Benchmark: Last item in array Slice vs Length - 1 vs .at

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]; }
.at
for (let i = 0; i < 1000; i++) { const v = arr.at(-1); }

Rendered benchmark preparation results:

Suite status: <idle, ready to run>

Previous results

Test case name	Result
Slice
Length - 1
.at

Fastest: N/A

Slowest: N/A

Latest run results:

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Autogenerated LLM Summary (model llama3.1:latest, generated one year ago):

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Let's dive into what this benchmark is testing.

**Benchmark Overview**

This benchmark compares three different ways to access the last item in an array of 1 million elements:

1. `arr.slice(-1)[0]` (Slice method)
2. `arr[arr.length - 1]` (Index access with length minus one)
3. `arr.at(-1)` (.at method)

The goal is to determine which approach is the fastest in terms of execution time.

**Test Cases**

Each test case runs a loop for 1000 iterations, and within each iteration, it accesses the last item in the array using one of the three methods mentioned above.

Here are the details:

1. **Slice**: This test uses `arr.slice(-1)[0]`, which creates a new array containing only the last element (via `slice`) and then accesses that element via index `[0]`.
2. **Length - 1**: This test uses `arr[arr.length - 1]`, which accesses the array's length property and subtracts 1 to get the index of the last item.
3. **.at**: This test uses `arr.at(-1)`, a new method introduced in ECMAScript 2022 (ES2022), which takes an index as an argument and returns the element at that position.

**Library**

None mentioned in this specific benchmark.

**Special JS Features or Syntax**

Yes, two special features are used:

1. **Array.prototype.slice()**: This method creates a shallow copy of a portion of an array.
2. **Array.at()` : Introduced in ECMAScript 2022 (ES2022), this method returns the element at the specified index.

**Pros/Cons of each approach**

Here's a brief summary:

1. **Slice**:
	* Pros: Simple and easy to understand, works across different browsers.
	* Cons: Creates an extra array object, which can be memory-intensive for large arrays.
2. **Length - 1**:
	* Pros: Fast and efficient, as it only involves a simple index calculation.
	* Cons: Can be slower than other methods when dealing with very large arrays or sparse arrays.
3. **.at**:
	* Pros: Introduced in ES2022, provides a more elegant way to access array elements directly.
	* Cons: May not work in older browsers that don't support the `.at()` method.

Other alternatives:

1. **Pop()**: This method removes and returns the last element of an array. While not exactly what we're testing here, it's another way to access the last item in an array.
2. **Array.slice().length - 1**: A variation on the `Slice` approach that uses the `slice()` method without `[0]`, which might be slightly faster.

Keep in mind that these results are specific to this benchmark and may not reflect real-world performance differences, as they depend on various factors like browser engine optimizations and array sizes.

Related benchmarks:

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

Comparing performance of: Slice vs Length - 1 vs .at

Created: 2 years ago by: Guest

Jump to the latest result

Slice

Length - 1

.at

Suite status: <idle, ready to run>

Fastest: N/A

Slowest: N/A

No previous run results

Autogenerated LLM Summary (model llama3.1:latest, generated one year ago):