Type checking

// Check to see if our number is actually a string

if ( typeof num  == "string" )

// If it is, then parse a number out of it

num = parseInt( num );

// Check to see if our array is actually a string

if ( typeof arr == "string" )

// If that's the case, make an array, splitting on commas

arr = arr.split(",");

The use of the typeof statement helps to lead us into the topic of type-checking. Since

JavaScript is (currently) a dynamically typed language, this proves to be a very useful and

important topic. There are a number of different ways to check the type of a variable; we’re

going to look at two that are particularly useful.

The first way of checking the type of an object is by using the obvious-sounding typeof

operator. This utility gives us a string name representing the type of the contents of a variable.

This would be the perfect solution except that for variables of type object or array, or a custom

object such as user, it only returns object, making it hard to differentiate between all objects.

The second way of checking the type of an object is by referencing a property of all

JavaScript objects called constructor. This property is a reference to the function used to

originally construct this object.

// Check to see if our number is actually a string

if ( num.constructor == String )

// If it is, then parse a number out of it

num = parseInt( num );

// Check to see if our string is actually an array

if ( str.constructor == Array )

// If that's the case, make a string by joining the array using commas

str = str.join(',');

Table 2-1 shows the results of type-checking different object types using the two differ

methods that I’ve described. The first column in the table shows the object that we’re tryin

find the type of. The second column is the result of running typeof Variable (where Variabl

the value contained in the first column). The result of everything in this column is a string.

Finally, the third column shows the result of running Variable.constructor against the objec

contained in the first column. The result of everything in this column is an object.

Table 2-1. Type-Checking Variables

Variable typeof    Variable     Variable.constructor

{ an: “object” }   object       Object

[ “an”, “array” ]  object       Array

function(){}       function     Function

“a string”         string       String

55                 number       Number

true               boolean      Boolean

new User()         object       User