Is a Boolean Return Value Sufficient for JavaScript Array Sorting Comparison Functions?

Sorting in JavaScript: Can Returning a Boolean Suffice for a Comparison Function?

The question arises from a common misconception that sorting an array simply by returning a boolean (true or false) in the comparison function is sufficient. While this approach may have seemingly worked in some cases, it is incorrect and can lead to unpredictable sorting results.

Why Returning a Boolean is Insufficient

The purpose of a comparison function in sorting is to determine the relative order of two elements in an array. For a correct and consistent sorting, the comparison function should return a number indicating the order:

• 0: The elements are equal and can be considered interchangeable in the ordering (e.g., [1, 1] remains unchanged).
• Positive number: The first element (a) is considered greater than the second (b) and should come after it in the sorted array.
• Negative number: The first element is considered smaller than the second and should come before it in the sorted array.

Returning a boolean (true or false) alone does not accurately convey this information. A boolean value of true, for example, implies that the elements are equal, but it does not specify whether a should precede b or vice versa.

Consequences of Using a Boolean Comparison Function

Using a comparison function that only returns a boolean can result in undefined or unexpected sorting behavior. Different sorting implementations may interpret the result differently, leading to inconsistencies across browsers or environments.

For example, the following code may not sort the array as intended:

arr = [1, 0, 2];
arr.sort((a, b) => a > b); // Returns a boolean
console.log(arr); // Output: [0, 1, 2] or [1, 0, 2] (depending on browser/environment)

In this case, Chrome and Internet Explorer 11 interpret a > b as a >= b, resulting in [0, 1, 2]. However, Opera 12 interprets it as simply a > b, resulting in [1, 0, 2].

Correct Comparison Function Implementation

To ensure correct and consistent sorting, it is essential to define a comparison function that returns the appropriate numeric value:

• For numbers, simply return the difference between the two elements: (a - b).
• For strings or other comparable types, return 1, 0, or -1 to indicate "greater than", "equal to", or "less than", respectively.
• For complex object types, use a custom comparison function that retrieves and compares the relevant properties.

Conclusion

Returning a boolean in a comparison function for sorting in JavaScript may appear to work in some scenarios, but it is not the intended way and can lead to unpredictable results. By using the correct implementation, developers can ensure that arrays are sorted in the desired order, regardless of the browser or environment.

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