That’s the same, because Object.fromEntries expects an iterable object as the argument. Not necessarily an array. And the standard iteration for map returns same key/value pairs as map.entries.So we get a plain object with same key/values as the map. A Set is a special type collection – “set of values” (without keys), where each value may occur only once. Jun 02, 2015 How to create multidimensional array with key value pair JS Hey everyone i'm currently learning more about javascript and wanted to know how to do something like this in PHP which is a multidimensional array with key pairs.
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The
length
property of Array
objects is one that many who are relatively new to JavaScript do not understand. Many mistakenly believe that the length tells you exactly how many entries there are in an array whereas this is only true of some arrays. Some beginners do not even realize that length
is a writable property of arrays. To clarify just exactly how the length
property works, let’s take a look at what happens when we either change its value ourselves or run something that updates the array that also results in the length changing.Let’s start at the beginning. A JavaScript array has a property called
length
and optionally has numbered properties with names between 0 and 4294967294 inclusive. It also has a number of methods for manipulating the properties some of which we will look at as a part of our examination of how the length property works. Note that JavaScript does not support associative arrays and so while you can add named properties to an array, they do not form a part of the array and will be ignored by all the array methods. They also will not affect the length.To make it easier to show exactly what happens to the array properties as we process various statements, we will run the following function after each piece of code. This will log the length of the array and all of the numbered properties to the browser’s console.
![Generate Key When I Map Array Javascript Generate Key When I Map Array Javascript](/uploads/1/2/5/8/125870325/974117321.gif)
Creating an Array
We will begin by looking at different ways to create an array in JavaScript. The first two of these examples create arrays where only the length is set and there are no numbered entries at all. The second two create numbered entries from 0 to one less than the length.
An array where the length is greater than the amount of numbered properties is known as a sparse array while one with the length equal to the number of numbered properties is a dense array.
Note that the array literal notation (where you define a new array using just empty brackets) is preferred when creating new arrays.
The array methods that process the numbered properties (
forEach
in our case) will only process those that exist. If you instead process the array using a for
or while
loop then the loop will also attempt to process those properties that don’t exist and the array will identify those entries that don’t exist as being undefined
. Your code would then be unable to distinguish between the last of the above examples and the first two. You should always use the array methods for processing an array where you are not certain that you are dealing with a dense array.Changing the Length
The following examples look at what happens if we set a new length for the array that is less than the current length.
Note that when creating an array using
[]
notation each entry consists of a value followed by a comma. Where the value is omitted then no property is created for that position. The last comma may only be omitted if there is a value supplied for that property as otherwise the length will be reduced by one.Removing Entries
JavaScript provides three methods pop, shift and splice that can remove entries from the array and which therefore reduce the length of the array. In each case the value (or values) removed are returned by the call.
Adding Entries
We can add a new entry into an array simply by specifying a position in the array for which a numbered property does not yet exist. We can also use one of the three methods JavaScript provides (push, unshift and splice) for inserting new entries and, where necessary, moving the old ones.
Replacing Entries
Where we assign a new value to an entry that already exists, then that entry simply gets a new value and the rest of the array is unaffected. Also by combining the variants of the
splice()
method that we have already looked at we can replace existing entries or fill gaps in the array.Conclusion
The above examples should have given you a better idea of how the
length
property of an array works. This can be greater or equal to the number of entries in the array. Where it is equal we have a dense array and where it is greater we have a sparse array. Exactly what a particular array method does can depend on whether there is actually a property corresponding to a given position in a sparse array. If we change the length of an array it removes any numbered properties in the array that are in positions that are greater than the new length. If the length was equal to the amount of numbered properties and we increase the length then we convert a dense array to a sparse one. The array methods for deleting and adding properties in the array will move the existing entries around where necessary and will also retain and move any gaps between the properties.Related
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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
.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.Generate Key When I Map Array Javascript In Java
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
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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
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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.