Which is better (for your CV), be a contributor to Angular.js (example) or create your something.js that left-pads and right-pads at the same time?

I think a better example would be: be a contributor to Angular.js or be the creator of Aurelia.js.

From what I can tell, something.js if the amount of code is equivalent.

Having a patch or two in angular doesn't seem to move the needle.

No. Because nobody knows about your something.js

Getting something on Angular means you were able to get your changes approved and merged (team work)

Situation is better if you can have a bigger project or more people using your something.js

Well, left pad was quite famous. And especially famous when the author removed it. You never know.

Was it famous before the author removed it?

It actually helps if you contribute to an existing project, because it is like, you worked with others on an existing codebase and contributed features in a big project which has high visiblity and is being used in real life by real people.

Much of what we work is all with people and on large codebase, so if you are contributing to existing project, it is actually helpful for interviews. Also it takes weeks to get a PR accepted, so if you actually were successful in getting your feature accepted, it shows your resolve to contribute the feature because you want to contribute and not because you want to do a PR stunt for your career (and not get annoyed by the constant changes which the authors ask, which are close to a million)