Logical OR

The logical OR operator is || in JavaScript:


var result = true || false;

Logical OR behaves in the following truth table:

OPERAND 1	OPERAND 2	RESULT
true	true	true
true	false	true
false	true	true
false	false	false

When logical OR working with non-Boolean values:

OPERAND 1	OPERAND 2	RESULT
object	any value	return the first operand
false	any value	return OPERAND 2
object	object	return the first operand
null	null	null
NaN	NaN	NaN
undefined	undefined	undefined

The logical OR operator is short-circuited. The short-circuited operation means that if the first operand determines the result, the second operand is not evaluated. If the first operand evaluates to true, the second operand is not evaluated.

 
<!DOCTYPE html>
<html>
<head>
    <title>Example</title>
    <script type="text/javascript">
    
        var found = true; 
        var i=1;
        var result = found || i++;
        document.writeln(result); //true
        
        found = false; 
        
        var result = found || i++;
        document.writeln(result); //1
        document.writeln(i);  //2
        var result = found || i++; 
        document.writeln(result); //2
        document.writeln(i); //3

       
    </script>
</head>
<body>
</body>
</html>

Click to view the demo

You can use short-circuited OR operator to avoid assigning a null or undefined value to a variable.


var myObject = preferredObject || backupObject;

If preferredObject isn't null, then it's assigned to myObject; if preferredObject is null, then backupObject is assigned to myObject.