Variable Typing

Like PHP, JavaScript is a very loosely typed language; the type of a variable is determined only when a value is assigned and can change as the variable appears in different contexts. Usually, you don’t have to worry about the type; JavaScript figures out what you want and just does it.

Take a look at Example 13-4, in which:

  1. The variable n is assigned the string value '838102050', the next line prints out its value, and the typeof operator is used to look up the type.

  2. n is given the value returned when the numbers 12345 and 67890 are multiplied together. This value is also 838102050, but it is a number, not a string. The type of the variable is then looked up and displayed.

  3. Some text is appended to the number n and the result is displayed.

Example 13-4. Setting a variable’s type by assignment
n = '838102050'        // Set 'n' to a string
document.write('n = ' + n + ', and is a ' + typeof n + '<br />')

n = 12345 * 67890;     // Set 'n' to a number
document.write('n = ' + n + ', and is a ' + typeof n + '<br />')

n += ' plus some text' // Change 'n' from a number to a string
document.write('n = ' + n + ', and is a ' + typeof n + '<br />')

The output from this script looks like:

n = 838102050, and is a string
n = 838102050, and is a number
n = 838102050 plus some text, and is a string

If there is ever any doubt about the type of a variable, or you need to ensure a variable has a particular type, you can force it to that type using statements such as the following ...

Get Learning PHP, MySQL, JavaScript, and CSS, 2nd Edition now with the O’Reilly learning platform.

O’Reilly members experience books, live events, courses curated by job role, and more from O’Reilly and nearly 200 top publishers.