Why Do Boolean Return Values Cause Unexpected Results in JavaScript's Sort Comparison Functions?

Sorting in JavaScript: Boolean Return Values in Comparison Functions

The Issue

Despite common practice, returning a boolean value (true or false) in comparison functions used for sorting in JavaScript is incorrect and can lead to unexpected results.

The Reason

Sorting algorithms expect comparison functions to return numbers, specifically:

• A positive number if the first argument should be sorted after the second
• 0 if the two arguments are considered equal
• A negative number if the first argument should be sorted before the second

Returning a boolean value (false equates to 0 and true equates to 1) fails the transitivity requirement for consistent comparison functions. This requirement ensures that the sorting algorithm can make inferences about the ordering of elements based on previous comparisons.

A Surprising Demonstration

Consider the comparison function:

function compare(a, b) {
    return a > b;
}

This function returns true (or 1) when 'a' is greater than 'b' but incorrectly returns false (or 0) even when 'b' is greater than 'a'. Consequently, the sorting algorithm treats 'a' and 'b' as equal, leading to unpredictable sort orders.

Counterexamples

Despite passing some basic tests, the above comparison function fails under certain circumstances, such as:

[0, 1, 0].sort(compare);  // Result: [0, 1, 0] (expected: [0, 0, 1])
[1, 1, 0, 2].sort(compare);  // Result: [0, 1, 2, 1] (expected: [0, 1, 1, 2])

Correct Comparison Functions

For a lexicographic sort (ascending), use no comparison function, as elements will be automatically converted to strings for comparison.

For numerical sorting, use:

function compareNumbers(a, b) {
    return a - b;
}

For more complex comparisons, use conditional statements to return the appropriate number:

function compareObjects(a, b) {
    if (a.property > b.property) return 1;
    if (a.property < b.property) return -1;
    return 0;
}

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