if (this.data == null) {
                  
                  
                        this.data = Object.create(null);
                  
                  
                      }
                  
                  
                      let val = this.data[key];
                  
                  
                      if (val === undefined && def !== undefined) val = this.data[key] = def;
                  
                  
                      return val;
                  
                  
                    } 
                  
                  
                  Can smb explain what def?: any mean?
                  
                  
                
def?: any is the same as: def: any | undefined
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