Question

I am reading Javascript the Good Parts and came across the following snippet under Chapter 5 Inheritance:

var coolcat = function (spec) {
   var that = cat(spec),
             super_get_name = that.superior('get_name');
   that.get_name = function (n) {
    return 'like ' + super_get_name() + ' baby'; return that;
    }
  }

I am confused by the coma after cat(spec) in line 2. What does the line do exactly? (line 2 +line 3)
Thanks

Answer 1

That’s just a shortcut for declaring two variables in one statement, it is equivalent to this:

var that           = cat(spec);
var super_get_name = that.superior('get_name');

The comma is actually an operator in JavaScript:

The comma operator evaluates both of its operands (from left to right) and returns the value of the second operand.

A var statement is made up of one or more expressions of the form:

varname [= value]

where the square brackets indicate an optional component. The general var statement looks like this:

var varname1 [= value1 [, varname2 [, varname3 … [, varnameN]]]];

You’ll usually only see the comma operator used in var statements and for loops:

for(var i = 0, x = complicated_array[0]; i < complicated_array.length; x = complicated_array[++i])

but it can be used in other places.