gistfile1.js

for (let V = INIT; TEST; UPD) STMT

===>

{
    let %tmp = INIT, %first = true;
    while (true) {
        // A fresh binding for V gets either the initializer-expression value(s)
        // or the value(s) from the end of the previous loop iteration.
        let V = %tmp;

        // Run the update-expression, if necessary.
        if (%first)
            %first = false;
        else
            UPD;

        // Check the test-expression.
        if (!(TEST))
            break;

        // Run the body of the loop. Make sure the values in V
        // are saved for the next iteration, even if STMT hits a ContinueStatement.
        try {
            STMT;
        } finally {
            %tmp = V;
        }
    }
}

The above desugaring looks complicated, but the implementation is much simpler. I think SpiderMonkey will be able to switch from per-loop binding to per-iteration binding by adding a single extra bytecode instruction per loop. (In the case where no closures capture the loop variable, even that can be optimized away, but I'm not sure we'll bother.)