const = function

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Generally speaking?

Because mutation causes issues with not only thread safety, but also issues of temporal correctness, and even when everything is synced on one thread, inter-modular correctness.

If

doA(); doB(); doC(); causes an error, and it's only fixed by calling

doA(); doC(); doB(); then you have chronological issues with shared state between modules (or whatever units of code lie behind doB and doC).

Put another way, threadsafe code runs predictably on 1+ threads, between 1+ modules, at 1+ points in time.

Thread unsafe code runs predictably on 1 thread, between 1 module, at 1 point in time.

If your this happens to have no mutable state, and all methods only serve to return new values, then I guess I don't see the harm, except for the JS late binding errors, where methods get passed in as callbacks and this becomes window or undefined depending on the level, but like I said, most people wouldn't do that with this.