How can I add the elements of one JavaScript array to another without creating a new array?

It seems challenging to merge the elements of one existing JavaScript array into another, similar to Python’s extend method. I am looking for a solution to perform the following operation:

let x = [1, 2];
let y = [3, 4, 5];
// What should I do here?
console.log(x); // This should output: [1, 2, 3, 4, 5]

While I am aware of the x.concat(y) method, this approach generates a new array rather than modifying the original one. I’m interested in a method that operates efficiently when x is much larger than y, avoiding a complete copy of x.

If you want to merge the elements of one existing JavaScript array into another without creating a new array, you can use the Array.prototype.push method alongside the spread operator. This method efficiently appends all elements of one array to another one, modifying the original array in place.

Here’s how you can achieve that:

let x = [1, 2];
let y = [3, 4, 5];

// Use push with the spread operator
y.push(...x);

console.log(x); // This will output: [1, 2, 3, 4, 5]

Explanation:

  • The spread operator in y.push(…x); unpacks the elements of array x into individual arguments for the push function.
  • The push method then appends these unpacked elements to the end of array y, modifying it directly.

This approach is beneficial when working with arrays of different lengths, particularly when you want to avoid creating additional arrays and optimize performance. Additionally, it maintains the order of elements, seamlessly extending array y with those from x.

You can use the Array.prototype.splice method to achieve this. It modifies the array in place, adding elements from one array into another without creating a new array:

let x = [1, 2];
let y = [3, 4, 5];

// Use splice to insert elements of y into x
x.splice(x.length, 0, ...y);

console.log(x); // Outputs: [1, 2, 3, 4, 5]

Explanation:

  • splice(x.length, 0, ...y) inserts all elements of y at the end of x, modifying x in place.
  • The spread operator (...) unpacks y into individual elements to be inserted by splice. This keeps it efficient and straightforward.

To add the elements of array y to array x without creating a new array, you can use a simple forEach loop. This method allows you to efficiently push the elements of y into x, preserving the order and modifying x in place. Here’s how you can do it:

let x = [1, 2];
let y = [3, 4, 5];

// Using forEach to append y's elements to x
y.forEach(element => x.push(element));

console.log(x); // Outputs: [1, 2, 3, 4, 5]

Explanation:

  • The forEach loop iterates over each element of array y.
  • The push method adds each iterated element to the end of array x.

This approach is straightforward and avoids the creation of additional arrays, making it suitable for scenarios where you aim to optimize for memory use and efficiency, especially when one array is much larger than the other.

Another possible approach to merge the elements of one JavaScript array into another without creating a new array is to use the Array.prototype.push.apply() method. This method allows you to append all elements from one array to another in an efficient way, directly modifying the original array.

let x = [1, 2];
let y = [3, 4, 5];

// Use apply to merge y into x efficiently
Array.prototype.push.apply(x, y);

console.log(x); // Outputs: [1, 2, 3, 4, 5]

Explanation:

  • Array.prototype.push.apply(x, y) invokes the push method on array x, using y as the list of arguments.
  • This method takes advantage of apply() to treat the elements of y as individual arguments to push, thus appending them all at once.
  • By directly appending y to x, it prevents the creation of a new array and modifies x in place.

This approach is particularly effective when dealing with large arrays, as it leverages native JavaScript functions for optimal performance, thus maintaining memory efficiency. Furthermore, it preserves the order of elements while extending the original array with ease.