typeof type guards in Typescript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to check if the variable is a string using typeof.

Typescript

if (typeof value === [1]) {
  console.log("It's a string!");
}

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

A"string"

B"number"

C"boolean"

D"object"

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'string' without quotes.

Comparing to 'String' with uppercase S.

Using typeof value === string (missing quotes).

The typeof operator returns "string" for string values, so we compare with "string".

2fill in blank

medium

Complete the code to check if the variable is a number using typeof.

Typescript

if (typeof num === [1]) {
  console.log("It's a number!");
}

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

A"undefined"

B"string"

C"boolean"

D"number"

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Number' with uppercase N.

Comparing to 'num' instead of 'number'.

Missing quotes around 'number'.

typeof returns "number" for numeric values, so we compare with "number".

3fill in blank

hard

Fix the error in the typeof check to correctly identify a boolean.

Typescript

if (typeof flag === [1]) {
  console.log("It's a boolean!");
}

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

A"string"

B"bool"

C"boolean"

D"Boolean"

Attempts:

3 left

💡 Hint

Common Mistakes

Using uppercase "Boolean" instead of lowercase.

Using "bool" which is not a valid typeof result.

Missing quotes around the type string.

The typeof operator returns "boolean" for boolean values, all lowercase.

4fill in blank

hard

Fill both blanks to create a type guard that checks if input is a string or a number.

Typescript

if (typeof input === [1] || typeof input === [2]) {
  console.log("Input is string or number");
}

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

A"string"

B"boolean"

C"number"

D"object"

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect type strings like "bool" or "object".

Using && instead of || which requires both types at once.

Missing quotes around type strings.

We check if input is either "string" or "number" using typeof comparisons.

5fill in blank

hard

Fill all three blanks to create a function that returns the type of the value as a string using typeof.

Typescript

function getType(value: unknown): string {
  return typeof [1] [2] [3];
}

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

Avalue

B+

C""

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Using '-' instead of '+' for concatenation.

Not using quotes around the empty string.

Using a variable name other than 'value'.

The function returns typeof value concatenated with an empty string to ensure the return type is string.

Practice

1. What does the typeof operator do in TypeScript when used as a type guard?

easy

A. It declares a new variable with a specific type.

B. It converts a variable to a string type.

C. It checks the runtime type of a variable to narrow its type inside a condition.

D. It creates a new type alias for a variable.

Solution

1. Step 1: Understand the purpose of typeof

   The typeof operator checks the actual type of a value at runtime, like 'string' or 'number'.

2. Step 2: Recognize its use as a type guard

   Using typeof in an if condition helps TypeScript know the exact type inside that block, making code safer.

3. Final Answer:

   It checks the runtime type of a variable to narrow its type inside a condition. -> Option C

4. Quick Check:

   typeof narrows type [OK]

Hint: typeof checks actual type at runtime to narrow types [OK]

Common Mistakes:

• Thinking typeof changes the variable's type
• Confusing typeof with type declarations
• Assuming typeof works at compile time only

2. Which of the following is the correct syntax to use typeof as a type guard in TypeScript?

easy

A. if (typeof x === 'string') { /* code */ }

B. if (typeof x = 'string') { /* code */ }

C. if (typeof x == string) { /* code */ }

D. if (typeof x === string) { /* code */ }

Solution

1. Step 1: Check the correct comparison operator

   The strict equality operator === is required to compare typeof result with a string.

2. Step 2: Verify the string literal format

   The type name must be a string literal like 'string', 'number', etc., inside quotes.

3. Final Answer:

   if (typeof x === 'string') { /* code */ } -> Option A

4. Quick Check:

   Correct typeof syntax uses === and quotes [OK]

Hint: Use === and quotes around type name in typeof checks [OK]

Common Mistakes:

• Using single = instead of ===
• Not putting type name in quotes
• Using == instead of ===

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

function checkValue(x: string | number) {
  if (typeof x === 'string') {
    return x.toUpperCase();
  } else {
    return x.toFixed(2);
  }
}
console.log(checkValue(3.14159));

medium

A. '3.14'

B. '3.14' as a string

C. '3.14' as a number

D. '3.14' as a string with two decimals

Solution

1. Step 1: Identify the type of input

   The input is 3.14159, which is a number, so the else block runs.

2. Step 2: Understand the else block behavior

   The else block calls toFixed(2) on the number, which returns a string with 2 decimals: '3.14'.

3. Final Answer:

   '3.14' as a string with two decimals -> Option D

4. Quick Check:

   Number input uses toFixed(2) = '3.14' [OK]

Hint: toFixed(2) returns string with 2 decimals [OK]

Common Mistakes:

• Thinking toFixed returns a number
• Confusing string and number outputs
• Ignoring the else block

4. Find the error in this TypeScript code using typeof type guard:

function process(value: string | boolean) {
  if (typeof value === 'boolean') {
    return value.toUpperCase();
  } else {
    return value;
  }
}

medium

A. The typeof check should be for 'string' instead of 'boolean'.

B. Calling toUpperCase() on a boolean value causes an error.

C. The else block should convert value to boolean.

D. There is no error; the code is correct.

Solution

1. Step 1: Analyze the typeof condition

   The condition checks if value is a boolean.

2. Step 2: Check method call on boolean

   Inside the if block, it calls toUpperCase(), which exists only on strings, not booleans, causing an error.

3. Final Answer:

   Calling toUpperCase() on a boolean value causes an error. -> Option B

4. Quick Check:

   toUpperCase() invalid on boolean [OK]

Hint: Check method exists on type before calling [OK]

Common Mistakes:

• Assuming booleans have string methods
• Mixing up type guard conditions
• Ignoring method compatibility

5. Given this function, what is the correct way to safely handle a variable that can be string, number, or boolean using typeof type guards?

function formatValue(val: string | number | boolean) {
  // Fill in the blanks
}

hard

A. Use separate if blocks: if typeof val === 'string', else if 'number', else if 'boolean', then handle each accordingly.

B. Use a single if block checking if typeof val === 'string' and treat others as strings.

C. Convert val to string directly without typeof checks.

D. Use typeof val === 'object' to handle all types.

Solution

1. Step 1: Recognize multiple possible types

   The variable can be string, number, or boolean, so each needs separate handling.

2. Step 2: Use multiple typeof checks

   Use if-else if blocks checking typeof val === 'string', 'number', and 'boolean' to safely handle each type.

3. Final Answer:

   Use separate if blocks: if typeof val === 'string', else if 'number', else if 'boolean', then handle each accordingly. -> Option A

4. Quick Check:

   Multiple typeof checks for multiple types [OK]

Hint: Check each type separately with typeof for safety [OK]

Common Mistakes:

• Handling all types as strings blindly
• Using typeof 'object' for primitives
• Skipping boolean type handling