Unknown type vs any type in Typescript - When to Use Which

The Big Idea

What if blindly trusting your data could break your whole app without warning?

The Scenario

Imagine you are sorting through a big box of mixed items without knowing what each item is. You try to use or fix them without checking, hoping everything works fine.

The Problem

This approach is risky and slow because you might break something or waste time fixing errors caused by wrong assumptions. Without knowing what you have, mistakes happen often.

The Solution

Using the unknown type in TypeScript forces you to check what you have before using it, making your code safer. Unlike any, which lets you do anything and can hide errors, unknown makes you pause and confirm, preventing bugs.

Before vs After

✗ Before

let data: any = fetchData();
console.log(data.name.toUpperCase());

✓ After

let data: unknown = fetchData();
if (typeof data === 'object' && data !== null && 'name' in data) {
  console.log((data as { name: string }).name.toUpperCase());
}

What It Enables

This concept enables you to write safer programs that catch mistakes early by making you verify unknown data before using it.

Real Life Example

When receiving user input or data from the internet, you don't know its shape. Using unknown helps you check the data carefully before trusting it, avoiding crashes or wrong results.

Key Takeaways

any lets you do anything but can hide errors.

unknown forces you to check before use, making code safer.

Using unknown helps catch bugs early and write more reliable programs.

Practice

1. What is the main difference between unknown and any types in TypeScript?

easy

A. unknown requires type checking before use, any does not.

B. any requires type checking before use, unknown does not.

C. unknown and any behave exactly the same.

D. any is safer than unknown.

Solution

1. Step 1: Understand unknown behavior

   unknown means the value can be anything, but you must check its type before using it to avoid errors.

2. Step 2: Understand any behavior

   any disables type checking, so you can use the value without checks, which can cause runtime errors.

3. Final Answer:

   unknown requires type checking before use, any does not. -> Option A

4. Quick Check:

   unknown = safe, any = unchecked [OK]

Hint: Remember: unknown forces checks, any skips them [OK]

Common Mistakes:

• Thinking any is safer than unknown
• Assuming unknown allows direct use without checks
• Believing unknown and any behave the same

2. Which of the following is the correct way to assign a value of type unknown to a variable of type string?

easy

A. let a: unknown = "hello"; let b: string = a as string;

B. let a: unknown = "hello"; let b: string = a.toString();

C. let a: unknown = "hello"; let b: string = a + '';

D. let a: unknown = "hello"; let b: string = a;

Solution

1. Step 1: Understand assignment rules for unknown

   You cannot assign unknown directly to a more specific type without a type assertion or check.

2. Step 2: Check options for correct syntax

   let a: unknown = "hello"; let b: string = a as string; uses a type assertion as string, which is the correct way to tell TypeScript the value is a string.

3. Final Answer:

   let b: string = a as string; -> Option A

4. Quick Check:

   Use as to convert unknown safely [OK]

Hint: Use 'as' to convert unknown to a specific type [OK]

Common Mistakes:

• Assigning unknown directly without assertion
• Using incorrect type assertion syntax
• Calling methods on unknown without checks

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

let value: unknown = 42;
if (typeof value === 'number') {
  console.log(value + 8);
} else {
  console.log('Not a number');
}

medium

A. TypeError at runtime

B. Not a number

C. 50

D. Compilation error

Solution

1. Step 1: Check the type of value

   The variable value is unknown but assigned 42, a number.

2. Step 2: Evaluate the if condition and output

   The condition typeof value === 'number' is true, so it logs value + 8 which is 42 + 8 = 50.

3. Final Answer:

   50 -> Option C

4. Quick Check:

   Type check passes, output is 50 [OK]

Hint: Check type before using unknown to avoid errors [OK]

Common Mistakes:

• Assuming unknown can be used without type check
• Expecting runtime error without type check
• Confusing unknown with any behavior

4. Identify the error in this TypeScript code snippet:

let data: unknown = 'hello';
let length: number = data.length;

medium

A. No error, code runs fine.

B. Cannot access property 'length' on type 'unknown'.

C. Type 'string' is not assignable to type 'unknown'.

D. Variable 'length' should be type 'string'.

Solution

1. Step 1: Understand property access on unknown

   You cannot access properties directly on a variable typed as unknown without type checking or assertion.

2. Step 2: Identify the error message

   Trying to access data.length causes a compile-time error: "Cannot access property 'length' on type 'unknown'."

3. Final Answer:

   Cannot access property 'length' on type 'unknown'. -> Option B

4. Quick Check:

   Accessing unknown properties causes error [OK]

Hint: Check type before accessing unknown properties [OK]

Common Mistakes:

• Assuming unknown behaves like any
• Ignoring compiler error on property access
• Wrongly typing variables

5. You receive data from an external API typed as any. You want to safely use it as a string only if it really is a string. Which approach is best?

hard

A. Assign the any data directly to a string variable.

B. Cast the any data to unknown and then to string without checks.

C. Ignore types and use the data as is.

D. Use a type guard to check if data is a string before using it.

Solution

1. Step 1: Understand risks of any

   any disables type checking, so using it directly can cause runtime errors if the data is not as expected.

2. Step 2: Use type guard for safety

   Checking the type with a type guard (e.g., typeof data === 'string') ensures safe usage only when the data is a string.

3. Final Answer:

   Use a type guard to check if data is a string before using it. -> Option D

4. Quick Check:

   Type guard ensures safe use of any data [OK]

Hint: Always check any data type before use [OK]

Common Mistakes:

• Assigning any directly without checks
• Casting any to unknown then string without validation
• Ignoring type safety for external data