Common bug patterns

Recognize common bugs like off-by-one, undefined access and wrong comparison.

The off-by-one error

An off-by-one error comes from a boundary being one too few or one too many. It most often appears when <= is written instead of < (or the other way). For example, for (let i = 0; i <= arr.length; i++) walks the array one step too far and reaches arr[arr.length], which is undefined.

The correct form is usually i < arr.length. Always think about whether the boundary condition should include equality.

Undefined access and wrong comparison

Two other common bugs: undefined access and wrong comparison. When you access a property that does not exist on an object you get undefined; operating on it throws a TypeError. Check that it exists first.

The == operator coerces types and can surprise you (0 == "" is true). If the type matters too, use ===. It does not mislead equality.

Exercise Fix the boundary error

Define a function called sumFirst(nums, k). It should sum and return the first k elements of the array. Use a for (let i = 0; i < k; i++) loop (use < not <= to avoid off-by-one). Then assign the result of sumFirst([10, 20, 30, 40], 3) to a variable result and log it with console.log(result).

Show example solution

function sumFirst(nums, k) {
  let total = 0;
  for (let i = 0; i < k; i++) {
    total += nums[i];
  }
  return total;
}

const result = sumFirst([10, 20, 30, 40], 3);
console.log(result);

Test yourself

Match the bug pattern

i <= arr.length

Walks the array one step too far
i < arr.length

Walks the array exactly
arr[arr.length]

Is the undefined value
arr[0]

Is the first element
Which is an off-by-one error?
- for (let i = 0; i <= arr.length; i++)
- for (let i = 0; i < arr.length; i++)
- const first = arr[0]
- console.log(arr)
Show answer

for (let i = 0; i <= arr.length; i++) — Correct. With <= the array is walked one step too far.

Add strict equality

if (count ____ 5) {
  console.log("five");
}

Show answer

if (count === 5) {
  console.log("five");
}

Next lesson: Summary - Debugging

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