Why Type narrowing with typeof in Typescript? - Purpose & Use Cases

The Big Idea

Discover how a simple check can save your code from confusing bugs!

The Scenario

Imagine you have a box with different kinds of fruits mixed together. You want to pick only the apples, but you have to check each fruit by guessing its type without any clear label.

The Problem

Without a clear way to check the type, you might pick the wrong fruit or waste a lot of time guessing. This is like writing code that treats all data the same, causing errors or unexpected results.

The Solution

Type narrowing with typeof lets you check the exact type of a value before using it. This way, your code knows what to expect and can handle each type correctly and safely.

Before vs After

✗ Before

function process(value: any) {
  // No type check
  console.log(value.toFixed(2));
}

✓ After

function process(value: unknown) {
  if (typeof value === 'number') {
    console.log(value.toFixed(2));
  }
}

What It Enables

It enables writing safer and clearer code that adapts to different data types without errors.

Real Life Example

When building a calculator app, you might get inputs as numbers or strings. Using typeof helps you handle each input correctly, avoiding crashes.

Key Takeaways

Manual type guessing is slow and risky.

typeof checks the type clearly and safely.

Type narrowing helps your code handle data correctly.

Practice

1. What does the typeof operator do in TypeScript?

easy

A. It converts a variable to a string.

B. It changes the type of a variable.

C. It declares a new type.

D. It checks the type of a variable at runtime.

Solution

1. Step 1: Understand the purpose of typeof

   The typeof operator is used to check the type of a variable during the program's execution (runtime).

2. Step 2: Differentiate from other options

   It does not change, declare, or convert types; it only inspects the type.

3. Final Answer:

   It checks the type of a variable at runtime. -> Option D

4. Quick Check:

   typeof checks type at runtime [OK]

Hint: Remember: typeof tells you the type, it doesn't change it. [OK]

Common Mistakes:

• Thinking typeof changes the variable's type
• Confusing typeof with type declaration
• Assuming typeof converts values to strings

2. Which of the following is the correct syntax to narrow a variable value to a string using typeof?

easy

A. if (typeof value === 'string') { }

B. if (typeof value = 'string') { }

C. if (typeof value == string) { }

D. if (typeof value === string) { }

Solution

1. Step 1: Check correct equality operator and string literal

   The correct way to compare is using triple equals === and the type name as a string literal, like 'string'.

2. Step 2: Identify syntax errors in other options

   if (typeof value = 'string') { } uses single equals which is assignment, not comparison. Options A and C miss quotes around 'string'.

3. Final Answer:

   if (typeof value === 'string') { } -> Option A

4. Quick Check:

   Use === and quotes for typeof checks [OK]

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

Common Mistakes:

• Using single = instead of ===
• Forgetting quotes around type names
• Using == instead of ===

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

function checkType(x: string | number) {
  if (typeof x === 'string') {
    return x.toUpperCase();
  } else {
    return x + 10;
  }
}

console.log(checkType('hello'));
console.log(checkType(5));

medium

A. HELLO and 510

B. hello and 15

C. HELLO and 15

D. Error at runtime

Solution

1. Step 1: Analyze the input 'hello'

   Since 'hello' is a string, typeof x === 'string' is true, so x.toUpperCase() returns 'HELLO'.

2. Step 2: Analyze the input 5

   Since 5 is a number, the else block runs, returning 5 + 10 = 15.

3. Final Answer:

   HELLO and 15 -> Option C

4. Quick Check:

   String upper + number addition = HELLO, 15 [OK]

Hint: Check typeof to pick string or number logic correctly. [OK]

Common Mistakes:

• Concatenating number and string instead of adding
• Calling string methods on numbers
• Confusing toUpperCase output

4. Identify the error in this TypeScript code using typeof for type narrowing:

function process(value: string | boolean) {
  if (typeof value === 'boolean') {
    return value.toUpperCase();
  } else {
    return value ? 'Yes' : 'No';
  }
}

medium

A. Calling toUpperCase on a boolean value

B. Using typeof with boolean type

C. Missing return type annotation

D. Incorrect if-else structure

Solution

1. Step 1: Check the type narrowing condition

   The condition typeof value === 'boolean' correctly narrows value to boolean.

2. Step 2: Identify method call on wrong type

   Inside this block, calling value.toUpperCase() is invalid because booleans do not have this method.

3. Final Answer:

   Calling toUpperCase on a boolean value -> Option A

4. Quick Check:

   Boolean has no toUpperCase method [OK]

Hint: Check method calls match the narrowed type. [OK]

Common Mistakes:

• Assuming booleans have string methods
• Ignoring type narrowing effect
• Confusing boolean and string logic

5. You have a function that accepts input: string | number | boolean. You want to return:
- The length of the string if input is a string,
- The number multiplied by 2 if input is a number,
- The string 'True' or 'False' if input is a boolean.

Which of the following implementations correctly uses typeof for type narrowing?

hard

A. function handle(input: string | number | boolean) {
if (typeof input === 'boolean') {
return input.length;
} else if (typeof input === 'string') {
return input * 2;
} else {
return input ? 'True' : 'False';
}
}

B. function handle(input: string | number | boolean) {
if (typeof input === 'string') {
return input.length;
} else if (typeof input === 'number') {
return input * 2;
} else {
return input ? 'True' : 'False';
}
}

C. function handle(input: string | number | boolean) {
if (typeof input === 'number') {
return input.length;
} else if (typeof input === 'boolean') {
return input * 2;
} else {
return input ? 'True' : 'False';
}
}

D. function handle(input: string | number | boolean) {
if (typeof input === 'string') {
return input.toUpperCase();
} else if (typeof input === 'number') {
return input + 2;
} else {
return input ? 'True' : 'False';
}
}

Solution

1. Step 1: Check string handling

   function handle(input: string | number | boolean) { if (typeof input === 'string') { return input.length; } else if (typeof input === 'number') { return input * 2; } else { return input ? 'True' : 'False'; } } returns input.length for strings, which is correct. function handle(input: string | number | boolean) { if (typeof input === 'string') { return input.toUpperCase(); } else if (typeof input === 'number') { return input + 2; } else { return input ? 'True' : 'False'; } } returns input.toUpperCase(), which is not requested.

2. Step 2: Check number handling

   function handle(input: string | number | boolean) { if (typeof input === 'string') { return input.length; } else if (typeof input === 'number') { return input * 2; } else { return input ? 'True' : 'False'; } } multiplies number by 2, as required. function handle(input: string | number | boolean) { if (typeof input === 'string') { return input.toUpperCase(); } else if (typeof input === 'number') { return input + 2; } else { return input ? 'True' : 'False'; } } adds 2, which is incorrect. Options B and C misuse length or multiplication on wrong types.

3. Step 3: Check boolean handling

   function handle(input: string | number | boolean) { if (typeof input === 'string') { return input.length; } else if (typeof input === 'number') { return input * 2; } else { return input ? 'True' : 'False'; } } returns 'True' or 'False' strings correctly. Others mix up boolean logic or return wrong types.

4. Final Answer:

   function handle(input: string | number | boolean) { if (typeof input === 'string') { return input.length; } else if (typeof input === 'number') { return input * 2; } else { return input ? 'True' : 'False'; } } -> Option B

5. Quick Check:

   Match typeof checks with correct operations [OK]

Hint: Match each typeof check with correct operation for that type. [OK]

Common Mistakes:

• Calling string methods on numbers or booleans
• Using length on numbers or booleans
• Mixing up operations for each type