Truthiness narrowing in Typescript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to check if the variable value is truthy.

Typescript

if ([1]) {
  console.log("Value is truthy");
}

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

Avalue == null

B!value

Cvalue === false

Dvalue

Attempts:

3 left

💡 Hint

Common Mistakes

Using negation like !value instead of value

Checking equality with false instead of truthiness

In TypeScript, simply using the variable name in the if condition checks if it is truthy.

2fill in blank

medium

Complete the code to narrow the type by checking if input is truthy before accessing its property.

Typescript

function printLength(input: string | null) {
  if ([1]) {
    console.log(input.length);
  }
}

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

Ainput.length > 0

Binput

Cinput === null

Dinput === undefined

Attempts:

3 left

💡 Hint

Common Mistakes

Checking input === null which is true only when input is null

Checking input.length > 0 without ensuring input is not null

Checking input directly in the if condition narrows the type to string because null is falsy.

3fill in blank

hard

Fix the error by completing the condition to narrow data to a truthy value before accessing data.value.

Typescript

function process(data: { value: number } | null) {
  if ([1]) {
    console.log(data.value);
  }
}

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

Adata !== null

Bdata.value !== null

Cdata.value > 0

Ddata === undefined

Attempts:

3 left

💡 Hint

Common Mistakes

Checking data.value !== null causes error if data is null

Checking data === undefined misses the null case

Checking data !== null ensures data is not null, so accessing data.value is safe.

4fill in blank

hard

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

Typescript

const words = ["apple", "cat", "banana", "dog"];
const lengths = { [1]: [2] for (const word of words) if (word.length > 3) };

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

Aword

Bword.length

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping keys and values

Using word.length as key which is a number

The dictionary keys are the words themselves, and the values are their lengths. The condition filters words longer than 3 characters.

5fill in blank

hard

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

Typescript

const data = { a: 1, b: 0, c: 3 };
const filtered = { [1]: [2] for (const [[3], v] of Object.entries(data)) if (v > 0) };

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

Ak.toUpperCase()

Bv

Ck

Dv.toString()

Attempts:

3 left

💡 Hint

Common Mistakes

Using v as the key variable

Not converting keys to uppercase

Using v.toString() instead of v

The keys are converted to uppercase, values are kept as numbers, and the loop variable is k for keys.

Practice

1. What does truthiness narrowing in TypeScript help you do inside an if statement?

easy

A. It forces TypeScript to ignore type errors.

B. It converts all values to strings automatically.

C. It tells TypeScript the value is not null, undefined, or falsy, so you can use it safely.

D. It makes the code run faster by skipping checks.

Solution

1. Step 1: Understand what truthiness means

   Truthiness means a value is considered true in a boolean context, so it is not null, undefined, 0, empty string, or false.

2. Step 2: Apply truthiness narrowing in TypeScript

   Inside an if statement, TypeScript narrows the type by excluding falsy values, letting you safely use the value without extra checks.

3. Final Answer:

   It tells TypeScript the value is not null, undefined, or falsy, so you can use it safely. -> Option C

4. Quick Check:

   Truthiness narrowing = safe value use [OK]

Hint: If inside if(value), value is truthy and safe to use [OK]

Common Mistakes:

• Thinking it converts values to strings
• Believing it skips type checks
• Assuming it improves runtime speed

2. Which of the following TypeScript if statements correctly uses truthiness narrowing?

easy

A. if (value == false) { console.log(value); }

B. if (value === 0) { console.log(value); }

C. if (value = true) { console.log(value); }

D. if (value !== null && value !== undefined) { console.log(value); }

Solution

1. Step 1: Check each if condition for truthiness narrowing

   if (value !== null && value !== undefined) { console.log(value); } explicitly checks for not null and not undefined, which is a common way to narrow truthy values.

2. Step 2: Identify incorrect syntax or logic

   if (value == false) { console.log(value); } checks if value is false (falsy), which is opposite of truthiness narrowing. if (value = true) { console.log(value); } uses assignment (=) instead of comparison (===). if (value === 0) { console.log(value); } checks if value equals 0, which is falsy, so it does not narrow to truthy.

3. Final Answer:

   if (value !== null && value !== undefined) { console.log(value); } -> Option D

4. Quick Check:

   Correct syntax and logic = if (value !== null && value !== undefined) { console.log(value); } [OK]

Hint: Use !== null && !== undefined for safe truthiness checks [OK]

Common Mistakes:

• Using assignment (=) instead of comparison (===)
• Checking for falsy instead of truthy
• Confusing equality operators

3. What will be the output of this TypeScript code?

function printLength(str?: string) {
  if (str) {
    console.log(str.length);
  } else {
    console.log('No string');
  }
}
printLength('hello');
printLength('');
printLength();

medium

A. 5, No string, No string

B. 5, 0, undefined

C. 5, No string, undefined

D. 5, 0, No string

Solution

1. Step 1: Analyze the if condition with truthiness narrowing

   The condition if (str) is true only if str is truthy. An empty string ('') is falsy, so it goes to else.

2. Step 2: Check each function call output

   printLength('hello') prints 5 because 'hello' is truthy and length is 5.
   printLength('') prints 'No string' because '' is falsy.
   printLength() prints 'No string' because str is undefined (falsy).

3. Final Answer:

   5, No string, No string -> Option A

4. Quick Check:

   Falsy empty string triggers else [OK]

Hint: Empty string is falsy, so else block runs [OK]

Common Mistakes:

• Assuming empty string is truthy
• Expecting undefined to print length
• Confusing falsy values with null only

4. Find the error in this TypeScript code using truthiness narrowing:

function process(value: string | null) {
  if (value !== null) {
    console.log(value.toUpperCase());
  } else {
    console.log('No value');
  }
}
process(null);
process(undefined);

medium

A. The if condition should check for undefined as well as null.

B. The code will cause a runtime error when value is undefined.

C. The toUpperCase method is not valid for strings.

D. There is no error; the code works fine.

Solution

1. Step 1: Check the type of value and condition

   Value can be string or null, but the function is called with undefined, which is not handled.

2. Step 2: Identify missing check for undefined

   The if condition only excludes null, so if value is undefined, it will cause an error when calling toUpperCase.

3. Final Answer:

   The if condition should check for undefined as well as null. -> Option A

4. Quick Check:

   Check all falsy types to avoid errors [OK]

Hint: Check both null and undefined for safe narrowing [OK]

Common Mistakes:

• Ignoring undefined values
• Assuming null check covers undefined
• Not testing all input cases

5. Given this TypeScript code, what is the safest way to narrow the type of input to avoid errors when accessing input.length?

function handleInput(input: string | null | undefined | number) {
  if (/* condition here */) {
    console.log(input.length);
  } else {
    console.log('Invalid input');
  }
}

hard

A. if (input) { console.log(input.length); }

B. if (typeof input === 'string' && input) { console.log(input.length); }

C. if (input !== null) { console.log(input.length); }

D. if (input != undefined) { console.log(input.length); }

Solution

1. Step 1: Identify all possible types of input

   Input can be string, null, undefined, or number. Only strings have a length property.

2. Step 2: Narrow input to string and check truthiness

   Using typeof input === 'string' ensures input is a string type. Adding && input excludes empty strings (falsy) if needed.

3. Final Answer:

   if (typeof input === 'string' && input) { console.log(input.length); } -> Option B

4. Quick Check:

   Check type and truthiness for safe property access [OK]

Hint: Check type string before accessing length [OK]

Common Mistakes:

• Only checking truthiness without type
• Ignoring number type causing errors
• Assuming null check covers undefined