Benchmark: Last item in array Slice vs Length - 1

Script Preparation code:

var arr = [];
for (let i = 0; i < 1000000; i++) {
  arr.push(i);
}

Tests:

Slice
for (let i = 0; i < 1000; i++) { const v = arr.slice(-1)[0]; }
Length - 1
for (let i = 0; i < 1000; i++) { const v = arr[arr.length - 1]; }

Historical Performance Data aggregated from 48 community runs

Desktop Results:

Chrome 48 runs

Slice

15K/s

Length - 1

11K/s

Fastest: Slice (1.4x faster)

View exact numbers

✓ Slice: 14,996 ops/sec
• Length - 1: 10,600 ops/sec

Results show median performance across all community submissions. Run this benchmark yourself to verify on your device.

Rendered benchmark preparation results:

Suite status: <idle, ready to run>

Previous results

Test case name	Result
Slice
Length - 1

Fastest: N/A

Slowest: N/A

Latest run results:

Run details: (Test run date: one year ago)

User agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:137.0) Gecko/20100101 Firefox/137.0

Browser/OS: Firefox 137 on Mac OS X 10.15

View result in a separate tab

Length - 1

1.2M/s

Slice

32K/s

View exact numbers

Test name	Executions per second
✓ Length - 1	1,195,875 Ops/sec
Slice	31,880 Ops/sec

Autogenerated LLM Summary (model llama3.2:3b, generated one year ago):

LLMs can make mistakes. Check important info.

Let's break down the benchmark and analyze what's being tested.

**Benchmark Definition**

The provided JSON represents a JavaScript microbenchmark that compares two approaches to access the last element of an array:

1. `Slice(-1)`: Using the `slice()` method with a negative index to get the last element.
2. `Length - 1` (or simply `arr[arr.length - 1]`): Directly accessing the last element using its index.

**Options Compared**

The two options being compared are:

* `Slice(-1)`: This approach uses the `slice()` method with a negative index to get the last element. The `-1` index means "start from the end of the array" and return the last element.
* `Length - 1` (or simply `arr[arr.length - 1]`): This approach directly accesses the last element using its index, which is calculated by subtracting 1 from the length of the array.

**Pros and Cons**

Here are some pros and cons of each approach:

* **Slice(-1)**
	+ Pros:
		- More intuitive and easier to read for developers who are familiar with array methods.
		- Can be more efficient in certain situations, as it only creates a new array slice object.
	+ Cons:
		- May be slower due to the overhead of creating a new array slice object.
		- Requires a valid index, which can lead to errors if the array is empty or has invalid indices.
* **Length - 1 (or simply `arr[arr.length - 1]`)**
	+ Pros:
		- Faster and more efficient in most cases, as it only accesses an existing element.
		- Does not require creating a new object or modifying the original array.
	+ Cons:
		- Less intuitive for developers who are not familiar with array indexing.
		- May be less readable due to its simplicity.

**Library and Special JS Features**

The benchmark uses no external libraries. There are no special JavaScript features being tested in this benchmark, as both approaches rely on standard JavaScript syntax.

**Alternative Approaches**

If you wanted to explore alternative approaches, here are a few:

* Using `Array.prototype.at()` (ES2019+): Instead of using a negative index or accessing the element by its index, you can use the `at()` method to access an element at a specific position in the array. This approach is more intuitive and easier to read, but may have performance implications due to its complexity.
* Using a custom function: You could write a custom function that takes the array and its length as arguments, and returns the last element. However, this would likely be less efficient than using either of the two approaches being tested.

Keep in mind that the choice of approach depends on your specific use case, performance requirements, and personal preference.

Last item in array Slice vs Length - 1 (version: 0)

Comparing performance of: `Slice vs Length - 1`

Created: 5 years ago by: Guest

Go to the latest result

Slice

Length - 1

Historical Performance Data aggregated from 48 community runs

Desktop Results:

Chrome 48 runs

Suite status: <idle, ready to run>

Fastest: N/A

Slowest: N/A

Autogenerated LLM Summary (model llama3.2:3b, generated one year ago):

Last item in array Slice vs Length - 1 (version: 0)

Comparing performance of: Slice vs Length - 1

Created: 5 years ago by: Guest

Go to the latest result

Slice

Length - 1

Historical Performance Data aggregated from 48 community runs

Desktop Results:

Chrome 48 runs

Suite status: <idle, ready to run>

Fastest: N/A

Slowest: N/A

Autogenerated LLM Summary (model llama3.2:3b, generated one year ago):

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Comparing performance of: `Slice vs Length - 1`