Any situation where the program depends on the expression producing a new string each time it is evaluated, rather than returning the same string. The program may be modifying the string, on the assumption that nothing else has access to it, since it is brand new. The program could also be relying on the string having a unique identity, not comparing equal to any previously seen object. (E.g. assuming that each time the expression is evaluated, it produces an object that can serve as a unique key in an EQ hash table).
Any situation in which these behaviors cannot be ruled out (because the object escapes beyond the scope of analysis), the optimization cannot be applied.
Ah, well all JS strings are always immutable and only value-referable (you have no access to the underlying memory location), so that’s not a concern here.
What about the identity side of it? Does the JS specification say that an operation like "a" + "b" is not required to create a new object? Regardless of whether there is such a spec, you can write code that is sensitive to the difference.
It looks like JS doesn't expose equality operator which can distinguish different strings. Thus "abc" and "abc" are the same object, no matter how they are produced, even if under the hood they are separate instances.
Pretty much, though some would contest calling strings “objects”.
The spec indeed goes through some trouble to ensure they are pure value-types and do not exhibit any reference-like semantics, for instance by prohibiting their use as keys of WeakMaps and WeakSets - along with numbers, booleans, nullish values, and bignums.
In the context of a discussion on JS, the spec assigns a very specific meaning to the term “object”, one which value-types do not satisfy. That is, indeed, the entire point of this thread, and to suddenly switch terminology then ramble about how you’re better than “script kiddies” screams rhetorical incompetence.
It's clear that you don’t know JS. I was trying to help you understand one interesting design decision they make, but you’ve taken to personal attacks against the entire community in a way that I don’t find worth humoring. Good day.
I already understand the design space and all the implications of strings being immutable and indistinguishable objects if they are the same character sequences; where JS sits in that space is just trivia (which I easily looked up myself).
Indeed, I don't know JS well enough to have a rote memory of where it sits in every design space, or which words it specification does not use for what things. There are plenty of languages in whose specifications object has both the broader meaning, and the more specific OOP meaning. People don't get confused somehow.
Conversely, not everything we can say about a Javascript program necessarily has vocabulary in the Javascript spec; we're going to end up using outside words one way or another.
(I'm not interested in Javascript, actually; this is just a "drive by" for me, because I am interested in the submission topic. All that I will ever know about Javascript will always come from doing the minimal amount of research to answer some question out of some kind of necessity.)
Any situation in which these behaviors cannot be ruled out (because the object escapes beyond the scope of analysis), the optimization cannot be applied.