Unfamiliar use of square brackets in calling a function

Question

In the middle of this page, I find the code below.

var plus = function(x,y){ return x + y };
var minus = function(x,y){ return x - y };

var operations = {
  \'+         


        

Answer 1

here operations is an object where the symbols + and - refers to two functions.

operations[operation] will return a reference to function plus where value of operation is + and then the following () will invoke the function