- Javascript Generate Array Of Numbers
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Let’s step away from the individual data structures and talk about the iterations over them. In the previous chapter we saw methods map.keys, map.values, map.entries. These methods are generic, there is a common agreement to use them for data structures. In a language like php this would be considered a multidimensional array with key value pairs, or an array within an array. I'm assuming because you asked about how to loop through a key value array you would want to know how to get an object (key=value array) like the person object above to have, let's say, more than one person. See the Pen JavaScript -Generate an array between two integers of 1 step length-array-ex- 41 by w3resource (@w3resource) on CodePen. Improve this sample solution and post your code through Disqus. Previous: Write a JavaScript function to generate an array of specified length, filled with integer numbers, increase by one from starting position.
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Introduction
From the classic
forloop
to the forEach()
method, there are various techniques and methods used to iterate through datasets in JavaScript. One of the most popular methods is the .map()
method. .map()
creates an array from calling a specific function on each item in the parent array. .map()
is a non-mutating method in that it creates a new array as opposed to mutating methods, which only make changes to the calling array.This method can have many uses when working with arrays. In this tutorial, we’ll look at four noteworthy uses of the
.map()
in JavaScript: calling a function of array elements, converting strings to arrays, rendering lists in JavaScript libraries, and reformatting array objects.Calling a Function on Each Item in an Array
![Code Code](/uploads/1/2/6/0/126071029/265447329.jpg)
.map()
accepts a callback function as one of its arguments, and an important parameter of that function is the current value of the item being processed by the function. This is a required parameter. With this parameter, we can modify each individual item in an array and create a new function off of it. Here’s an example:
This can be simplified further to make it cleaner with:
Having code like
sweetArray.map(makeSweeter)
makes your code a bit more readable.Converting a String to an Array
.map()
is known to belong to the Array prototype. Let’s use it to convert a String to an Array. We are not developing the method to work for strings here. Rather, we will use the special .call()
method.Everything in JavaScript is an object and methods are functions attached to these objects.
.call()
allows us to use the context of one object on another. Therefore, we would be copying the context of .map()
in an array over to a string..call()
can be passed arguments of the context to be used and parameters for the arguments of the original function.Here’s an example:
Here, we used the context of
.map()
on a String and passed an argument of the function that .map()
expects. This functions like the
.split()
method of a String only that each individual string characters can be modified before being returned in an array.Rendering Lists in JavaScript Libraries
JavaScript libraries like React use
.map()
to render items in a list. This requires JSX syntax, however, as the .map()
method is wrapped in JSX syntax. Here’s an example of a React component:
This is a stateless component in React, which renders a
div
with a list. The individual list items are rendered using .map()
to iterate over the names array initially created. This component is rendered using ReactDOM on the DOM element with id of root
.Reformatting Array Objects
.map()
can be used to iterate through objects in an array and, in a similar fashion to traditional arrays, modify the content of each individual object and return a new array. This modification is done based on what is returned in the callback function. Here’s an example:
Here we modified each object in the array using the bracket and dot notation. This use case can be employed to process or condense received data before being saved or parsed on a frontend application.
Conclusion
In this tutorial, we looked at four uses of the
.map()
method in JavaScript. In combination with other methods, the functionality of .map()
can be extended. For more information, see our How To Use Array Methods in JavaScript: Iteration Methods article.JavaScript arrays are used to store multiple values in a single variable.
Example
Try it Yourself »What is an Array?
An array is a special variable, which can hold more than one value at a time.
If you have a list of items (a list of car names, for example), storing the cars in single variables could look like this:
var car1 = 'Saab';
var car2 = 'Volvo';
var car3 = 'BMW';
var car2 = 'Volvo';
var car3 = 'BMW';
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?
The solution is an array!
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.
Syntax:
Example
Try it Yourself »Spaces and line breaks are not important. A declaration can span multiple lines:
Example
Try it Yourself »Using the JavaScript Keyword new
The following example also creates an Array, and assigns values to it:
Example
Try it Yourself »The two examples above do exactly the same. There is no need to use
For simplicity, readability and execution speed, use the first one (the array literal method).
new Array()
.For simplicity, readability and execution speed, use the first one (the array literal method).
Access the Elements of an Array
You access an array element by referring to the index number.
This statement accesses the value of the first element in
cars
:Example
var cars = ['Saab', 'Volvo', 'BMW'];
document.getElementById('demo').innerHTML = cars[0];
Try it Yourself »document.getElementById('demo').innerHTML = cars[0];
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
var cars = ['Saab', 'Volvo', 'BMW'];
cars[0] = 'Opel';
document.getElementById('demo').innerHTML = cars[0];
Try it Yourself »cars[0] = 'Opel';
document.getElementById('demo').innerHTML = cars[0];
Access the Full Array
With JavaScript, the full array can be accessed by referring to the array name:
Example
var cars = ['Saab', 'Volvo', 'BMW'];
document.getElementById('demo').innerHTML = cars;
Try it Yourself »document.getElementById('demo').innerHTML = cars;
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:
Try it Yourself »Objects use names to access its 'members'. In this example,
person.firstName
returns John:Object:
var person = {firstName:'John', lastName:'Doe', age:46};
Try it Yourself »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:
myArray[0] = Date.now;
myArray[1] = myFunction;
myArray[2] = myCars;
myArray[1] = myFunction;
myArray[2] = myCars;
Array Properties and Methods
The real strength of JavaScript arrays are the built-in array properties and methods:
Examples
var x = cars.length; // The length property returns the number of elements
var y = cars.sort(); // The sort() method sorts arrays
var y = cars.sort(); // The sort() method sorts arrays
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
var fruits = ['Banana', 'Orange', 'Apple', 'Mango'];
fruits.length; // the length of fruits is 4
Try it Yourself »fruits.length; // the length of fruits is 4
The
length
property is always one more than the highest array index. Generate api key for rest api.Accessing the First Array Element
Example
fruits = ['Banana', 'Orange', 'Apple', 'Mango'];
var first = fruits[0];
Try it Yourself »var first = fruits[0];
Accessing the Last Array Element
Example
fruits = ['Banana', 'Orange', 'Apple', 'Mango'];
var last = fruits[fruits.length - 1];
Try it Yourself »var last = fruits[fruits.length - 1];
Looping Array Elements
The safest way to loop through an array, is using a
for
loop:Example
var fruits, text, fLen, i;
fruits = ['Banana', 'Orange', 'Apple', 'Mango'];
fLen = fruits.length;
text = '<ul>';
for (i = 0; i < fLen; i++) {
text += '<li>' + fruits[i] + '</li>';
}
text += '</ul>';
Try it Yourself »fruits = ['Banana', 'Orange', 'Apple', 'Mango'];
fLen = fruits.length;
text = '<ul>';
for (i = 0; i < fLen; i++) {
text += '<li>' + fruits[i] + '</li>';
}
text += '</ul>';
You can also use the
Array.forEach()
function:Example
var fruits, text;
fruits = ['Banana', 'Orange', 'Apple', 'Mango'];
text = '<ul>';
fruits.forEach(myFunction);
text += '</ul>';
function myFunction(value) {
text += '<li>' + value + '</li>';
}
fruits = ['Banana', 'Orange', 'Apple', 'Mango'];
text = '<ul>';
fruits.forEach(myFunction);
text += '</ul>';
function myFunction(value) {
text += '<li>' + value + '</li>';
}
Javascript Generate Array Of Numbers
Try it Yourself »Adding Array Elements
The easiest way to add a new element to an array is using the
push()
method:Example
var fruits = ['Banana', 'Orange', 'Apple', 'Mango'];
fruits.push('Lemon'); // adds a new element (Lemon) to fruits
Try it Yourself »fruits.push('Lemon'); // adds a new element (Lemon) to fruits
New element can also be added to an array using the
length
property:Example
var fruits = ['Banana', 'Orange', 'Apple', 'Mango'];
fruits[fruits.length] = 'Lemon'; // adds a new element (Lemon) to fruits
Try it Yourself »fruits[fruits.length] = 'Lemon'; // adds a new element (Lemon) to fruits
WARNING !
Adding elements with high indexes can create undefined 'holes' in an array:
Example
var fruits = ['Banana', 'Orange', 'Apple', 'Mango'];
fruits[6] = 'Lemon'; // adds a new element (Lemon) to fruits
Try it Yourself »fruits[6] = 'Lemon'; // adds a new element (Lemon) to fruits
Associative Arrays
Generate Key When I Map Array Javascript Free
Many programming languages support arrays with named indexes.
Gta v cd key generator v2 0 download. 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.
Javascript Map Array Keys
Example
var person = [];
person[0] = 'John';
person[1] = 'Doe';
person[2] = 46;
var x = person.length; // person.length will return 3
var y = person[0]; // person[0] will return 'John'
Try it Yourself »person[0] = 'John';
person[1] = 'Doe';
person[2] = 46;
var x = person.length; // person.length will return 3
var y = person[0]; // person[0] will return 'John'
WARNING !!
If you use named indexes, JavaScript will redefine the array to a standard object.
After that, some array methods and properties will produce incorrect results.
If you use named indexes, JavaScript will redefine the array to a standard object.
After that, some array methods and properties will produce incorrect results.
Example:
var person = [];
person['firstName'] = 'John';
person['lastName'] = 'Doe';
person['age'] = 46;
var x = person.length; // person.length will return 0
var y = person[0]; // person[0] will return undefined
Try it Yourself »person['firstName'] = 'John';
person['lastName'] = 'Doe';
person['age'] = 46;
var x = person.length; // person.length will return 0
var y = person[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.
Avoid new Array()
There is no need to use the JavaScript's built-in array constructor
new
Array().Use
[]
instead.These two different statements both create a new empty array named points:
var points = new Array(); // Bad
var points = []; // Good
var points = []; // Good
These two different statements both create a new array containing 6 numbers:
var points = new Array(40, 100, 1, 5, 25, 10); // Bad
var points = [40, 100, 1, 5, 25, 10]; // Good
Try it Yourself »var points = [40, 100, 1, 5, 25, 10]; // Good
The
new
keyword only complicates the code. It can also produce some unexpected results: var points = new Array(40, 100); // Creates an array with two elements (40 and 100)
What if I remove one of the elements?
var points = new Array(40); // Creates an array with 40 undefined elements !!!!!
Try it Yourself »How to Recognize an Array
A common question is: How do I know if a variable is an array?
The problem is that the JavaScript operator
typeof
returns 'object
': var fruits = ['Banana', 'Orange', 'Apple', 'Mango'];
typeof fruits; // returns object
Try it Yourself »typeof fruits; // returns object
The typeof operator returns object because a JavaScript array is an object.
Solution 1:
To solve this problem ECMAScript 5 defines a new method
Array.isArray()
:The problem with this solution is that ECMAScript 5 is not supported in older browsers.
Solution 2:
To solve this problem you can create your own
isArray()
function: function isArray(x) {
return x.constructor.toString().indexOf('Array') > -1;
}
Try it Yourself »return x.constructor.toString().indexOf('Array') > -1;
}
The function above always returns true if the argument is an array.
Or more precisely: it returns true if the object prototype contains the word 'Array'.
Solution 3:
Quick heal total security 2015 serial key generator. The
instanceof
operator returns true if an object is created by a given constructor: var fruits = ['Banana', 'Orange', 'Apple', 'Mango'];
fruits instanceof Array; // returns true
Try it Yourself »fruits instanceof Array; // returns true