V8 optimization killersEdit

Things which cause the entire containing function to be deoptimized (ie. compiled with the generic compiler instead of the optimizing compiler, which can be 100x slower):

  • generator functions
  • for/of statements
  • try/catch statements
  • try/finally statements
  • compound let assignment
  • compound const assignment
  • object literals with a __proto__ property, or get or set declarations
  • debugger
  • eval()
  • with
  • any use of arguments except arguments.length, arguments[i] (where i is an inbounds integer index) and foo.apply(bar, arguments)
  • switch with more than 128 case clauses
  • for/in if:
    • "key" is not a strictly local variable
    • object is in dictionary mode (ie. because of properties added/deleted outside the constructor)
    • object has enumerable properties in its prototype chain
    • object has array indices (true for arrays, obviously)