extend existing js array with another array

## Extending Existing JavaScript Arrays with Another Array: A Comprehensive Tutorial

JavaScript provides several ways to extend an existing array with the elements of another array. The method you choose depends on whether you want to modify the original array in place (mutating) or create a new, extended array (non-mutating), and on the specific features of the arrays you're working with. This tutorial will cover the common and less common techniques, including their nuances and potential pitfalls.

**I. Mutating Methods (Modifying the Original Array)**

These methods directly alter the original array, which can be more efficient for large arrays when memory usage is a concern. However, it's crucial to be aware of this mutation as it can have unintended consequences if other parts of your code depend on the original array's contents.

**1. `push()` with the Spread Operator (...)**

- **Description:** The `push()` method adds one or more elements to the *end* of an array and returns the new length of the array. The spread operator (`...`) expands the elements of the second array into individual arguments for the `push()` method.

- **Syntax:**



- **Example:**



- **Explanation:**

- `...arrayToAppend` effectively transforms `[4, 5, 6]` into `4, 5, 6`.
- `push(4, 5, 6)` then adds these individual elements to the end of `originalArray`.

- **Advantages:**

- Concise and readable.
- Relatively efficient.

- **Disadvantages:**

- Modifies the original array (mutation).
- In older browsers that don't support the spread operator, you'll need to use the `apply()` method (see below).

- **Browser Compatibility:** The spread operator (`...`) is widely supported in modern browsers, but may need polyfilling in older environments.

**2. `apply()` with `push()`**

- **Description:** This approach uses the `apply()` method of the `push()` function to achieve the same result as the spread operator, but it is les ...

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