Add

The add operator is +.

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

Click to view the demo

Rules for add operator:

Operand 1	Operand 2	Result
NaN	any value	NaN
any value	NaN	NaN
Infinity	Infinity	Infinity
-Infinity	-Infinity	-Infinity
Infinity	-Infinity	NaN
+0	+0	+0
-0	+0	+0
-0	-0	-0
string	string	, string concatenation
string	any value	other operand is converted to a string and the do the string concatenation

If either operand is an object, number, or Boolean, its toString() method is called to get a string value. undefined becomes "undefined" and null becomes "null" for add operator.

 
<!DOCTYPE html>
<html>
<head>
    <title>Example</title>
    <script type="text/javascript">
            var result1 = 5 + 5;             //two numbers 
            document.writeln(result1); //10 
            var result2 = 5 + "5";             //a number and a string 
            document.writeln(result2); //"55" 

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

Click to view the demo

Most common mistakes

 
<!DOCTYPE html>
<html>
<head>
    <title>Example</title>
    <script type="text/javascript">
    
        var num1 = 5; 
        var num2 = 10; 
        var message = "Result: " + num1 + num2; 
        document.writeln(message); //"Result: 510" 
               
    </script>
</head>
<body>
</body>
</html>

Click to view the demo

To perform the arithmetic calculation and then append that to the string:

 
<!DOCTYPE html>
<html>
<head>
    <title>Example</title>
    <script type="text/javascript">
    
        var num1 = 5; 
        var num2 = 10; 
        var message = "Result: " + (num1 + num2); 
        document.writeln(message); //"Result: 15" 

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

Click to view the demo