typeof type guards in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a typeof type guard in TypeScript?

A typeof type guard is a way to check the type of a variable at runtime using the typeof operator. It helps TypeScript narrow down the type inside conditional blocks.

beginner

How does typeof help with type safety in TypeScript?

By checking the type of a variable with typeof, TypeScript can safely allow operations specific to that type inside the guarded block, preventing errors.

beginner

Example: What does this code do?

if (typeof value === 'string') {
  console.log(value.toUpperCase());
}

This code checks if value is a string. Inside the if block, TypeScript knows value is a string, so calling toUpperCase() is safe.

intermediate

Which types can typeof check in TypeScript?

typeof can check primitive types like 'string', 'number', 'boolean', 'bigint', 'symbol', 'undefined', and 'object' (including null).

intermediate

Why can't typeof distinguish between null and object?

Because in JavaScript, typeof null returns 'object'. So typeof cannot differentiate null from other objects.

What does typeof return for a string variable?

A 'text'

B 'char'

C 'string'

D 'str'

typeof returns the string 'string' for string variables.

Which of these is NOT a valid typeof result in JavaScript/TypeScript?

A 'undefined'

B 'boolean'

C 'number'

D 'array'

typeof never returns 'array'. Arrays are objects, so it returns 'object'.

Why use typeof type guards in TypeScript?

A To check variable types at runtime and narrow types safely

B To declare new types

C To convert types automatically

D To create classes

typeof type guards check types at runtime and help TypeScript narrow types for safer code.

What will typeof null return?

A 'object'

B 'undefined'

C 'null'

D 'boolean'

Due to JavaScript design, typeof null returns 'object'.

Which code snippet correctly uses a typeof type guard?

A if (typeof x === 'stringify') { console.log(x); }

B if (typeof x === 'number') { console.log(x.toFixed(2)); }

C if (typeof x === 'integer') { console.log(x); }

D if (typeof x === 'array') { console.log(x.length); }

Only 'number' is a valid typeof result. 'array', 'integer', and 'stringify' are invalid.

Explain how typeof type guards help TypeScript understand variable types.

Think about how checking a variable's type at runtime helps avoid errors.

List the primitive types that typeof can detect and mention one limitation.

Recall the common JavaScript types and the quirk with null.

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