Unknown type vs any type in Typescript - Quick Revision & Key Differences

Recall & Review

beginner

What is the any type in TypeScript?

The any type allows a variable to hold any value without type checking. It disables TypeScript's type safety for that variable.

beginner

What is the unknown type in TypeScript?

The unknown type represents any value but requires type checking before using it. It is safer than any because it forces you to check the type before operations.

intermediate

How does unknown improve safety compared to any?

unknown forces you to check the type before using the value, preventing errors. any skips checks and can cause runtime errors.

intermediate

Can you assign a value of type any to a variable of type unknown?

Yes, you can assign any to unknown because unknown accepts any value safely.

intermediate

Can you assign a value of type unknown to a variable of type any?

Yes, unknown can be assigned to any because any accepts all types without checks.

Which type requires you to check the type before using the value?

A any

B string

C unknown

D number

unknown requires type checks before usage, making it safer than any.

What happens if you use any type in TypeScript?

A You can assign any value and use it without checks

B TypeScript enforces strict type checks

C You must check the type before use

D It only accepts string values

any disables type checking and allows any value without restrictions.

Can you assign a value of type unknown directly to a variable of type string?

A No, you must check or cast first

B Yes, always

C Only if the value is a string literal

D Only in JavaScript, not TypeScript

You must check or cast unknown before assigning it to a specific type like string.

Which type is safer to use when you don't know the exact type yet?

A any

B void

C never

D unknown

unknown is safer because it forces type checks before use.

What is a key difference between any and unknown?

A <code>any</code> requires type checks, <code>unknown</code> does not

B <code>unknown</code> requires type checks, <code>any</code> does not

C Both behave exactly the same

D <code>any</code> is safer than <code>unknown</code>

unknown requires type checks before use, while any disables checks.

Explain the difference between any and unknown types in TypeScript.

Think about safety and when you can use the value without checks.

When should you prefer unknown over any in your TypeScript code?

Consider safety and error prevention.

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