Data can be stored and accessed in many different ways, both in Javascript and other languages. This section will teach you how to manipulate arrays, as well as access and copy the information within them.

It will also teach you how to manipulate and access the data within Javascript objects, using both dot and bracket notation.When you're done with this section, you should understand the basic properties and differences between arrays and objects, as well as how to choose which to use for a given purpose

The below is an example of the simplest implementation of an array data structure. This is known as a one-dimensional array, meaning it only has one level, or that it does not have any other arrays nested within it. Notice it contains booleans, strings, and numbers, among other valid JavaScript data types:

All array's have a length property, which as shown above, can be very easily accessed with the syntax Array.length

A more complex implementation of an array can be seen below. This is known as a multi-dimensional array, or an array that contains other arrays. Notice that this array also contains JavaScript objects, which we will examine very closely in our next section, but for now, all you need to know is that arrays are also capable of storing complex objects.

The fundamental feature of any data structure is, of course, the ability to not only store data, but to be able to retrieve that data on command. So, now that we've learned how to create an array, let's begin to think about how we can access that array's information.

When we define a simple array as seen below, there are 3 items in it:

In an array, each array item has an index. This index doubles as the position of that item in the array, and how you reference it. However, it is important to note, that JavaScript arrays are zero-indexed, meaning that the first element of an array is actually at the zeroth position, not the first.

In order to retrieve an element from an array we can enclose an index in brackets and append it to the end of an array, or more commonly, to a variable which references an array object. This is known as bracket notation.

For example, if we want to retrieve the "a" from ourArray and assign it to a variable, we can do so with the following code:

In addition to accessing the value associated with an index, you can also set an index to a value using the same notation:

Using bracket notation, we have now reset the item at index 1 from "b", to "not b anymore" .

An array's length, like the data types it can contain, is not fixed. Arrays can be defined with a length of any number of elements, and elements can be added or removed over time; in other words, arrays are mutable. In this challenge, we will look at two methods with which we can programmatically modify an array: Array.push() and Array.unshift().

Both methods take one or more elements as parameters and add those elements to the array the method is being called on; the push() method adds elements to the end of an array, and unshift() adds elements to the beginning. Consider the following:

Notice that we can also pass variables, which allows us even greater flexibility in dynamically modifying our array's data.

Both push() and unshift() have corresponding methods that are nearly functional opposites: pop() and shift(). As you may have guessed by now, instead of adding, pop() removes an element from the end of an array, while shift() removes an element from the beginning. The key difference between pop() and shift() and their cousins push() and unshift(), is that neither method takes parameters, and each only allows an array to be modified by a single element at a time.

let's take a look:

We can also return the value of the removed element with either method like this:

Ok, so we've learned how to remove elements from the beginning and end of arrays using shift() and pop(), but what if we want to remove an element from somewhere in the middle? Or remove more than one element at once? Well, that's where splice() comes in. splice() allows us to do just that: remove any number of consecutive elements from anywhere in an array.

splice() can take up to 3 parameters, but for now, we'll focus on just the first 2. The first two parameters of splice() are integers which represent indexes, or positions, of the array that splice() is being called upon. And remember, arrays are zero-indexed, so to indicate the first element of an array, we would use 0. splice()'s first parameter represents the index on the array from which to begin removing elements, while the second parameter indicates the number of elements to delete. For example:

splice() not only modifies the array it's being called on, but it also returns a new array containing the value of the removed elements:

Remember in the last challenge we mentioned that splice() can take up to three parameters? Well, we can go one step further with splice() — in addition to removing elements, we can use that third parameter, which represents one or more elements, to add them as well

This can be incredibly useful for quickly switching out an element, or a set of elements, for another. For instance, let's say you're storing a color scheme for a set of DOM elements in an array, and want to dynamically change a color based on some action:

This function takes an array of hex values, an index at which to remove an element, and the new color to replace the removed element with. The return value is an array containing a newly modified color scheme! While this example is a bit oversimplified, we can see the value that utilizing splice() to its maximum potential can have.

The next method we will cover is slice(). rather than modifying an array, copies, or extracts, a given number of elements to a new array, leaving the array it is called upon untouched. slice() takes only 2 parameters — the first is the index at which to begin extraction, and the second is the index at which to stop extraction (extraction will occur up to, but not including the element at this index). Consider this:

In effect, we have created a new array by extracting elements from an existing array.

While slice() allows us to be selective about what elements of an array to copy, among several other useful tasks, ES6's new spread operator allows us to easily copy all of an array's elements, in order, with a simple and highly readable syntax. The spread syntax simply looks like this: ...

In practice, we can use the spread operator to copy an array like so:

Another huge advantage of the spread operator, is the ability to combine arrays, or to insert all the elements of one array into another, at any index. With more traditional syntaxes, we can concatenate arrays, but this only allows us to combine arrays at the end of one, and at the start of another. Spread syntax makes the following operation extremely simple:

Using spread syntax, we have just achieved an operation that would have been more more complex and more verbose had we used traditional methods.

Since arrays can be changed, or mutated, at any time, there's no guarantee about where a particular piece of data will be on a given array, or if that element even still exists. Luckily, JavaScript provides us with another built-in method, indexOf(), that allows us to quickly and easily check for the presence of an element on an array. indexOf() takes an element as a parameter, and when called, it returns the position, or index, of that element, or -1 if the element does not exist on the array.

For example:

Sometimes when working with arrays, it is very handy to be able to iterate through each item to find one or more elements that we might need, or to manipulate an array based on which data items meet a certain set of criteria. JavaScript offers several built in methods that each iterate over arrays in slightly different ways to achieve different results (such as every(), forEach(), map(), etc.), however the technique which is most flexible and offers us the greatest amount of control is a simple for loop.

Consider the following:

At their most basic, objects are just collections of key-value pairs, or in other words, pieces of data mapped to unique identifiers that we call properties or keys. Let's take a look at a very simple example:

The above code defines an object called User that has four properties, each of which map to a specific value. If we wanted to know the number of followers User has, we can access that property by writing: has, we can access that property by writing:

This is called dot notation. Alternatively, we can also access the property with brackets, like so:

Notice that with bracket notation, we enclosed followers in quotes. This is because the brackets actually allow us to pass a variable in to be evaluated as a property name (hint: keep this in mind for later!). Had we passed followers in without the quotes, the JavaScript engine would have attempted to evaluate it as a variable, and a ReferenceError: followers is not defined would have been thrown.

Now let's take a look at a slightly more complex object. Object properties can be nested to an arbitrary depth, and their values can be any type of data supported by JavaScript, including arrays and even other objects. Consider the following:

nestedObject has three unique keys: id, whose value is a number, date whose value is a string, and data, whose value is an object which has yet another object nested within it. While structures can quickly become complex, we can still use the same notations to access the information we need.

In the first object challenge we mentioned the use of bracket notation as a way to access property values using the evaluation of a variable. For instance, imagine that our foods object is being used in a program for a supermarket cash register. We have some function that sets the selectedFood and we want to check our foods object for the presence of that food. This might look like:

This code will evaluate the value stored in the selecteFood variable and return the value of that key in the foods object, or undefined if it is not present. Bracket notation is very useful because sometimes object properties are not known before runtime or we need to access them in a more dynamic way.

Now you know what objects are and their basic features and advantages. In short, they are key-value stores which provide a flexible, intuitive way to structure data, and, they provide very fast lookup time. Throughout the rest of these challenges, we will describe several common operations you can perform on objects so you can become comfortable applying these useful data structures in your programs.

In earlier challenges, we have both added to and modified an object's key-value pairs. Here we will see how we can remove a key-value pair from an object.

Let's revisit our foods foods object example one last time. If we wanted to remove the apples key, we can remove it by using the delete delete keyword like this:

Now we can add, modify, and remove keys from objects. But what if we just wanted to know if an object has a specific property? JavaScript provides us with two different ways to do this. One uses the hasOwnProperty() method and the other uses the in keyword. If we have an object users with a property of Alan, we could check for its presence in either of the following ways:

Sometimes you may need to iterate through all the keys within an object. This requires a specific syntax in JavaScript called a for...in statement. For our users object, this could look like:

In this statement, we defined a variable user, and as you can see, this variable was reset during each iteration to each of the object's keys as the statement looped through the object, resulting in each user's name being printed to the console.

NOTE:

Objects do not maintain an ordering to stored keys like arrays do; thus a keys position on an object, or the relative order in which it appears, is irrelevant when referencing or accessing that key.

We can also generate an array which contains all the keys stored in an object using the Object.keys() method and passing in an object as the argument. This will return an array with strings representing each property in the object. Again, there will be no specific order to the entries in the array.

see also