Sealing prototypes is easy - V8 already "pseudo"seals them: it removes the map checks against prototypes and instead deoptimizes the code depending on hidden cl

Sealing prototypes is easy - V8 already "pseudo"seals them: it removes the map checks against prototypes and instead deoptimizes the code depending on hidden classes when somebody changes them. Similar pseudo-sealing technique can be applied to objects: when optimizing some code you can assume that if some hidden class is a leaf in the transition tree it is no likely to change - then make the code depend on this assumption. This would mean you only need to check this once. I think the most interesting part that stricter-minus-types mode brings to the table (at least it's informally described part) are hole-less arrays. This is something that is missing from the ES as it is described now: new Array(N) is producing a hole-full array. V8 is capable of tracking whether array is containing holes or not - but the way to efficiently preallocate holeless version is simply missing from the ECMAScript.