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JavaScript Arrays

In this tutorial you will learn how to create and manipulate arrays in JavaScript.

What is an Array

Arrays are complex variables that allow us to store more than one value or a group of values under a single variable name. JavaScript arrays can store any valid value, including strings, numbers, objects, functions, and even other arrays, thus making it possible to create more complex data structures such as an array of objects or an array of arrays.

Let's suppose you want to store the name of colors in your JavaScript code. Storing the color names one by one in a variable could look something like this:

var color1 = "Red";
var color2 = "Green";
var color3 = "Blue";

But what happens if you need to store the state or city names of a country in variables and this time this not just three may be hundred. It is quite hard and boring to store each of them in a separate variable. Also, using so many variables simultaneously and keeping track of them all will be a very difficult task. And here array comes into play. Arrays solve this problem by providing an ordered structure for storing multiple values or a group of values.

Creating an Array

The simplest way to create an array in JavaScript is enclosing a comma-separated list of values in square brackets ([]), as shown in the following syntax:

var myArray = [element0, element1, ..., elementN];

Array can also be created using the Array() constructor as shown in the following syntax. However, for the sake of simplicity previous syntax is recommended.

var myArray = new Array(element0, element1, ..., elementN);

Here are some examples of arrays created using array literal syntax:

var colors = ["Red", "Green", "Blue"]; 
var fruits = ["Apple", "Banana", "Mango", "Orange", "Papaya"];
var cities = ["London", "Paris", "New York"];
var person = ["John", "Wick", 32];
 

Note: An array is an ordered collection of values. Each value in an array is called an element, and each element has a numeric position in an array, known as its index.


Accessing the Elements of an Array

Array elements can be accessed by their index using the square bracket notation. An index is a number that represents an element's position in an array.

Array indexes are zero-based. This means that the first item of an array is stored at index 0, not 1, the second item is stored at index 1, and so on. Array indexes start at 0 and go up to the number of elements minus 1. So, array of five elements would have indexes from 0 to 4.

The following example will show you how to get individual array element by their index.

var fruits = ["Apple", "Banana", "Mango", "Orange", "Papaya"];
 
document.write(fruits[0]); // Prints: Apple
document.write(fruits[1]); // Prints: Banana
document.write(fruits[2]); // Prints: Mango
document.write(fruits[fruits.length - 1]); // Prints: Papaya

Note: In JavaScript, arrays are really just a special type of objects which has numeric indexes as keys. The typeof operator will return "object" for arrays.


Getting the Length of an Array

The length property returns the length of an array, which is the total number of elements contained in the array. Array length is always greater than the index of any of its element.

var fruits = ["Apple", "Banana", "Mango", "Orange", "Papaya"];
document.write(fruits.length); // Prints: 5

Looping Through Array Elements

You can use for loop to access each element of an array in sequential order, like this:

var fruits = ["Apple", "Banana", "Mango", "Orange", "Papaya"];
 
// Iterates over array elements
for(var i = 0; i < fruits.length; i++) {    
    document.write(fruits[i] + "<br>"); // Print array element
}

ECMAScript 6 has introduced a simpler way to iterate over array element, which is for-of loop. In this loop you don't have to initialize and keep track of the loop counter variable (i).

Here's the same example rewritten using the for-of loop:

var fruits = ["Apple", "Banana", "Mango", "Orange", "Papaya"];
 
// Iterates over array elements
for(var fruit of fruits) {    
    document.write(fruit + "<br>"); // Print array element
}

You can also iterate over the array elements using for-in loop, like this:

var fruits = ["Apple", "Banana", "Mango", "Orange", "Papaya"];
 
// Loop through all the elements in the array 
for(var i in fruits) {  
    document.write(fruits[i] + "<br>");
}

Note: The for-in loop should not be used to iterate over an array where the index order is important. The for-in loop is optimized for iterating over object's properties, you should better use a for loop with a numeric index or for-of loop.


Adding New Elements to an Array

To add a new element at the end of an array, simply use the push() method, like this:

var colors = ["Red", "Green", "Blue"]; 
colors.push("Yellow");
 
document.write(colors); // Prints: Red,Green,Blue,Yellow
document.write(colors.length); // Prints: 4

Similarly, to add a new element at the beginning of an array use the unshift() method, like this:

var colors = ["Red", "Green", "Blue"]; 
colors.unshift("Yellow");
 
document.write(colors); // Prints: Yellow,Red,Green,Blue
document.write(colors.length); // Prints: 4

You can also add multiple elements at once using the push() and unshift() methods, like this:

var colors = ["Red", "Green", "Blue"];
colors.push("Pink", "Voilet");
colors.unshift("Yellow", "Grey");
 
document.write(colors); // Prints: Yellow,Grey,Red,Green,Blue,Pink,Voilet
document.write(colors.length); // Prints: 7

Removing Elements from an Array

To remove the last element from an array you can use the pop() method. This method returns the value that was popped out. Here's an example:

var colors = ["Red", "Green", "Blue"];
var last = colors.pop();
 
document.write(last); // Prints: Blue
document.write(colors.length); // Prints: 2

Similarly, you can remove the first element from an array using the shift() method. This method also returns the value that was shifted out. Here's an example:

var colors = ["Red", "Green", "Blue"];
var first = colors.shift();
 
document.write(first); // Prints: Red
document.write(colors.length); // Prints: 2

Tip: The push() and pop() methods runs faster than unshift() and shift(). Because push() and pop() methods simply add and remove elements at the end of an array therefore the elements do not move, whereas unshift() and shift() add and remove elements at the beginning of the array that require re-indexing of whole array.


Adding or Removing Elements at Any Position

The splice() method is a very versatile array method that allows you to add or remove elements from any index, using the syntax arr.splice(startIndex, deleteCount, elem1, ..., elemN).

This method takes three parameters: the first parameter is the index at which to start splicing the array, it is required; the second parameter is the number of elements to remove (use 0 if you don't want to remove any elements), it is optional; and the third parameter is a set of replacement elements, it is also optional. The following example shows how it works:

var colors = ["Red", "Green", "Blue"];
var removed = colors.splice(0,1); // Remove the first element
 
document.write(colors); // Prints: Green,Blue
document.write(removed); // Prints: Red (one item array)
document.write(removed.length); // Prints: 1
 
removed = colors.splice(1, 0, "Pink", "Yellow"); // Insert two items at position one
document.write(colors); // Prints: Green,Pink,Yellow,Blue
document.write(removed); // Empty array
document.write(removed.length); // Prints: 0
 
removed = colors.splice(1, 1, "Purple", "Voilet"); // Insert two values, remove one
document.write(colors); //Prints: Green,Purple,Voilet,Yellow,Blue
document.write(removed); // Prints: Pink (one item array)
document.write(removed.length); // Prints: 1

The splice() method returns an array of the deleted elements, or an empty array if no elements were deleted, as you can see in the above example. If the second argument is omitted, all elements from the start to the end of the array are removed. Unlike slice() and concat() methods, the splice() method modifies the array on which it is called on.


Creating a String from an Array

There may be situations where you simply want to create a string by joining the elements of an array. To do this you can use the join() method. This method takes an optional parameter which is a separator string that is added in between each element. If you omit the separator, then JavaScript will use comma (,) by default. The following example shows how it works:

var colors = ["Red", "Green", "Blue"];
 
document.write(colors.join()); // Prints: Red,Green,Blue
document.write(colors.join("")); // Prints: RedGreenBlue
document.write(colors.join("-")); // Prints: Red-Green-Blue
document.write(colors.join(", ")); // Prints: Red, Green, Blue

You can also convert an array to a comma-separated string using the toString(). This method does not accept the separator parameter like join(). Here's an example:

var colors = ["Red", "Green", "Blue"];
document.write(colors.toString()); // Prints: Red,Green,Blue

Extracting a Portion of an Array

If you want to extract out a portion of an array (i.e. subarray) but keep the original array intact you can use the slice() method. This method takes 2 parameters: start index (index at which to begin extraction), and an optional end index (index before which to end extraction), like arr.slice(startIndex, endIndex). Here's an example:

var fruits = ["Apple", "Banana", "Mango", "Orange", "Papaya"];
var subarr = fruits.slice(1, 3);
document.write(subarr); // Prints: Banana,Mango

If endIndex parameter is omitted, all elements to the end of the array are extracted. You can also specify negative indexes or offsets —in that case the slice() method extract the elements from the end of an array, rather then the begining. For example:

var fruits = ["Apple", "Banana", "Mango", "Orange", "Papaya"];
 
document.write(fruits.slice(2)); // Prints: Mango,Orange,Papaya
document.write(fruits.slice(-2)); // Prints: Orange,Papaya
document.write(fruits.slice(2, -1)); // Prints: Mango,Orange

Merging Two or More Arrays

The concat() method can be used to merge or combine two or more arrays. This method does not change the existing arrays, instead it returns a new array. For example:

var pets = ["Cat", "Dog", "Parrot"];
var wilds = ["Tiger", "Wolf", "Zebra"];
 
// Creating new array by combining pets and wilds arrays
var animals = pets.concat(wilds); 
document.write(animals); // Prints: Cat,Dog,Parrot,Tiger,Wolf,Zebra

The concat() method can take any number of array arguments, so you can create an array from any number of other arrays, as shown in the following example:

var pets = ["Cat", "Dog", "Parrot"];
var wilds = ["Tiger", "Wolf", "Zebra"];
var bugs = ["Ant", "Bee"];
 
// Creating new array by combining pets, wilds and bugs arrays
var animals = pets.concat(wilds, bugs); 
document.write(animals); // Prints: Cat,Dog,Parrot,Tiger,Wolf,Zebra,Ant,Bee

Searching Through an Array

If you want to search an array for a specific value, you can simply use the indexOf() and lastIndexOf(). If the value is found, both methods return an index representing the array element. If the value is not found, -1 is returned. The indexOf() method returns the first one found, whereas the lastIndexOf() returns the last one found.

var fruits = ["Apple", "Banana", "Mango", "Orange", "Papaya"];
 
document.write(fruits.indexOf("Apple")); // Prints: 0
document.write(fruits.indexOf("Banana")); // Prints: 1
document.write(fruits.indexOf("Pineapple")); // Prints: -1

Both methods also accept an optional integer parameter from index which specifies the index within the array at which to start the search. Here's an example:

var arr = [1, 0, 3, 1, false, 5, 1, 4, 7];
 
// Searching forwards, starting at from- index
document.write(arr.indexOf(1, 2)); // Prints: 3
 
// Searching backwards, starting at from index
document.write(arr.lastIndexOf(1, 2)); // Prints: 0

You can also use includes() method to find out whether an array includes a certain element or not. This method takes the same parameters as indexOf() and lastIndexOf() methods, but it returns true or false instead of index number. For example:

var arr = [1, 0, 3, 1, false, 5, 1, 4, 7];
 
document.write(arr.includes(1)); // Prints: true
document.write(arr.includes(6)); // Prints: false
document.write(arr.includes(1, 2)); // Prints: true
document.write(arr.includes(3, 4)); // Prints: false

If you want to search an array based on certain condition then you can use the JavaScript find() method which is newly introduced in ES6. This method returns the value of the first element in the array that satisfies the provided testing function. Otherwise it return undefined.

var arr = [1, 0, 3, 1, false, 5, 1, 4, 7];
 
var result = arr.find(function(element) {
  return element > 4;
});
document.write(result); // Prints: 5

There is one more method similar to find() is findIndex() method, which returns the index of a found element in the array instead of its value. For example:

var arr = [1, 0, 3, 1, false, 5, 1, 4, 7];
 
var result = arr.findIndex(function(element) {
  return element > 6;
});
document.write(result); // Prints: 8

The find() method only looks for the first element that satisfies the provided testing function. However, if you want to find out all the matched elements you can use the filter() method.

The filter() method creates a new array with all the elements that successfully passes the given test. The following example will show you how this actually works:

var arr = [1, 0, 3, 1, false, 5, 1, 4, 7];
 
var result = arr.filter(function(element) {
  return element > 4;
});
document.write(result); // Prints: 5,7
document.write(result.length); // Prints: 2

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