[chrisseaton]

These checks can be completely free, in terms of runtime cost, through deoptimisation.

[gfxgirl]

How? You have to do some kind of check with let, and you don't with const.

[miloignis]

Not sure if V8 does this, but a general strategy used in HotSpot is to compile assuming the good case and install a "trap" to deoptimize the code if the condition is violated. So you could compile the let like a const with no checks on the use of the variable, but have a trap such that if any code ever tries to assign to that variable, all code compiled under the assumption that it was constant is throw out.