The fill() function alters all the elements in an array by assigning them the same fixed value. By default, it starts from the first index (0) and goes up to the end index (which is the length of the array). After the operation, the function returns the modified array.
```js
let arr = [1, 2, 3, 4, 5];

// fill the entire array with value 0
arr.fill(0);

console.log(arr); // [0, 0, 0, 0, 0]

// fill a specific portion of the array with value 10
arr.fill(10, 1, 3);

console.log(arr); // [0, 10, 10, 0, 0]
```

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Arrays are generally described as "list-like objects"; they are basically single objects that contain multiple values stored in a list. Array objects can be stored in variables and dealt with in much the same way as any other type of value, the difference being that we can access each value inside the list individually, and do super useful and efficient things with the list, like loop through it and do the same thing to every value. Maybe we've got a series of product items and their prices stored in an array, and we want to loop through them all and print them out on an invoice, while totaling all the prices together and printing out the total price at the bottom.

What is an array?

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A JavaScript array's length property and numerical properties are connected.

Several of the built-in array methods (e.g., `join()`, `slice()`, `indexOf()`, etc.) take into account the value of an array's length property when they're called.

Other methods (e.g., `push()`, `splice()`, etc.) also result in updates to an array's length property.
```js
const fruits = [];
fruits.push("banana", "apple", "peach");
console.log(fruits.length); // 3
```

Relationship between length and numerical properties

The  ArrayBuffer is an object that is utilized to express a raw binary data buffer of a fixed length that is generic in nature. It is essentially an array of bytes, which is commonly referred to as a "byte array" in other programming languages. It is not possible to directly modify the contents of an ArrayBuffer. Rather, you need to create one of the typed array objects or a DataView object that can represent the buffer in a particular format. You can then use this to read and write the contents of the buffer. To create a new ArrayBuffer of a specified length in bytes, you can use the `ArrayBuffer()` constructor. Additionally, you can obtain an array buffer from preexisting data, such as a Base64 string or a local file.  

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