-end interviews, but I don't think it's fair to assume that this is what the poster above meant. Only "basic knowledge" was mentioned.