In JavaScript, we can create an array with a specific length and pre-filled values using various approaches. This can be useful when we need to initialize an array with default or placeholder values before populating it with actual data.

Method 1: Using the Array() Constructor and fill() Method

The Array constructor can be used to create an array of a specific length. The fill() method can fill this array with a specified value.

Syntax:

const filledArray = new Array(length).fill(value);

Example: Below is the implementation of the above approach

const length = 5; 
const value = 5;
const filledArray = new Array(length).fill(value);
console.log(filledArray); 

[ 5, 5, 5, 5, 5 ]

Method 2: Using a Loop to Initialize Values

We can use a loop to iterate through the desired length and assign values.

Syntax:

const filledArray = Array.from();
for (let i = 0; i < length; i++) {
    filledArray.push(value);
}

Example: Below is the implementation of the above approach

const length = 5;
const filledArray = new Array();
for (let i = 0; i < length; i++) {
    filledArray.push(i + 1);
}
console.log(filledArray);

[ 1, 2, 3, 4, 5 ]

Method 3: Using the map() Method

The map() method can be used to create an array by applying a function to each element.

Syntax:

const filledArray = Array.from({ length }).map(() => value);

Example: Below is the implementation of the above approach

const length = 5; 
const filledArray = 
    Array.from({ length })
        .map(() => 5);
console.log(filledArray); 

[ 5, 5, 5, 5, 5 ]

Using Array.from with a Mapping Function

Using Array.from with a mapping function, you can create an array of a specific length with pre-filled values. The mapping function defines the values to fill the array. This approach offers concise and flexible syntax for generating arrays with custom initial values.

Example: In this example we generates a new array filledArray of length 5, filled with the value 0 using Array.from() and an arrow function. It then logs filledArray to the console.

const length = 5;
const value = 0;
const filledArray = Array.from({ length }, () => value);

console.log(filledArray); 

[ 0, 0, 0, 0, 0 ]

Method 5: Using the Spread Operator

In this approach, we use the spread operator (…) to expand an array and then use the Array.prototype.map method to fill it with a specific value. This method provides a concise and flexible way to create an array with pre-filled values.

Syntax:

const filledArray = [...Array(length)].map(() => value);

Example:

const length = 5;
const value = 0;
const filledArray = [...Array(length)].map(() => value);

console.log(filledArray);

[ 0, 0, 0, 0, 0 ]