JavaScript 2.0-The Complete Reference, Second ... - freecodingtutorial

Being weakly typed is both a blessing and a curse for JavaScript. While weak typing appears to
free the programmer from having to declare types ahead of time, it does so at the expense of
introducing subtle typing errors. For example, given the following script that manipulates
various string and number values, we will see type conversions cause potential ambiguities:
document.write(4*3);
document.write("");
document.write("5" + 5);
document.write("");
document.write("5" - 3);
document.write("");
document.write(5 * "5");
The output of this example when included in an HTML document is shown here:
Notice in most of the examples the string was converted to a number before calculation and the
correct result was produced. Of course, if we would have attempted to do something like "cat" –
3, we would have seen a result of NaN because the string "cat" would convert to NaN and then
the subtraction would produce NaN as well. However, in the case of the addition of "5" + 5, the
answer was actually the string "55" rather than a number 10. The reason the addition didn‘t
work is that the plus sign serves two meanings, both as addition and as string concatenation.
Type conversion, coupled with overloaded operators like +, can create all sorts of confusion for
the beginning and advanced programmer alike, so we spend a great deal of time on the subject
in Chapter 3. Fortunately, the rules presented there are relatively logical and there are many
ways to convert data predictably in JavaScript using methods like parseFloat() and to even
check the value of a variable using the typeof operator. For example,
var x = "5";
alert (typeof x);
correctly identifies text after x as a string value, as shown here: