8000 Private `$derived` class fields break SSR · Issue #15960 · sveltejs/svelte · GitHub
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huntabyte opened this issue May 19, 2025 · 0 comments · Fixed by #15964
Closed

Private $derived class fields break SSR #15960

huntabyte opened this issue May 19, 2025 · 0 comments · Fixed by #15964

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@huntabyte
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huntabyte commented May 19, 2025 •
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Describe the bug

On initial load/SSR, private class fields that use $derived break with the following:
TypeError: Cannot read properties of undefined (reading '<var name>')

Introduced in svelte@5.31.0

Reproduction

<script lang="ts">
    class SomeState {
        opts: { baz: boolean };
        #foo = $derived.by(() => this.opts.baz);

        constructor(opts: { baz: boolean }) {
            this.opts = opts;
        }
    }

    new SomeState({ baz: true });
</script>

Stackblitz: https://stackblitz.com/github/AdrianGonz97/dependent-on-private-derived-repro?file=README.md
GitHub Repo: https://github.com/AdrianGonz97/dependent-on-private-derived-repro

Logs



System Info


(StackBlitz)


  Binaries:
    Node: 20.19.1 - /usr/local/bin/node
    Yarn: 1.22.19 - /usr/local/bin/yarn
    npm: 10.8.2 - /usr/local/bin/npm
    pnpm: 8.15.6 - /usr/local/bin/pnpm
  npmPackages:
    svelte: ^5.0.0 => 5.31.1


Severity


blocking an upgrade

      		
    

      
  





        



            
            

              
            

        

      


      
    








      

    



      

  
          

  

    
  


  

      

    @huntabyte
huntabyte



    mentioned this issue
    
      May 19, 2025
    





  


  

      
  
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          @huntabyte
huntabyte




        changed the title
Depending on private $derived in classes breaks on SSR

Private $derived class fields break SSR


      May 20, 2025

    

  



  

  

    
  


  

      

    @paoloricciuti
paoloricciuti



    mentioned this issue
    
      May 20, 2025
    





  


  

      
  
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  dummdidumm 

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      May 20, 2025
    
    

      


  

    

      


  

      
        @dummdidumm
  





      

        
          fix: make deriveds on the server lazy again
        

          
            
          

      


      

          

    
    
    

  

      


      

          

    
    
    

  

  

    
  



      


      
      

        
          e6c4e8c
        
      

    

  

    

      
Fixes a regression introduced in #15820: deriveds need to be lazily called on the server, too, since they can close over variables only later defined

Fixes #15960

    




    

  




  

  

    
  


  

      

    @dummdidumm
dummdidumm



    mentioned this issue
    
      May 20, 2025
    





  


  

      
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  This was referenced May 20, 2025





  


  

      
  
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  n00ki 

      added a commit
        to n00ki/sveltekit-omakase
      that referenced
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      May 22, 2025
    
    

      


  

    

      


  

      
        @n00ki
  





      

        
          📌 temporarily pin svelte version to 5.30.2
        

          
            
          

      


      

          

    
    
    

  

      


      

      


      
      

        
          4afa1be
        
      

    

  

    

      
until fixed: sveltejs/svelte#15960

    




    

  




  

  

    
  


  

      

  This was referenced May 22, 2025





  


  

      
  
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  Rich-Harris 

      pushed a commit
      that referenced
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      May 22, 2025
    
    

      


  

    

      


  

      
        @dummdidumm
      
        @paoloricciuti
  





      

        
          fix: make deriveds on the server lazy again (#15964)
        

          
            
          

      


      

          

    
    
    

  

    


      


      

          

    
    
    

  

  

    
  



      


      
      

        
          ac046df
        
      

    

  

    

      
* fix: make deriveds on the server lazy again

Fixes a regression introduced in #15820: deriveds need to be lazily called on the server, too, since they can close over variables only later defined

Fixes #15960

* fix: handle basic assignment of deriveds on the server

* fix: use `build_assignment_value` for deriveds assignments

* use once

* allow writing to public deriveds on server

---------

Co-authored-by: paoloricciuti <ricciutipaolo@gmail.com>

    




    

  




  

  

    
  


  

      

    @huntabyte
huntabyte



    mentioned this issue
    
      May 24, 2025
    





  


  

      
  
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      fix: make deriveds on the server lazy again

    
      sveltejs/svelte
    





      
    

  




      

    

  
      

  

    

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