purpose of .bind(this) at end of ajax callback?

It ensure’s this will be the correct object inside the callback. See Function.prototype.bind().

An alternative specific to react is to do:

myAjaxFunction: function(){
  $.getJSON('/something', this.handleData);
},
handleData: function(data){
  this.setState({data: data});
}

This works because React handles binding of component methods for you.

If you ran your original code in without bind, you’d get this error: TypeError: undefined is not a function because this === window in the callback;

or in strict mode: TypeError: Cannot read property 'setState' of undefined, where this === undefined in the callback.