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Collections are a way for you to organize kata so that you can create your own training routines. Every collection you create is public and automatically sharable with other warriors. After you have added a few kata to a collection you and others can train on the kata contained within the collection.
Get started now by creating a new collection.
I think that's only a concern if you are writing code for a real-time application and are at the same time very resource-restrained. Otherwise, this shouldn't matter when running code on modern computers.
It is an expression version of "if...else". So, "foo === 10 ? console.log("foo is ten") : console.log("foo is not ten");" is similar to "if (foo === 10) {console.log("foo is ten");} else {console.log("foo is not ten");}"
Read more on this here (MDN provides the best documentationf for JavaScript): https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Conditional_Operator
All in all, it's just a nicer and shorter version of an "if" statement.