What's this?

You’ve probably seen this (no pun intended) at the top of some javascript functions:

var _self = this;

This is because the meaning of this (the calling context) can change, for example when callbacks call back and promises are fulfilled, by which time this has changed.

However, here is a case I ran in today that suprised me (I have stripped this down to toy functions, but you get the idea):

MYOBJECT.prototype.myFunction = function(a) {
  var q;
  Object.keys(a).forEach(function(k) {
        q += "AND " + this.anotherFunction(k);
      });
}

MYOBJECT.prototype.anotherFunction = function(a) {
   return(a);
}

Experienced javascript programmers no doubt can spot that this in that context is (roughly speaking) the calling array, not MYOBJECT. Here or course is the solution:

MYOBJECT.prototype.myFunction = function(a) {
  var _self = this;
  var q;
  Object.keys(a).forEach(function(k) {
        q += "AND " + _self.anotherFunction(k);
      });
}

MYOBJECT.prototype.anotherFunction = function(a) {
   return(a);
}

The meaning of _self remains unadulterated because of the scope in which it is defined.

More info here