Unknown type vs any type in Typescript

Introduction

We use unknown and any types to handle values when we don't know their exact type. They help us write flexible code but in different ways.

When you get data from an external source like user input or a web API and you don't know its type yet.

When you want to allow any type but still want to check the type before using it.

When you want to skip type checking completely and trust the value is correct.

When you want safer code by forcing checks before using a value.

When you want to quickly disable type checking during development.

Syntax

Typescript

let value: unknown;
let anything: any;

unknown means the value could be anything but you must check its type before using it.

any means the value can be anything and TypeScript will not check it at all.

Examples

With unknown, you must check the type before using it as a number.

Typescript

let data: unknown = 5;
// Need to check type before using
if (typeof data === 'number') {
  console.log(data + 10); // OK
}

With any, TypeScript lets you use the value without checks.

Typescript

let data: any = 5;
console.log(data + 10); // No error, even if data is not a number

unknown forces you to check the type before accessing properties.

Typescript

let value: unknown = 'hello';
// console.log(value.length); // Error: Object is of type 'unknown'.

if (typeof value === 'string') {
  console.log(value.length); // OK
}

any skips all type checks, so you can access properties freely.

Typescript

let value: any = 'hello';
console.log(value.length); // OK, no error

Sample Program

This program shows how to safely use a value of type unknown by checking its type first.

Typescript

function processValue(value: unknown) {
  if (typeof value === 'string') {
    console.log(`String of length ${value.length}`);
  } else if (typeof value === 'number') {
    console.log(`Number plus 10 is ${value + 10}`);
  } else {
    console.log('Unknown type');
  }
}

processValue('hello');
processValue(5);
processValue(true);

Important Notes

unknown is safer than any because it forces you to check types before use.

any disables all type checking and can lead to bugs if used carelessly.

Use unknown when you want flexibility but still want type safety.

Summary

unknown means the value could be anything but you must check its type before using it.

any means the value can be anything and TypeScript will not check it at all.

Prefer unknown for safer code and any only when you need to skip checks.

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