Why Isn't a Boolean Return Value Enough for JavaScript's Array.sort Comparison Function?

Shouldn't Returning a Boolean Be Enough for a Comparison Function in JavaScript?

JavaScript's Array.sort method allows custom comparison functions for sorting elements, and a common question arises: can a simple comparison function returning a boolean suffice for sorting? The answer is unequivocally no.

Why Boolean Comparison is Insufficient

A comparison function in JavaScript should adhere to the following rules:

• Return a number: greater than 0 if a should be sorted after b, 0 if equal, and less than 0 if a should be sorted before b.
• Be consistent: If compare(a, b) > 0, then compare(b, a) < 0, and if compare(a, b) == 0, then compare(b, a) == 0.

However, returning a boolean does not guarantee consistency. For example, if compare(a, b) returns false (or equivalently, 0), it implies that a is either equal to or smaller than b. This violates the transitivity property required for sorting algorithms.

Counterexample

Consider this comparison function:

function compare(a, b) {
  return a > b;
}<p>It returns a boolean: true if a is greater than b, and false otherwise.</p>
<p>Using this function in sort, we have:</p>
<pre class="brush:php;toolbar:false">[0, 1, 0].sort(compare); // [0, 1, 0] or [1, 0, 0] (varies between implementations)
[1, 1, 0, 2].sort(compare); // [0, 1, 2, 1] or [1, 1, 0, 2] (varies between implementations)

The results are inconsistent because the comparison function does not enforce transitivity.

Correct Comparison Functions

To ensure consistent sorting, use comparison functions that return the correct numeric values:

• Lexicographic Sort: Use sort() without a custom function for simple string or number sorting.
• Numeric Sort: Use (a, b) => a - b for sorting numbers in ascending order.
• Custom Sort: Implement comparison functions that follow the rules outlined above, considering the specific properties you want to sort by.

By following these guidelines, you can ensure reliable and predictable sorting behavior in your JavaScript code.

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