Overview

MeasureThat.net is a platform that allows users to create and run JavaScript microbenchmarks. The provided benchmark definition json represents the test case, which compares various approaches for checking if an array-like object is actually an array.

Benchmark Definition

The benchmark definition json has three main sections:

1. Script Preparation Code: This code sets up two identical arrays a using the Array(10).fill().map((el,i) => i) method, which creates a new array with 10 elements that increment by index.
2. Html Preparation Code: There is no HTML preparation code provided.
3. Benchmark Definition: The benchmark definition specifies six test cases:

• if(a instanceof Array) ++c;
• if(Array.isArray(a)) ++c;
• if(a.length > -1) ++c;
• if(a.length !== undefined) ++c;
• if(a.map) ++c;
• if(a.constructor === Array) ++c;

Each test case is a simple if-statement that checks the condition and increments a counter variable c if true.

Options Compared

The benchmark compares six different approaches to check if an array-like object is actually an array:

1. instanceof: Checks if the object is an instance of the Array constructor.
2. Array.isArray(): A method provided by the Array prototype that checks if an object is an array.
3. Length Check: Checks if the length property of the object is greater than -1, which is a common heuristic for checking if an object is an array.
4. Undefined Length Check: Similar to the previous approach, but also checks if the length property is not undefined.
5. Map Check: Checks if the map() method is defined on the object, which is another common way to check if an object is an array-like object.
6. Constructor Check: Checks if the constructor property of the object is equal to the Array constructor.

Pros and Cons

Here's a brief pros and cons summary for each approach:

1. instanceof:
   • Pros: Simple and reliable, but can be slower due to the overhead of the constructor lookup.
   • Cons: May not work correctly for certain types of objects (e.g., arrays created with Array.from()).
2. Array.isArray():
   • Pros: Fast and widely supported, but may not work correctly for certain types of objects (e.g., non-array-like objects with a defined length property).
   • Cons: May be slower than the other approaches due to the method call overhead.
3. Length Check: Quick and simple, but may not be accurate for all cases (e.g., if the object's length is -1 or NaN).
4. Undefined Length Check:
   • Similar pros and cons as the previous approach, with an added layer of complexity due to the additional check.
5. Map Check: Fast and reliable, but may not work correctly for certain types of objects (e.g., if the map() method is defined but returns a non-array value).
6. Constructor Check:
   • Pros: Simple and reliable, with good performance characteristics.
   • Cons: May be slower than some other approaches due to the constructor lookup overhead.

Other Considerations

The benchmark also considers various factors such as:

• Device platform and operating system
• Browser version and rendering engine
• Number of executions per second

These factors can affect the results and provide a more accurate representation of real-world performance characteristics.

Alternatives

Some alternative approaches to checking if an array-like object is actually an array include:

1. Using Object.is(a, Array) (if supported by the browser)
2. Checking for specific properties or methods that are unique to arrays
3. Using a library like Lodash's isArrayLike() function

However, these alternatives may not be as widely supported or perform as well as the approaches compared in this benchmark.