I think there's an opportunity to name variables after their invariant.

For example, in the typical sum example, the mutated variable is usually called "sum" or "total". But you could also call it sum_from_0_to_i, and then a reader can immediately see that the result is the full sum because i equals the array length. It's like a proof-by-variable-name.

The same trick tends to work as well with less trivial invariants. You get long variable names, but not horribly so.

You can do the same for the accumulator in a reduce/fold operation. Way too often variables in reduce calls (and other recursive functions) have horrible names, making the entire reduce needlessly hard to understand. Naming the accumulator properly solves this:

Eg

    const total = array.reduce(
        (totalUptoPrevious, current) => totalUptoPrevious + current,
        0
    );

So much easier to follow! I've often had to look up, when reading a reduce, which argument is the accumulator in this particular language or library. When you use the invariant for the accumulator name, you fix this entirely.