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Inside JavaScript: Objects and Arrays

Target Audience

This blog is aimed at developers who are either learning JavaScript or have been working with it for a while and would like to deepen their understanding.

In JavaScript, Everything is an Object

Well, everything that is, except for primitive values such a numbers and Booleans etc

But even things like functions are special objects with an additional executable part

So, what’s an object?

An object is two things basically:

  1. It is an unordered collection of name/value pairs, and
  2. It is a container for a set of methods and properties inherited from a parent object known as a prototype (we’ll discuss prototypical inheritance in another blog)

Here’s the simplest way to create an object…

// Create an empty object
var person1 = {}

Creating Object Properties

We can now bind any value we like to any object property we like. However, depending on what names we want to assign to these properties, we might need to write the coding using one of two different syntaxes.

If the property name does not need to contain any characters that have reserved meanings such as - or + or @, then you can use the refinement operator . (that’s the fancy technical name for the dot in between the object name and the property name):

// Create an empty object
var person1 = {}

person1.firstName = "Harry"
person1.lastName  = "Hawk"
person1.hobbies   = ["swimming","cycling"]

person1.listHobbies = function() {
  return this.hobbies.join(" ");
}

But Objects Also Have Array-like Properties…

But what if you need to have a property whose name contains a reserved character? In this case, you can use the array syntax. This is possible because all JavaScript objects can be treated as if they are arrays containing named elements (not numbered elements).

// Create an empty object
var person2 = {}

person2["first-name"] = "Harry"
person2["last-name"]  = "Hawk"
person2["hobbies"]    = ["swimming","cycling"]

person2["listHobbies"] = function() {
  return this.hobbies.join(" ")
}

Notice that the person2 object now contains two properties (first-name and last-name) whose names contain the reserved minus “-” character. Due to the presence of this reserved character in the name, it is now impossible to refer to these properties using the refinement operator (.). Any attempt to do so would create a syntax error:

> console.log(person2.last-name)
Uncaught ReferenceError: name is not defined

But by switching to the array syntax, this now becomes perfectly valid JavaScript:

> console.log(person2["last-name"])
Hawk

Deleting Object Properties

A property can be removed from an object using the delete keyword; but here, you must be careful not to confuse object properties with variables:

// Declare a global variable.  This is not a property of the global object
var aGlobalVariable = "I'm a global variable"

// Declare a property belonging to the global object (notice that the 'var' keyword is missing)
aGlobalProperty     = "I'm a global property"

// Delete can only operate on properties, not variables, so delete here returns false
delete aGlobalVariable    // false
aGlobalVariable           // "I'm a global variable" (this value still exists!)

// The property is deleted because it belongs to the global object - delete therefore returns true
delete aGlobalProperty    // true
aGlobalProperty           // undefined (this property no longer exists in the global object)

JavaScript Arrays Are Also Objects…

JavaScript arrays are in fact just JavaScript objects—but with a special twist

IMPORTANMT

JavaScript Array elements are simply object properties whose names are the string representation of the element’s index number

// Create an array
var listOfThings = ["ball","cup","pen","car"]

// Array elements can now be accessed either by their numeric index, 
// or by the string representation of the numeric index
listOfThings[1]         // "cup"
listOfThings["1"]       // "cup"

// But attempts to use the refinement operator (dot notation) make no sense
listOfThings.1          // Uncaught SyntaxError: Unexpected number
listOfThings."1"        // Uncaught SyntaxError: Unexpected string

So, here we can see that the element at index 1 can be referenced using either the integer 1 or by the string representation of that integer "1". These two references are equivalent.

Q:    Ok, so what about adding new elements to the array?
A:    Again, there are multiple ways of doing this.

We could simply push an element onto the end of the array:

listOfThings.push("dog")           // ["ball","cup","pen","car","dog"]

Or we could add a new element at a specific numerical index given by an integer:

listOfThings[5] = "tree"           // ["ball","cup","pen","car","dog","tree"]

Or we could add a new element at a specific numerical index given by a character string:

listOfThings["6"] = "snorkel"      // ["ball","cup","pen","car","dog","tree","snorkel"]

Don’t Get Array Index Numbers Confused with Property Names!

Here’s another JavaScript gotcha…

Since an array is a special object whose element names are simply the string representation of numbers, you might think that you could add an element to the array using a named property like this:

var listOfThings = ["ball","cup","pen","car"]
listOfThings.length                          // 4

// Create an "array element" using a non-numeric property name
listOfThings["first"] = "Some value"
listOfThings["first"]                        // "Some value"
listOfThings.first                           // "Some value"

listOfThings.length                          // 4 - Huh?
listOfThings = ["ball","cup","pen","car"]    // But didn't we just add a new array element?

The problem here is that we have added a new property called first to the listOfThings object, but that property is not recognised as being an element of the array because its name is not the string representation of a number.

In fact, all the above coding did was add a new property to the object, not a new element to the array.

So, the mistake here was to confuse the properties of object listOfItems with the elements of array listOfItems.

IMPORTANT

The elements of an Array object are nothing more than a specially named subset of that object’s properties

What the Array .length Property Does Not Mean…

The .length property of an array does not always return the expected value. For instance:

// Create an empty array
var someArray = []

someArray.length   // 0     Ok, that's fine

// Add a new element at some arbitrary index
someArray[4] = "Surprise!"

someArray.length   // 5  Uh!?

someArray          // [undefined, undefined, undefined, undefined, "Surprise!"]

The .length property will always return a value 1 higher than the value of the current highest index – irrespective of whether any of the intervening elements exist or not!

Hope that helps!

Chris W

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