Unknown type vs any type in Typescript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a variable with the unknown type.

Typescript

let value: [1];

Drag options to blanks, or click blank then click option'

Aunknown

Bany

Cvoid

Dnever

Attempts:

3 left

💡 Hint

Common Mistakes

Using any instead of unknown disables type safety.

Confusing void with unknown.

The unknown type is used when the type is not known yet, unlike any which disables type checking.

2fill in blank

medium

Complete the code to assign a value of type unknown to a variable.

Typescript

let data: unknown = [1];

Drag options to blanks, or click blank then click option'

Atrue

B'hello'

C42

Dnull

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to assign a value that is not valid syntax.

Confusing unknown with any in assignment.

You can assign any value to a variable of type unknown. Here, 42 is a valid number.

3fill in blank

hard

Fix the error in the code by adding a type check before assignment.

Typescript

let value: string;
let input: unknown = 'test';
if (typeof input === [1]) {
  value = input;
}

Drag options to blanks, or click blank then click option'

A'number'

B'object'

C'boolean'

D'string'

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning unknown directly without type check.

Using wrong type string in the check.

Before assigning input of type unknown to value of type string, we check if input is a string.

4fill in blank

hard

Fill both blanks to safely assign an unknown value to a number variable.

Typescript

let num: number;
let val: unknown = 10;
if (typeof val === [1]) {
  num = val [2] 0;
}

Drag options to blanks, or click blank then click option'

A'number'

B+

C-

D'string'

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong type string in the check.

Missing arithmetic operation causing type errors.

Check if val is a number, then assign it to num. The + 0 ensures num is a number type.

5fill in blank

hard

Fill all three blanks to convert an any type to a string safely.

Typescript

let input: any = 123;
let output: string;
if (typeof input === [1]) {
  output = input [2] '';
} else {
  output = String([3]);
}

Drag options to blanks, or click blank then click option'

A'string'

B+

Cinput

Dinput.toString()

Attempts:

3 left

💡 Hint

Common Mistakes

Not checking type before assignment.

Using wrong conversion method.

Check if input is a string, then concatenate with empty string to keep it string. Otherwise, convert input to string using String(input).

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