How boundary conditions cause most logic bugs

Boundaries Break Code

The majority of logic bugs occur at the edges of valid input ranges, not in the middle. Arrays at index 0 and length-1. Loops that run zero times. Null inputs to functions that assume non-null. Knowing boundary conditions exist is not enough -- you must actively test them.

The Boundary Checklist

For any function, enumerate these cases before testing: empty input (empty array, empty string, zero), single element input, maximum valid input, minimum valid input, and input just outside valid bounds. For loops: does it run correctly when the list has 0 items? 1 item? 2 items?

function firstPositive(arr) {
  for (let i = 0; i < arr.length; i++) {
    if (arr[i] > 0) return arr[i];
  }
  // Bug: returns undefined when no positive exists
  // Fix: return null or throw, never implicit undefined
}

// Boundary tests:
console.log(firstPositive([]));       // undefined -- boundary failure
console.log(firstPositive([-1,-2]));  // undefined -- boundary failure
console.log(firstPositive([0]));      // undefined -- 0 not positive, correct
console.log(firstPositive([1]));      // 1 -- correct

Off-By-One Is Its Own Category

Off-by-one errors are so common they deserve explicit attention. Every time you write less-than vs less-than-or-equal in a loop, stop and ask: should the last index be included? Write the boundary value in a comment next to the condition. This single habit eliminates a large class of bugs.