JS referencing prototype methods

Question

Like many, I am using prototype inheritance to define object methods. This is a small sample of code that I believe should work:

// bag object
function bag() {
   this.list = [];
}

// bag methods
bag.prototype = {

   stack: function (e) {
      this.list.unshift(e);
      return this;
   },

   stackAll: function (es) {
      es.forEach(this.stack); // refer stack method
      return this;
   }
}

let b = new bag();
b.stackAll([1,2,3]); // error: no method this.stack
console.log(b);

Depending on interpreters I have had either of two errors: - not finding the method this.stack (when called from stackAll) - not finding this.list (when referred to in stack() from stackAll)

I'm puzzled. I'm interested in an alternative that would work, and a rationale why it doesn't.