To define array in javascript

JavaScript Arrays

An array is a special variable, which can hold more than one value:

Why Use Arrays?

If you have a list of items (a list of car names, for example), storing the cars in single variables could look like this:

However, what if you want to loop through the cars and find a specific one? And what if you had not 3 cars, but 300?

An array can hold many values under a single name, and you can access the values by referring to an index number.

Creating an Array

Using an array literal is the easiest way to create a JavaScript Array.

It is a common practice to declare arrays with the const keyword.

Learn more about const with arrays in the chapter: JS Array Const.

Example

Spaces and line breaks are not important. A declaration can span multiple lines:

Example

You can also create an array, and then provide the elements:

Example

Using the JavaScript Keyword new

The following example also creates an Array, and assigns values to it:

Example

The two examples above do exactly the same.

There is no need to use new Array() .

For simplicity, readability and execution speed, use the array literal method.

Accessing Array Elements

You access an array element by referring to the index number:

Note: Array indexes start with 0.

[0] is the first element. [1] is the second element.

Changing an Array Element

This statement changes the value of the first element in cars :

Example

Converting an Array to a String

The JavaScript method toString() converts an array to a string of (comma separated) array values.

Example

const fruits = [«Banana», «Orange», «Apple», «Mango»];
document.getElementById(«demo»).innerHTML = fruits.toString();

Access the Full Array

With JavaScript, the full array can be accessed by referring to the array name:

Example

Arrays are Objects

Arrays are a special type of objects. The typeof operator in JavaScript returns «object» for arrays.

But, JavaScript arrays are best described as arrays.

Arrays use numbers to access its «elements». In this example, person[0] returns John:

Array:

Objects use names to access its «members». In this example, person.firstName returns John:

Object:

Array Elements Can Be Objects

JavaScript variables can be objects. Arrays are special kinds of objects.

Because of this, you can have variables of different types in the same Array.

You can have objects in an Array. You can have functions in an Array. You can have arrays in an Array:

Array Properties and Methods

The real strength of JavaScript arrays are the built-in array properties and methods:

Array methods are covered in the next chapters.

The length Property

The length property of an array returns the length of an array (the number of array elements).

Example

The length property is always one more than the highest array index.

Accessing the First Array Element

Example

Accessing the Last Array Element

Example

Looping Array Elements

One way to loop through an array, is using a for loop:

Example

const fruits = [«Banana», «Orange», «Apple», «Mango»];
let fLen = fruits.length;

You can also use the Array.forEach() function:

Example

const fruits = [«Banana», «Orange», «Apple», «Mango»];

Adding Array Elements

The easiest way to add a new element to an array is using the push() method:

Example

const fruits = [«Banana», «Orange», «Apple»];
fruits.push(«Lemon»); // Adds a new element (Lemon) to fruits

New element can also be added to an array using the length property:

Example

const fruits = [«Banana», «Orange», «Apple»];
fruits[fruits.length] = «Lemon»; // Adds «Lemon» to fruits

Adding elements with high indexes can create undefined «holes» in an array:

Example

const fruits = [«Banana», «Orange», «Apple»];
fruits[6] = «Lemon»; // Creates undefined «holes» in fruits

Associative Arrays

Many programming languages support arrays with named indexes.

Arrays with named indexes are called associative arrays (or hashes).

JavaScript does not support arrays with named indexes.

In JavaScript, arrays always use numbered indexes.

Example

const person = [];
person[0] = «John»;
person[1] = «Doe»;
person[2] = 46;
person.length; // Will return 3
person[0]; // Will return «John»

WARNING !!
If you use named indexes, JavaScript will redefine the array to an object.

After that, some array methods and properties will produce incorrect results.

Example:

const person = [];
person[«firstName»] = «John»;
person[«lastName»] = «Doe»;
person[«age»] = 46;
person.length; // Will return 0
person[0]; // Will return undefined

The Difference Between Arrays and Objects

In JavaScript, arrays use numbered indexes.

In JavaScript, objects use named indexes.

Arrays are a special kind of objects, with numbered indexes.

When to Use Arrays. When to use Objects.

• JavaScript does not support associative arrays.
• You should use objects when you want the element names to be strings (text).
• You should use arrays when you want the element names to be numbers.

JavaScript new Array()

JavaScript has a built-in array constructor new Array() .

But you can safely use [] instead.

These two different statements both create a new empty array named points:

These two different statements both create a new array containing 6 numbers:

The new keyword can produce some unexpected results:

Источник

How to Declare an Array in JavaScript – Creating an Array in JS

Joel Olawanle

In JavaScript, an array is one of the most commonly used data types. It stores multiple values and elements in one variable.

These values can be of any data type — meaning you can store a string, number, boolean, and other data types in one variable.

There are two standard ways to declare an array in JavaScript. These are either via the array constructor or the literal notation.

In case you are in a rush, here is what an array looks like declared both ways:

// Using array constructor let array = new array("John Doe", 24, true); // Using the literal notation let array = ["John Doe", 24, true]; 

You can continue reading this article to properly understand these methods alongside some other interesting options they possess.

How to Declare an Array with Literal Notation

This is the most popular and easiest way to create an array. It is a shorter and cleaner way of declaring an array.

To declare an array with literal notation you just define a new array using empty brackets. It looks like this:

You will place all elements within the square brackets and separate each item or element with a comma.

let myArray = ["John Doe", 24, true]; 

Arrays are zero-indexed, meaning you can access each element starting from zero or output the entire array.

console.log(myArray[0]); // 'John Doe' console.log(myArray[2]); // true console.log(myArray); // ['John Doe', 24, true] 

How to Declare an Array with the Array Constructor

You can also use the array constructor to create or declare an array. There are a lot of technicalities to declaring an array with the Array() constructor.

Just as you can store multiple values with diverse data types in one variable with the array literal notation, you can do the same with the array constructor.

let myArray = new Array(); console.log(myArray); // [] 

The above will create a new empty array. You can add values to the new array by placing them in between the brackets, separated by a comma.

let myArray = new Array("John Doe", 24, true); 

Just like you learned earlier, you can access each value using its index number, which starts from zero (0).

console.log(myArray[0]); // 'John Doe' console.log(myArray[2]); // true console.log(myArray); // ['John Doe', 24, true] 

When declaring arrays with the array constructor method, it is important to bear the following in mind.

• When you pass a single digit into an array constructor, it will fill the array with the number of empty values you entered.

let myArray = new Array(4); console.log(myArray); // [. ] 

But when you pass a single string or any other data type, it works well.

let myArray = new Array(true); console.log(myArray); // [true] 

let myArray = Array("John Doe", 24, true); 

Wrapping up!

In this article, you have learned how to declare an array in JavaScript. It is important to know that the array constructor is not really used, as it’s way more complicated than using the array literal notation.

Источник