Benchmark: last item in array [length-1] vs slice(-1) vs pop

Script Preparation code:

const elements = document.all;
window.arr = Array.from(elements);

Tests:

index[length - 1]
const arr = window.arr; const last = arr[arr.length-1];
Array.slice(-1)
const arr = window.arr; const last = arr.slice(-1)

Rendered benchmark preparation results:

Suite status: <idle, ready to run>

Previous results

Test case name	Result
index[length - 1]
Array.slice(-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_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/131.0.0.0 Safari/537.36

Browser/OS: Chrome 131 on Mac OS X 10.15.7

View result in a separate tab

Test name	Executions per second
index[length - 1]	155631248.0 Ops/sec
Array.slice(-1)	56764428.0 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 provided benchmark and explain what is being tested.

**Benchmark Overview**

The benchmark measures the performance of two ways to access the last element in an array:

1. Direct indexing (`arr[arr.length-1]`)
2. Using `Array.prototype.slice(-1)` method

**Test Case 1: Direct Indexing (`index[length - 1]`)**

In this test case, a constant reference to an array (`const arr = window.arr`) is created and stored in the scope. Then, a variable `last` is assigned the value of the last element of the array using direct indexing (`arr[arr.length-1]`). The test measures how fast this approach can be executed.

**Test Case 2: Array.prototype.slice(-1)**

In this test case, a constant reference to an array (`const arr = window.arr`) is created and stored in the scope. Then, a variable `last` is assigned the value of the last element of the array using the `Array.prototype.slice(-1)` method. This approach creates a new array with only the last element and returns it.

**Comparison**

Both approaches have their pros and cons:

* **Direct Indexing (`arr[arr.length-1]`)**
	+ Pros:
		- Accesses the last element directly, which is likely to be faster.
		- Uses less memory since no new array is created.
	+ Cons:
		- May not work as expected if the array is modified or extended after indexing.
		- Can lead to stack overflow if the array index exceeds the maximum allowed value.
* **Array.prototype.slice(-1)** method
	+ Pros:
		- Creates a new array with only the last element, which can be useful for other operations.
		- More flexible and safe than direct indexing since it doesn't rely on array indices.
	+ Cons:
		- Creates a new array, which may increase memory usage.
		- May be slower than direct indexing due to the overhead of creating and returning a new array.

**Library Usage**

The test case uses the `Array.from()` method, which is part of the ECMAScript standard. However, it's not explicitly used in this benchmark. Instead, it's assumed that the `window.arr` variable is an array created using this method during script preparation.

**JavaScript Features/Syntax**

This benchmark does not use any special JavaScript features or syntax beyond the standard ECMAScript.

**Other Alternatives**

If you want to access the last element of an array, other approaches include:

* Using the `at()` method (introduced in ECMAScript 2020) which returns a new iterator object that iterates over the elements in sequence:
```javascript
arr.at(-1);
```
* Using a simple loop to iterate through the array and extract the last element:
```javascript
for (let i = arr.length - 1; i >= 0; i--) {
    const lastElement = arr[i];
    // ...
}
```

Keep in mind that these alternatives may have different performance characteristics compared to direct indexing or the `Array.prototype.slice(-1)` method.

Related benchmarks:

last item in array [length-1] vs slice(-1) vs pop (version: 0)

Comparing performance of: index[length - 1] vs Array.slice(-1)

Created: 3 years ago by: Guest

Jump to the latest result

index[length - 1]

Array.slice(-1)

Suite status: <idle, ready to run>

Fastest: N/A

Slowest: N/A

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