The fill() method in JavaScript is used to fill all the elements of an array with a static value from a specified start index to an end index. This method modifies the original array and can be useful for initializing or resetting array values.

Syntax:

array.fill(value, start, end);

• value: The value to fill the array elements with. This can be any type of value, including objects or arrays.

• start (optional): The index at which to start filling the array. The default is 0 (the beginning of the array).

• end (optional): The index before which to stop filling the array (not inclusive). The default is the length of the array.

Return Value:

• The modified array, with elements filled with the specified value.

Key Points:

• The fill() method modifies the original array and returns it.
• If start is greater than or equal to the array length, the method does nothing.
• If end is less than or equal to start, the method fills nothing.
• Negative values for start or end are treated as offsets from the end of the array.

Example 1: Basic usage

let numbers = [1, 2, 3, 4, 5];
numbers.fill(0);  // Fill the entire array with 0

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

Example 2: Specifying start and end indices

let numbers = [1, 2, 3, 4, 5];
numbers.fill(0, 1, 4);  // Fill from index 1 to index 4 (not inclusive)

console.log(numbers);  // [1, 0, 0, 0, 5]

Example 3: Using negative indices

let numbers = [1, 2, 3, 4, 5];
numbers.fill(0, -3);  // Fill starting from the third last element to the end

console.log(numbers);  // [1, 2, 0, 0, 0]

Example 4: Filling an empty array

let emptyArray = new Array(5);  // Create an array with 5 empty slots
emptyArray.fill('x');  // Fill with 'x'

console.log(emptyArray);  // ['x', 'x', 'x', 'x', 'x']

Example 5: Filling with an object

let obj = { key: 'value' };
let array = new Array(3).fill(obj);  // Fill with a reference to the same object

console.log(array);  // [{ key: 'value' }, { key: 'value' }, { key: 'value' }]

// Changing the object in one place affects all references
array[0].key = 'new value';
console.log(array);  // [{ key: 'new value' }, { key: 'new value' }, { key: 'new value' }]

Summary:

• The fill() method is a powerful way to fill an array with a specified value, making it easy to initialize or reset array contents.
• It is useful for preparing arrays for further manipulation or when you need to set default values.
• Since it modifies the original array, care should be taken to avoid unintentional changes to existing data.