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

My JavaScript book says that object properties need be named with arbitrary names. But '+' and '-' are not names. From the original question, it is inferred that object properties just need be keyed, not named.