The document of comma operator is very simple.

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

I think the document means this.

 
                expr1, expr2
                /   \
             expra,  exprb
              /   \
          exprc , exprd
           ...
 

So the value of the expression is the value of the last expression.

 
>false || ("abd", abc = 2, abc = 3+4, !1,typeof window) == "object";
true
>abc
7
 

Comma can concatenate any number of expressions and evaluate them all, if the value of the expression is needed , the last expression is returned .

Comma used in for loop

At the end of for loop, only one statement can be executed , if you want to execute multiple expressions, use comma.

 
for (var i = 0, j = 10; i <= 10; i++, j--)
   console.log(i + j );