Execution order during initialization #15

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1. jeffmo commented on Aug 15, 2016

   

   jeffmo

   on Aug 15, 2016

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   Because ES classes generate the instance object/value starting at the base-most constructor, there is no good mechanism by which it would be possible to execute the fields on MyModel before the ModelBase constructor has executed.

   Desugaring this example into expando constructor-assignment helps to illustrate the problem:

   class ModelBase{
     constructor(initParams){
       for (let i in initParams) {
         this[i] = initParams[i];
       }
     }
   }
   
   class MyModel extends ModelBase{
     constructor(initParams){
       /** 
        * `this` is in TDZ here because `this` is only allocated after a call to super()
        * for derived classes. As such, it is not possible to assign to `this` here 
        * because `this` doesn't exist yet.
        */
       super(initParams);
       // Only at this point has `this` been allocated and bound
       this.a = null;
       this.b = 'foo';
       this.c = 123;
     }
   }
2. jeffmo

   closed this as completedon Aug 15, 2016