What happens if I compare two items of different data types (for example, a number and a string) in a conditional expression?

The JavaScript interpreter does its best to make the values a common format and compare them. In this case, it would convert them both to strings before comparing. In JavaScript 1.3 and later, you can use the special equality operator === to compare two values and their types—using this operator, the expression will be true only if the expressions have the same value and the same data type.

Why don't I get a friendly error message if I accidentally use = instead of == ?

In some cases, this will result in an error. However, the incorrect version often appears to be a correct statement. For example, in the statement if (a=1), the variable a will be assigned the value 1. The if statement is considered true, and the value of a is lost.

Why does the script in Listing 6.1 specify JavaScript 1.2?

The switch statement was added in JavaScript 1.2. Specifying that version prevents older browsers from attempting to execute the script and becoming confused.