This morning's "I made my very own WTF!?"

wtf.js

      // I'm in the middle of a method, and I decide that I'll use an
      // Immediately Invoked Function Expression ("IIFE") to create a new
      // block scope so that I don't have to pollute the entire thing with
      // resultJSON and contentJSON variables that would otherwise get
      // hoisted.
      //
      // The current object DOES have a toJSON method, and so does the object called content.
      
      // _.isFunction(this.toJSON) ==> true
      // _.isFunction(content.toJSON) => true
�
      return (function () {
        
        var resultJSON = this.toJSON(),
            contentJSON = content.toJSON();
        
        _.each(contentJSON, function (contentRow, rowIndex) {
          var resultRow = resultJSON[extant + upperLeft.y + rowIndex];
          
          _.each(contentRow, function (contentCell, columnIndex) {
            resultRow[extant + upperLeft.x + columnIndex] = contentCell;
          });
        });
        
        return QuadTree.fromJSON(resultJSON);
        
      })();
      
      // So what happens? 
      // TypeError: Cannot call method 'toJSON' of undefined
      // Investigation reveals that the offending line is 14,
      // "resultJSON = this.toJSON()"
      //
      // WTF!?

You folks are spot on! I changed it to .call(this), but same thing. Good eyes!!

What happens if you pass the context into your IIFE? I.E.

return (function (that) {
  var resultJSON = that.toJSON(),
      contentJSON = content.toJSON();

  _.each(contentJSON, function (contentRow, contentIndex) {
    // Lots of copying from contentJSON to resultJSON
    // ...
  });

  return QuadTree.fromJSON(resultJSON);
})(this);

Passing this to that works, as did:

return (function () {
  var resultJSON = this.toJSON(),
      contentJSON = content.toJSON();

  _.each(contentJSON, function (contentRow, contentIndex) {
    // Lots of copying from contentJSON to resultJSON
    // ...
  });

  return QuadTree.fromJSON(resultJSON);
}).call(this);