Dojo: The Definitive Guide

Type Checking

In a language with dynamic typing like JavaScript, it's often necessary (and a very good idea) to test the type of variable before performing any operations on it. Although it might not seem like much to test for the type of a variable, it isn't always a freebie, and in practice can simply result in annoyances and bugs because of subtle differences. Base provides a few handy functions to simplify the nuances entailed. Like the other issues we've touched on so far, there are subtleties amongst various browsers involving some of the finer points. The following list summarizes:

isString(/*Any*/ value)

Returns true if value is a String.

isArray(/*Any*/ value)

Returns true if value is an Array.

isFunction(/*Any*/ value)

Returns true if value is a Function.

isObject(/*Any*/ value)

Returns true if value is an Object (including an Array and Function ) or null.

isArrayLike(/*Any*/ value)

Returns true if value is an Array but also allows for more permissive possibilities. For example, the built-in arguments value that can be accessed from within a Function object is especially an oddball in that it does not support built-in methods such as push ; however, it is array-like in that it is a list of values that can be indexed.

isAlien(/*Any*/ value)

Returns true if value is a built-in function or native function such as an ActiveX component but does not respect the normal checks such as the instanceof Function.

Duck Typing

A concept commonly involved in dynamic programming languages like Python and JavaScript called duck typing provides a common thread for many of the functions just introduced. Duck typing is based upon the saying that if it walks like a duck and quacks like a duck, then it's a duck. Basically, what that means is that if a particular data member exhibits the minimal necessary properties to be a particular data type, then that's good enough to assume that it is that particular data type.

For example, the built-in arguments member qualifying as an array via the isArrayLike function hopefully ties this theme together. When you consider the inherent dynamism in a language that does not require you to declare a particular variable to always be a particular data type (dynamic binding), duck typing is a great vehicle to inspect the type of an object when necessary.

For example, invoking the typeof operator on an ordinary array such as [] returns object while Base's isArray function performs some duck type checks behind the scenes and returns true for an array such as [].

Tip

Duck typing is a fundamental programming concept in JavaScript and much of the toolkit, so this discussion is more practical to your day-to-day programming than you might imagine at first glance.

The bottom line is that Base's type checking functions can save you time and spare you from nonintuitive browser inconsistencies, so use them well, and use them often.