javascript create array with default values

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default array values

Is there a way to assign a default values to arrays in javascript?

ex: an array with 24 slots that defaults to 0

4 check out Array.prototype.fill – neaumusic Dec 26, 2015 at 6:52

18 Answers 18

You can use the fill function on an array:

Note: fill was only introduced in ECMAScript 2015 (alias "6"), so as of 2017 browser support is still very limited (for example no Internet Explorer).

2 How is map allowed on the first Array? Something about the way you construct the Array allows map to work, when it normally would not – neaumusic Dec 25, 2015 at 6:59

var A= [].repeat(0, 24);

the best and easiest solution is :

and result will be

you can put any thing you like in it:

Now if you want to change some of the current value in the array you can use

and output is

A little wordy, but it works.

If you use a "very modern" browser (check the compatibility table ), you can rely on fill :

For several reasons, the use of new Array() is not appreciated by a lot of developers; so you can do the same as follows:

The latter is much more compatible, and as you can see both solutions are represented by two lines of code.

I personally use:

If you are using ES6 and up you can use

this also works:

You have to use a loop.

13 Technically it's still a loop, moved into a function. – kennytm Jan 12, 2010 at 5:33

And now, more cleverly :) @see JavaScript Array reference Example on JSFiddle

1 Here's a version that gives you numbers as requested in the question: Array(24).join().split('').map(function(){return 0}) – JussiR Jun 11, 2014 at 11:05

Use Lodash ( https://lodash.com/docs ), a robust library for array manipulation which is available for browser too.

Another way to achieve the same -

Array.from(Array(24),()=>5)

Array.from accepts a second param as its map function.

Simplest one:

Your answer is redundant . – zzzzBov Sep 2, 2017 at 23:40

Syntax which i am using below is based on ECMA Script 6/es6

let arr=[];

arr.length=5; //Size of your array;

[...arr].map( Boolean ).map( Number ); //three dot operator is called spread Operator introduce in ECMA Script 6

------------Another way-------------

let variable_name=new Array(size).fill(initial_value)

for ex- let arr=new Array(5).fill(0) //fill method is also introduced in es6

Another way as per ECMA 5 or es5

var variable_name=Array.apply(null,Array(size)).map(Boolean).map(Number)

var arr=Array.apply(null,Array(5)).map(Boolean).map(Number);

All of them will give you same result : [0,0,0,0,0]

4 I have no idea why this was ever upvoted. He asked for an array full of 0s not the string version of 0. plus this code is horribly written, why use a regex replace when you can just do var a = new Array(6).join(0).split('') ? – Russ Bradberry May 7, 2012 at 19:22

If you want BigO(1) instead of BigO(n) please check below solution:

Default value of an array is undefined . So if you want to set default to 0, you have to loop all elements of array to set them to 0.

If you have to do that, it's ok. But I think it'll better if you check the value when want to get value of the element.

Define a function 'get': to get value of the array. myArray[index], if myArray[index] undefined, return '0', else return the value.

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How to Fill an Array with Defaults

The array built-in function, the fill and map array methods, and more.

There are cases when we want to create an array filled with default values. For small arrays, of course, we can just write down the values when declaring the array but for larger arrays, a nicer system may be needed.

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How to Fill an Array with Initial Values in JavaScript

JavaScript provides many ways to initialize arrays with initial data. Let's see in this post which ways are the most simple and popular.

Before I go on, let me recommend something to you.

If you want to significantly improve your JavaScript knowledge, take the amazingly useful course "Modern JavaScript From The Beginning 2.0" by Brad Traversy. Use the coupon code "DMITRI" and get 20% discount!

1. fill an array with primitives, 2.1 using array.fill(), 2.2 using array.from(), 2.3 using array.map() with spread operator, 3. conclusion.

Let's say that you'd like to initialize an array of length 3 with zeros.

The array.fill(initalValue) method available on the array instance is a convenient way to initialize an arrays: when the method is called on an array, the entire array is filled with initialValue , and the modified array is returned.

But you need to use array.fill(initialValue) in combination with Array(n) (the array constructor):

Try the demo.

Array(length) creates a sparse array with the length of 3 .

Then Array(length).fill(0) method fills the array with zeroes, returning the filled array: [0, 0, 0] .

Array(length).fill(initialValue) is a convenient way to create arrays with a desired length and initialized with a primitive value (number, string, boolean).

2. Fill an array with objects

What if you need to fill an array with objects? This requirement is slightly nuanced depending if you want the array filled with the initial object instances, or different instances.

If you don't mind initializing the array with the same object instance, then you could easily use array.fill() method mentioned above:

Array(length).fill({ value: 0 }) creates an array of length 3 , and assigns to each item the { value: 0 } (note: same object instance).

This approach creates an array having the same object instance. If you happen to modify any item in the array, then each item in the array is affected:

Altering the second item of the array filledArray[1].value = 3 alters all the items in the array.

In case if you want the array to fill with copies of the initial object, then you could use the Array.from() utility function.

Array.from(array, mapperFunction) accepts 2 arguments: an array (or generally an iterable), and a mapper function.

Array.from() invokes the mapperFunction upon each item of the array, pushes the result to a new array, and finally returns the newly mapped array.

Thus Array.from() method can easily create and initialize an array with different object instances:

Array.from() invokes the mapper function on each empty slot of the array and creates a new array with every value returned from the mapper function.

You get a filled array with different object instances because each mapper function call returns a new object instance.

If you'd modify any item in the array, then only that item would be affected, and the other ones remain unaffected:

filledArray[1].value = 3 modifies only the second item of the array.

You may be wondering: why use Array.from() and its mapper function, since the array has already an array.map() method?

Good question!

When using Array(length) to create array instances, it creates sparse arrays (i.e. with empty slots):

And the problem is that array.map() skips the empty slots:

Thus using directly Array(length).map(mapperFunc) would create sparse arrays.

Fortunately, you can use the spread operator to transform a sparse array into an array with items initialized with undefined . Then apply array.map() method on that array:

The expression [...Array(length)] creates an array with items initialized as undefined . On such an array, array.map() can map to new object instances.

I prefer the Array.from() approach to filling an array with objects because it involves less magic.

JavaScript provides a bunch of good ways to fill an array with initial values.

If you'd like to create an array initialized with primitive values, then the best approach is Array(length).fill(initialValue) .

To create an array initialized with object instances, and you don't care that each item would have the same object instance, then Array(length).fill(initialObject) is the way to go.

Otherwise, to fill an array with different object instances you could use Array.from(Array.length, mapper) or [...Array(length)].map(mapper) , where mapper is a function that returns a new object instance on each call.

What other ways to fill an array in JavaScript do you know?

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Mapping an array to a new array with default values in JavaScript

Suppose, we have an array of start times and stop times for a stopwatch like this −

We are required to write a JavaScript function that takes one such array. Our function needs to turn them into one final array that is the actual amount of time that passed for each entry.

Therefore, for the above array, the output should look like −

The code for this will be −

And the output in the console will be −

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Fill an Array with Initial Values in JavaScript

Posted by Sudipta Deb | May 9, 2022 | javascript , JavaScript Certification , Uncategorized , Web Technology | 1

Arrays can be initialised with data in a variety of ways in JavaScript. Let’s examine which methods are the easiest and most popular in this post.

Read this blog post or watch the video below to learn more about different ways to fill an array with initial values in JavaScript .

#1 Fill an array with primitives

Let’s say, you want to initialize an array of elements 5 with the number 1. You can use the array.fill(initialValue) method to initialize an array. This method will return the modified array.

The thing that you need to keep in mind is the usage of array.fill(initialValue) in combination with Array(n) constructor.

Array(length) creates the sparse array. If you are not aware of sparse array, please read my previous blog: Sparse vs Dense Arrays in JavaScript . So Array(length).fill(1) basically modifies the array by putting 1 in every index of the array and finally returning the array like [1, 1, 1, 1, 1].

Array(length).fill(initialValue) is a convenient way to create arrays with the desired length and initialized them with a primitive value (number, string, boolean).

#2 Fill an array with objects

What if you need to add objects to an array? Whether you want the array filled with the initial object instances or with other instances, this need is significantly complicated.

#2.1 array. fill (initialValue)

If you want to initialize the array with the same object instance, then you can use array.fill() method.

Array(length).fill({stage:1}) creates an array with length 4 and at the same time put the same object i.e. {stage : 1} on each index of the array.

The important thing that you need to remember is that each of the index is filled with same object instance. So if you want to change one index of the array, it will not only change that index, rather it will change every index of the array. Refer to the program below

#2.2 array. from ()

If you want to fill the array by creating copies from another array, then you can use array.from() method.

Array.from( array, mapperFunction ) accepts two arguments: an array (from which elements will be copied) and a mapperFunction (a function which will be called for each element of the original array).

Array.from() will invoke the mapperFunction on each item of the array and then pushes the result to a new array, and finally the newly mapped array.

With this approach, if you change some element of the array, only that element is changed, not the entire array.

#2.3 array. map () with spread operator

When you use Array.length() to create an array, it creates a sparse array and using map() function on the sparse array skips the empty indexes from the array.

But using the spread operator you can transform a sparse array into an array with items initialized with undefined . After that, you can apply array.map() method on that array.

The expression […Array(length)] creates the array where items are initialized with undefined . And then applying map() to create new instances.

There are several good approaches to fill an array with initial values in JavaScript.

If you want to create an array using primitive values, the best method is to use Array (length). fill(initialValue) .

If you want to create an array with object instances but don’t care if each item has the same object instance, use Array (length) . The method to use is fill(initialObject) .

Otherwise, you might use Array.from(Array.length, mapper) or […Array(length)] to populate an array with distinct object instances. map(mapper), where mapper is a function that, on each invocation, returns a new object instance.

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How to Create an Array with Default Value in Swift?

Swift – create an array with default value.

To create an array with default value in Swift, use Array initialiser syntax and pass this default value. We also need to pass the size of the Array for the Array initialiser.

The syntax of Array initialiser to create an array with specific default value, is

The following code snippet returns an Array of size 4 with default value of 0 . The datatype of this array is inferred from the repeating value, which is the default value.

In this example, we will create a String Array with default value of "abc" . So repeating in the Array initialiser would be "abc" . To use Array initialiser syntax, we need to pass count as well. Let us give a value of 8 for the count .

In this Swift Tutorial , we learned how to create an Array with specific default value using Array initialiser syntax.

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COMMENTS

default array values
Default value of an array is undefined . So if you want to set default to 0, you have to loop all elements of array to set them to 0. If you
How to Fill an Array with Defaults
There are cases when we want to create an array filled with default values. For small arrays, of course, we can just write down the values
How to Fill an Array with Initial Values in JavaScript
JavaScript provides a bunch of good ways to fill an array with initial values. If you'd like to create an array initialized with primitive
Array.prototype.fill()
The fill() method changes all elements in an array to a static value, from a start index (default 0) to an end index (default array.length).
Array() constructor
Syntax. new Array ; Array literal notation · const · = ; Array constructor with a single parameter · const · = ; Array constructor with multiple parameters · const · =
Mapping an array to a new array with default values in JavaScript
Mapping an array to a new array with default values in JavaScript ... const arr = [ { startTime: 1234, stopTime: 2345 }, { startTime: 3452
Create An Array with Default Values
Create An Array with Default Values · // Create an Array of Strings · var appleProgrammingLanguages: · // Thanks to Swift's type inference, you don't have to write
Fill an Array with Initial Values in JavaScript
If you want to initialize the array with the same object instance, then you can use array.fill() method. ... Array(length).fill({stage:1}) creates
How to Create an Array with Default Value in Swift?
To create an array with default value in Swift, use Array initialiser syntax and pass this default value. We also need to pass the size of the Array for the
Changing Array Default Values
Complete the following steps to manipulate the default values of an array. Create a 1D array shell in the front panel window. Add