Question

I have stumbled across a bit of code that I was hoping someone could explain to me.

This is used in the context of jqGrid.

onSelectRow: function(id){ 
     if(id && id!==lastsel){ 
          jQuery('#rowed3').jqGrid('restoreRow',lastsel);
          jQuery('#rowed3').jqGrid('editRow',id,true); 
          lastsel=id; 
     } 
}, 

Why use logical operators on a string in javascript as shown above? Is this simply a bug or is there some functionality here that I dont understand?

Code is taken from http://trirand.com/blog/jqgrid/rowedex3.html

Full example http://trirand.com/blog/jqgrid/jqgrid.html > Row editing > Using events

Answer 1

The variables should be numeri, though, the conditional would work either way.

if(id && id !== lastsel)

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1. The first id is saying that it must have a value. The value also has to be truthy. A value is truthy if it is not falsy, which means it just can’t be one of the following (borrowed from 11heavens.com):

   • false
   • null
   • undefined
   • The empty string ''
   • The number 0
   • The number NaN (NaN is of type number)

   Note: You will see the same kind of comparisons to true elsewhere in JavaScript, specifically for/while loops:

   /*1*/ while(id){...} // while id is true, do something
   /*2*/ for(;id;){...} // same thing, without the incrementation or variable definition
   

2. The second part is saying the id can’t be equal the lastsel, which is the last id that was used. !== is special in JavaScript meaning it must compare value and type, whereas != just compares value:

   • a == b : value of a equals value of b
   • a != b : value of a does not equal value of b
   • a === b : value of a equals value of b and type of a equals type of b
   • a !== b : value of a does not equal value of b and type of a does not equal type of b

Edit

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Additonally, it might help to think of the expression if you saw it in parentheses:
if( (id) && (id !== lastsel) )