Why understanding the boundary matters in Typescript - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to print numbers from 0 to 4.

Typescript

for(let i = 0; i [1] 5; i++) {
  console.log(i);
}

Drag options to blanks, or click blank then click option'

A<=

B<

C>

D>=

Attempts:

3 left

💡 Hint

Common Mistakes

Using <= 5 causes the loop to run one extra time.

We use < because the loop runs while i is less than 5, so it stops before 5.

2fill in blank

medium

Complete the code to check if a number is within the boundary 1 to 10 (inclusive).

Typescript

if(num [1] 1 && num [2] 10) {
  console.log('Inside boundary');
}

Drag options to blanks, or click blank then click option'

A>=

B<=

C>

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using > 1 excludes 1 from the boundary.

We use >= 1 and <= 10 to include both 1 and 10 in the boundary.

3fill in blank

hard

Fix the error in the loop boundary to avoid an infinite loop.

Typescript

let count = 0;
while(count [1] 5) {
  console.log(count);
  count--;
}

Drag options to blanks, or click blank then click option'

A<

B<=

C>=

D>

Attempts:

3 left

💡 Hint

Common Mistakes

Using < causes the loop to never end because count decreases.

Using > makes the condition count > 5 false from the start (0 > 5 is false), so the loop does not enter and avoids being infinite. With < or <=, since count is decremented, it moves further away from the exit condition, causing an infinite loop.

4fill in blank

hard

Fill both blanks to create a dictionary of word lengths for words longer than 3 characters.

Typescript

const words = ['apple', 'bat', 'carrot', 'dog'];
const lengths = Object.fromEntries(words.filter(w => w.length > 3).map(word => [[1], [2]]));

Drag options to blanks, or click blank then click option'

Aword

Bword.length

Cwords.filter(w => w.length > 3)

Dword => word.length

Attempts:

3 left

💡 Hint

Common Mistakes

Using the whole array as a key or value.

We use word as the key and word.length as the value to map each word to its length.

5fill in blank

hard

Fill all three blanks to create an object with uppercase keys and values greater than 0.

Typescript

const data = { a: 1, b: 0, c: 3 };
const result = Object.fromEntries(Object.entries(data).filter(([k,v]) => v > 0).map(([[3],v]) => [[1], [2]]));

Drag options to blanks, or click blank then click option'

Ak.toUpperCase()

Bv

Ck

DObject.entries

Attempts:

3 left

💡 Hint

Common Mistakes

Using original keys without uppercase.

We use k.toUpperCase() as the new key, v as the value, and k as the variable for keys in the loop.

Practice

1.

Why is it important to understand boundaries in TypeScript code?

easy

A. To keep code clear and prevent mistakes

B. To make code run faster

C. To use more memory

D. To write longer code

Solution

1. Step 1: Understand the role of boundaries

   Boundaries help separate parts of code to keep it organized and clear.

2. Step 2: Recognize the benefit of boundaries

   They prevent mistakes by controlling how parts of code interact.

3. Final Answer:

   To keep code clear and prevent mistakes -> Option A

4. Quick Check:

   Boundaries = clarity and fewer mistakes [OK]

Hint: Boundaries keep code clear and safe from errors [OK]

Common Mistakes:

• Thinking boundaries make code faster
• Believing boundaries increase memory use
• Assuming boundaries make code longer

2.

Which of the following is the correct way to define a boundary using a TypeScript interface?

interface UserBoundary {
  name: string;
  age: number;
}

easy

A. UserBoundary interface { name: string; age: number; }

B. interface UserBoundary = { name: string; age: number; }

C. interface UserBoundary { name: string; age: number; }

D. interface UserBoundary ( name: string; age: number; )

Solution

1. Step 1: Recall TypeScript interface syntax

   Interfaces use the keyword 'interface' followed by the name and curly braces with properties.

2. Step 2: Check each option's syntax

   interface UserBoundary { name: string; age: number; } matches correct syntax; others use wrong symbols or order.

3. Final Answer:

   interface UserBoundary { name: string; age: number; } -> Option C

4. Quick Check:

   Correct interface syntax = interface UserBoundary { name: string; age: number; } [OK]

Hint: Interfaces use 'interface Name { ... }' syntax [OK]

Common Mistakes:

• Using '=' instead of '{ }' for interface
• Placing 'interface' after the name
• Using parentheses instead of braces

3.

What will be the output of this TypeScript code?

<pre>interface Boundary { value: number; } function checkBoundary(obj: Boundary) { if (obj.value > 10) return 'Above boundary'; else return 'Within boundary'; } console.log(checkBoundary({ value: 15 }));

medium

A. "Within boundary"

B. "Above boundary"

C. undefined

D. TypeError

Solution

1. Step 1: Understand the function logic

   The function returns 'Above boundary' if obj.value is greater than 10, else 'Within boundary'.

2. Step 2: Check the input value

   The input object has value 15, which is greater than 10, so it returns 'Above boundary'.

3. Final Answer:

   "Above boundary" -> Option B

4. Quick Check:

   Value 15 > 10 = "Above boundary" [OK]

Hint: Compare value to boundary condition carefully [OK]

Common Mistakes:

• Confusing greater than with less than
• Expecting undefined return
• Thinking function throws error

4.

Find the error in this TypeScript code that tries to enforce a boundary on a number:

function setBoundary(value: number) {
  if (value < 0 || value > 100) {
    throw new Error('Value out of boundary');
  }
  return value;
}
console.log(setBoundary(150));

medium

A. The error message is missing a semicolon

B. The boundary check should use && instead of ||

C. The function should return a string, not number

D. No error; code works as expected

Solution

1. Step 1: Analyze the boundary condition

   The condition checks if value is less than 0 OR greater than 100, which correctly detects out-of-bound values.

2. Step 2: Check the function behavior

   If value is out of boundary, it throws an error; otherwise, returns the value. Calling with 150 triggers the error as expected.

3. Final Answer:

   No error; code works as expected -> Option D

4. Quick Check:

   Boundary check with || is correct [OK]

Hint: Use || to check if value is outside boundaries [OK]

Common Mistakes:

• Replacing || with && breaks boundary logic
• Expecting return type mismatch error
• Worrying about missing semicolon in error message

5.

You want to create a function that accepts an object with a number property score but only allows values between 0 and 100 inclusive. Which approach best enforces this boundary in TypeScript?

hard

A. Use a type guard function that checks score and throws an error if out of range

B. Use any type and trust the caller to pass correct values

C. Declare score as number without checks

D. Use a string type for score to avoid number boundaries

Solution

1. Step 1: Understand boundary enforcement

   To enforce boundaries, code must check values and handle out-of-range cases explicitly.

2. Step 2: Evaluate options

   Use a type guard function that checks score and throws an error if out of range uses a type guard function to check and throw errors, which actively enforces boundaries. Others either skip checks or use wrong types.

3. Final Answer:

   Use a type guard function that checks score and throws an error if out of range -> Option A

4. Quick Check:

   Active checks enforce boundaries best [OK]

Hint: Check values explicitly to enforce boundaries [OK]

Common Mistakes:

• Trusting caller without checks
• Using wrong data types to avoid boundaries
• Ignoring boundary enforcement in code