4 ms·

Reading the issue comments, I still don't quite get why this is the case. Anybody with a better handle on PL theory and/or JS internals care to ELI5?

• saagarjha 6y ago
  Perhaps the linked, seemingly related WebKit bug may be interesting? https://bugs.webkit.org/show_bug.cgi?id=199866 https://bugs.webkit.org/show_bug.cgi?id=199866
  • whoisburbansky 6y ago
    TLDR seems like var isn't scope-bound, but let is, and actually calculating the scope for let statements is what's causing the slowdown?
    • morelisp 6y ago
      Damn you Olin Shivers!
• winstonewert 6y ago
  wild speculation: let/const is implemented by creating a new scope with every let or const declaration. Looking up variable then ends up tracing linearly through all of these scopes. Bundlers end up creating a huge number of let/const and thereby scopes. Then every variable reference ends up scanning through this huge linear list of scopes to find the variable it refers to.
  • brundolf 6y ago
    Why would there be a whole new scope for every declaration, instead of just each block? To prevent access within the block but before the declaration?
    • nightcracker 6y ago
      Probably was simplest to implement, and then not looked at again.
    • winstonewert 6y ago
      Yes, I think creating a new scope for every let/const would be the simplest (but not very efficient) way to accomplishing that.