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

It is a dictionary access, which is like an array, but with a key instead of a numeric index.

operations['+'] will evaluate to the function plus, which is then called with the arguments plus(x,y).