What kind of Javascript function is this?

It's not a function, it's a function call. That expression looks up the property named by this.options.position on the object referenced by this and then calls the function that property's value refers to. So for instance, if this.options.position contains the string "one", then it's like doing position = this.one().

Example:

var obj = { options: {}, one: function() { console.log("This is function one"); return 1; }, two: function() { console.log("This is function two"); return 2; }, example: function() { var position = this[this.options.position](); console.log(position); } } obj.options.position = "one"; obj.example(); obj.options.position = "two"; obj.example();

This has nothing to do with functions, but with property access. This one is called bracket notation. It allows you to access a property whose name is determined at runtime.

For example, these two are equivalent:

this.left() // dot notation // var position = 'left'; this[position](); // bracket notation 

If you have worked with arrays, then you have come across bracket notation already:

var arr = [1,2,3]; console.log(arr[0]);