Why Returning Booleans in JavaScript Array Sort Comparison Functions is Wrong: A Guide to Correct Usage?

Custom Comparison Functions for Array Sorting in JavaScript: When Boolean Returns Aren't Enough

While custom comparison functions are often used to modify the default lexicographic ordering of arrays, a common misconception is that simply returning true or false from the function is sufficient. However, this approach is incorrect and can lead to unreliable sorting results.

Issues with Boolean Comparison Functions

The problem with returning a boolean from a comparison function is that it can violate the requirements for a "consistent comparison function" as defined in the JavaScript specification. Consistent comparison functions must return a number (specifically, -1, 0, or 1) indicating the relative ordering of the compared elements.

Returning a boolean instead of a number can result in unexpected behavior:

• Transitivity Violation: The function function(a, b) { return a > b; } returns false when b is larger than a, implying that b should be sorted before a. However, the function also returns false when a and b are equal, which contradicts the transitive property that states that if a is equal to b and b is less than c, then a must be less than c.
• Undefined Sorting Order: Returning false from the comparison function can lead to undefined sorting order. For example, the sort [1, 1, 0, 2].sort(function(a, b) { return a > b; }) could result in [0, 1, 1, 2] or [1, 1, 0, 2] depending on the implementation of the sorting algorithm.

Correct Comparison Functions

To ensure consistent and reliable sorting, it is crucial to define comparison functions that return a number representing the relative ordering of the compared elements:

• Generic Comparison Function: For lexicographic comparison, a generic function can be defined as function(a, b) { if (a < b) return -1; if (a > b) return 1; else return 0; }.
• Number Comparison: For comparing numbers, a simpler function can be used: function(a, b) { return a - b; }.
• Comparison by Specific Properties: To sort objects or arrays by a specific property, the function can access that property and compare its values: function(a, b) { return a.name.localeCompare(b.name); } for sorting by object names.

Conclusion

While it might be tempting to use boolean returns for comparison functions in JavaScript, it is essential to follow the specified requirements for consistent comparison functions. This ensures that sorting algorithms can properly compare elements and produce reliable and predictable results.

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