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  3. JavaScript — Create and Fill an Array Containing N Items

JavaScript — Create and Fill an Array Containing N Items

In JavaScript, an array is a dictionary of indexes to values. Typically, you’ll only see the array items in the format like [1, 2, 3]. The sequential order of the sample array has a fixed length of 3. In some situations, you want to dynamically create an array of a given length.

JavaScript comes with an array constructor that you can use to create a new array of a given length. You can also fill the array with a mapping function. This tutorial shows you how to instantiate and fill an array of a given length.

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Create and Fill an Array of Any Length in JavaScript

You can create an array in JavaScript using the static Array.from() method. This static method creates a new array from iterable or array-like objects which will be converted to an array. Iterable objects are a Map, a Set, or another array. Array-like objects contain a length property.

You can create an array filled with undefined using Array.from in combination with an object that contains a length property:

const arrayWith10Items = Array.from({ length: 3 })  
// [undefined, undefined, undefined]

Arrays are dictionaries that map an index to a value. You may use the Array#keys method to retrieve a sequential list of numbers representing the indexes used in the array:

const arrayWith10Items = [...Array.from({ length: 10 }).keys()]  
// [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

You might want the items to start at 1 and increment by one for each descendent. In such cases, you can use the Array.from() method with an array-like object providing a length option and a map function. The map function receives each array item and its index.

Here’s an example using the item’s index and adding one because the first array index begins at zero:

const arrayWith10Items = Array.from({ length: 10 }, (item, index) => index + 1)  
// [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

Enjoy creating and filling JavaScript arrays of a given length!

Mentioned Resources

Marcus Pöhls

Marcus is a fullstack JS developer. He’s passionate about the hapi framework for Node.js and loves to build web apps and APIs. Creator of Futureflix and the “learn hapi” learning path.

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