Push() Method

The JavaScript push() method is one of the most commonly used functions to append elements to the end of an array. However, have you ever wondered what happens under the hood? Understanding the internal workings of this method requires delving into the realm of Data Structures and Algorithms (DSA).

In this article, we'll explore how the push() method works internally using DSA concepts and why it's an efficient operation.

1. The Basics: What Does push() Do?

The push() method appends one or more elements to the end of an array and returns the new length of the array. For instance:

Here, the number 4 is appended at the end, and the updated length of the array is returned.

2. Arrays and Memory Allocation

In JavaScript, arrays are dynamic, meaning their size can change at runtime. However, understanding the underlying memory structure requires a comparison with traditional arrays in languages like C or Java.

• Contiguous Memory Block: Arrays are stored in contiguous memory blocks. When we create an array of size 3 ([1, 2, 3]), three consecutive memory slots are allocated.
• Dynamic Resizing: When you push() an element, if the array has extra space in its allocated memory, the element is added at the next available index. If the array is full, the engine needs to allocate a new block of memory with a larger size.

3. Internal Working of push()

Here's how the push() operation works step-by-step:

• Step 1: Check Available Capacity When you push() a new element, the array first checks if there's space in the current memory block.
• Step 2: Copying Existing Elements (When Resizing) If resizing is needed, the existing elements from the original array are copied to the new memory block. This operation involves looping through the array and copying each element to the new block.
• Step 3: Appending the New Element After resizing and copying, the new element is added to the next available index in the newly allocated memory, and the length is updated.

4. Time Complexity of push()

• Amortized O(1): On average, the time complexity of push() is constant, or O(1). This is because most push() operations only require appending the element to the end of the array, which is a simple and fast operation.
• O(n) in Worst Case: In the worst case, when resizing the array is required, the time complexity becomes O(n), where n is the number of elements in the array. This is due to the need to allocate a new memory block and copy the existing elements.

However, since resizing happens infrequently, the overall time complexity is considered amortized O(1).

5. Resizing Strategy: Doubling the Capacity

One important concept behind the efficiency of push() is the resizing strategy used. When an array's capacity is exceeded, instead of increasing the size by just one, the array size is typically doubled.

For example, if you start with an array of size 4 and you keep adding elements using push(), the capacity changes as follows:

• Initial size: 4
• After 5th element: size doubles to 8
• After 9th element: size doubles to 16

By doubling the size, we minimize the number of costly resizing operations, ensuring that push() remains efficient.

6. Comparison with Linked List Insertion

While arrays store elements in contiguous memory, linked lists use a different data structure where each element (node) points to the next one. In a singly linked list, appending an element to the end can be done in constant time O(1) without any resizing or shifting of elements.

However, linked lists come with their own trade-offs, such as increased memory overhead due to storing references (pointers) and slower element access compared to arrays.

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