Question

In the code below, because s is null d = “test”
but if s = “hello” then d would = “hello”.

Is this correct as it works? what is the correct way to use ||

var s = null;

var d = s || "test";

alert(d);

Answer 1

|| is “or” ; and understanding what happens here is a bit trickery

var a=false;
var b=true;
result=a||b

will give “result” true (as b is true). What happens is:

• ‘a’ is checked – it is false
• ‘b’ is checked AFTERWARDS (as no “true” result has been obtained yet, and ONE “true” result would suffice to make the whole || operator be true) – the value of it will be assigned to the left side

if you had

   var a=true;
   var b="test";
   result=a||b

result will yield true; as no other value needs to be checked by the logic of “||”

with

var a=null;
var b="test";
result=a||b;

a will be checked first – it is null, which converts to “false”. b is “test”, which is non-null, and converts to “true”. so the value of b will be assigned.

And yes, this is a correct way to use || ; the feature you use is also called short-circuit evaluation (as it stops evaluating the boolean expression as early as possible)