typeof type guards in Typescript

Introduction

We use typeof type guards to check the type of a value at runtime. This helps TypeScript understand what kind of data we are working with so it can avoid mistakes.

When you have a variable that can hold different types like string or number.

When you want to run different code depending on whether a value is a number or a string.

When you want to avoid errors by making sure a value is the right type before using it.

When you receive input from users or external sources and need to check its type.

When you want to write safer code that adapts to different data types.

Syntax

Typescript

if (typeof variable === 'type') {
  // code for this type
} else {
  // code for other types
}

The typeof operator returns a string like 'string', 'number', 'boolean', 'object', 'undefined', or 'function'.

Use triple equals === to compare the result of typeof to a string.

Examples

This checks if value is a string before running the code inside.

Typescript

if (typeof value === 'string') {
  console.log('It is a string');
}

This adds 10 only if num is a number.

Typescript

if (typeof num === 'number') {
  console.log(num + 10);
}

This runs different code depending on whether input is a boolean or not.

Typescript

if (typeof input === 'boolean') {
  console.log('Boolean value:', input);
} else {
  console.log('Not a boolean');
}

Sample Program

This function uses typeof type guards to check the type of value and print a message accordingly.

Typescript

function printValue(value: string | number | boolean) {
  if (typeof value === 'string') {
    console.log(`String: ${value}`);
  } else if (typeof value === 'number') {
    console.log(`Number: ${value}`);
  } else if (typeof value === 'boolean') {
    console.log(`Boolean: ${value}`);
  } else {
    console.log('Unknown type');
  }
}

printValue('hello');
printValue(42);
printValue(true);

Important Notes

typeof works well for primitive types like string, number, and boolean.

It does not distinguish between different object types (like arrays or null), so use other checks for those.

Always use === to compare the result of typeof to avoid unexpected bugs.

Summary

typeof type guards help check the type of a value at runtime.

They make your code safer by letting TypeScript know the exact type inside a condition.

Use them when your variable can hold different primitive types like string, number, or boolean.

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