For the first, I frequently (for my own projects) change the Array prototype to have

   Array.prototype.toString = function () {
      return '[' + this.join(', ') + ']';
   };

It would be convenient if Object.prototype.toString by default threw an exception, since the default breaks the embedding rule I described.

The second example is misleading, and it has nothing to do with the rules for addition: the {} at the beginning of the expression is parsed as a code block, played off the rules for what the value of a Javascript statement is. More realistically, we have

   ({}) + [] == "[object Object]"

but the original is, effectively,

   {
   }
   console.log(+[]);