Explanation needed for this piece of code?

Question

var p = this.getParams();
var pD = (o.params||{});
if (this.useJsonData) pD = (pD.jsonData||{});
this.cursor = (pD && pD[p.start]) ? pD[p.start] : 0;

And what is the difference between these two lines.

this.cursor = pD[p.start] || this.cursor || 0;

Is the first code fine or there is any fault in it.

Answer 1

The first one is a shortcut for an "if/else" statement. In detail, it means:

if(pD && pD[p.start])
   this.cursor = this.cursor;
else
   this.cursor = 0;

The second one is a boolean OR that delivers back True or False, and then this TRUE or FALSE gets assigned to this.cursor.

Answer 2

Your second line has a fault. What if pD is null? Also it will keep value of this.cursor that is true (not null or false, that is).

Otherwise, they are identical.

Answer 3

Look at this page: https://developer.mozilla.org/en/JavaScript/Guide/Expressions_and_Operators

Search for conditional operator, logical operators and short-circuit evaluation.