<p><pre><code>  &gt; It&#x27;s easier to reason about a variable if you know it will always be one value—even
  &gt; if the value may be mutated (which is unfortunate).
</code></pre>
True, but because const, let, and var are lexically-scoped, it’s super-easy to reason about everything that happens to variables. (Excepting bad JS environments where you can declare global constants, of course).<p>If I see PI = 3.14159265 somewhere in a file, it’s not a hardship to assume it will not be rebound, and to notice if it is rebound.<p>The super-important thing to reason about is value mutability, because that escapes lexical scope. We pass mutable values to library functions containing code we have never read. We integrate with code written by a colleague who is in another time zone and may accidentally change a routine to mutate an array we pass with .push instead of .concat.<p>“Easier to reason about” is massively important with mutability. Not so much with lexical bindings.