parseInt()

The parseInt() function ignores leading white space. The parseInt() function checks the string character by character.

If this first non-white space character isn't a number, the minus sign, or the plus sign, parseInt() returns NaN. The parseInt() function returns NaN for empty string, while Number() returns 0.

If the first character is a number, plus, or minus, then the checking continues until the end of the string is reached or a nonnumeric character is found. For instance, "1234asdf" is converted to 1234 because "asdf" is ignored.

 
<!DOCTYPE html>
<html>
<head>
    <title>Number Example</title>
    <script type="text/javascript">
        document.writeln(parseInt("1234asdf"));
        
    </script>
</head>
<body>
</body>
</html>

Click to view the demo

"1.2" is converted to 1 because the decimal point is not a valid integer character.

 
<!DOCTYPE html>
<html>
<head>
    <title>Number Example</title>
    <script type="text/javascript">
        document.writeln(parseInt("1.2"));
        
    </script>
</head>
<body>
</body>
</html>

Click to view the demo

The parseInt() function recognizes the decimal, octal, and hexadecimal formats.

 
<!DOCTYPE html>
<html>
<head>
    <title>Number Example</title>
    <script type="text/javascript">
        document.writeln(parseInt("0xA"));
        
    </script>
</head>
<body>
</body>
</html>

Click to view the demo

"" is converted to NaN.

 
<!DOCTYPE html>
<html>
<head>
    <title>Number Example</title>
    <script type="text/javascript">
        document.writeln(parseInt(""));
        
    </script>
</head>
<body>
</body>
</html>

Click to view the demo

If the string begins with "0" followed by a 0-7 number, it is interpreted as an octal value.

 
<!DOCTYPE html>
<html>
<head>
    <title>Number Example</title>
    <script type="text/javascript">
        document.writeln(parseInt("0123"));
        
    </script>
</head>
<body>
</body>
</html>

Click to view the demo

radix

parseInt() provides a second argument: the radix.

For hexadecimal format, the radix is 16.

 
<!DOCTYPE html>
<html>
<head>
    <title>Number Example</title>
    <script type="text/javascript">
        document.writeln(parseInt("0xAE", 16));
        
    </script>
</head>
<body>
</body>
</html>

Click to view the demo

The leading "0x" is not necessary by providing the hexadecimal radix:

 
<!DOCTYPE html>
<html>
<head>
    <title>Number Example</title>
    <script type="text/javascript">
        document.writeln(parseInt("AE", 16));
        
    </script>
</head>
<body>
</body>
</html>

Click to view the demo

Passing in a radix can change the outcome of the conversion.

 
<!DOCTYPE html>
<html>
<head>
    <title>Number Example</title>
    <script type="text/javascript">
        document.writeln(parseInt("10", 2)); //2 - parsed as binary 
        document.writeln(parseInt("10", 8)); //8 - parsed as octal 
        document.writeln(parseInt("10", 10));//10 - parsed as decimal 
        document.writeln(parseInt("10", 16));//16 - parsed as hexadecimal 

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

Click to view the demo