Small improvement discussion: Use `let` to lock variable previously declared as `var`

[dom96]

> I use this pattern a lot, although @dom96 either forgot to return tmp in his 
> expression, or suggests computeFoo returns, and not just modifies its argument

For what it's worth I just followed @cmc's block example and translated it into 
an expression instead of a block statement. In that example it only makes sense 
for `computeFoo` to return a value.