Have a look at the code below:

// mickey is undefined
var mickey;
var spidey = mickey || '5';

// you  will see "5"
alert( spidey );

Also, you can use this tip in:

function myFUnc( param ) 
{
  var newParam = param || 'default';
  // ...
}

The double pipe is great, don’t you? However, you have to be careful, because this tip can be dangerous. See some example below:

var a = 0; // zero or false can be dangerous
var b = a || '5';

alert( a ); // you will see "5"

var a = false;
var b = a || '5';

alert( b ); // you will see "5"

var a = 1;
var b = a || '5';

alert( b ); // you  will see "1"