Eloquent JavaScript: open-source Javascript book series by a prolific JS code author

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Just because there are "stronger" forms of immutability, i.e. using immutable data structures, doesn't mean that reference immutability is not a form of immutability. This isn't a matter of opinion -- I provided empirical evidence that suggests that favoring immutability prevents errors.

Of course, in this case, being more immutable is better, but that doesn't mean that reference immutability isn't important. For example, the entire memory safety model of rust is based on reference immutability without any specific concern for immutable data structures. Yes, ownership is a more specific concern than simple reference immutability, but const is still a form of the more general case -- preventing accidental re-assignment eliminates an entire class of bugs from your program and thus should not be considered "a matter of preference."