Why understanding the boundary matters in Typescript - Challenge Your Understanding

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a loop boundary in TypeScript

What will be the output of this TypeScript code snippet?

Typescript

let result = '';
for (let i = 0; i <= 3; i++) {
  result += i + ',';
}
console.log(result);

A "0,1,2,3,"

B "0,1,2,"

C "1,2,3,"

D "1,2,3,4,"

💡 Hint

Look carefully at the loop condition i <= 3. It includes the number 3.

The loop runs while i is less than or equal to 3, so it includes 0, 1, 2, and 3. The output string concatenates these numbers with commas.

🧠 Conceptual

intermediate

Why boundary conditions matter in array indexing

Why is it important to understand boundary conditions when accessing array elements in TypeScript?

A Because arrays automatically resize when accessing out-of-bound indexes.

B Because TypeScript ignores boundary conditions for arrays.

C Because accessing an index outside the array length causes undefined behavior or errors.

D Because array elements are always initialized to zero.

💡 Hint

Think about what happens if you try to get an element at an index that does not exist.

Accessing an index outside the array length returns undefined in TypeScript, which can cause bugs if not handled properly.

🔧 Debug

advanced

Identify the boundary error in this TypeScript function

What error will this function cause when called with an empty array?

Typescript

function getLastElement(arr: number[]): number | undefined {
  return arr[arr.length - 1];
}

console.log(getLastElement([]));

A Throws a runtime error: Index out of bounds

B Returns 0 by default

C Throws a compile-time error

D Returns undefined without error

💡 Hint

What is the value of arr.length - 1 when the array is empty?

When the array is empty, arr.length - 1 equals -1, so arr[-1] is undefined. No error is thrown, but the function returns undefined.

📝 Syntax

advanced

Which option correctly uses boundary checks in TypeScript?

Which code snippet correctly checks if an index is within the array boundary before accessing it?

A if (index > 0 && index <= arr.length) { console.log(arr[index]); }

B if (index >= 0 && index < arr.length) { console.log(arr[index]); }

C if (index >= 1 && index < arr.length) { console.log(arr[index]); }

D if (index > 0 && index < arr.length - 1) { console.log(arr[index]); }

💡 Hint

Remember array indexes start at 0 and go up to length - 1.

Option B correctly checks that the index is at least 0 and less than the array length, ensuring valid access.

🚀 Application

expert

Predict the output when boundary conditions are off by one

What will be the output of this TypeScript code?

Typescript

const arr = [10, 20, 30];
let sum = 0;
for (let i = 0; i <= arr.length; i++) {
  sum += arr[i] ?? 0;
}
console.log(sum);

A 60

B Throws runtime error

C NaN

D 30

💡 Hint

Notice the loop runs one step beyond the last index. What does arr[i] return then?

The loop runs from 0 to 3 (inclusive). arr[3] is undefined, so arr[i] ?? 0 adds 0. The sum is 10 + 20 + 30 + 0 = 60.