Pick type in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the Pick type do in TypeScript?

The Pick type creates a new type by selecting specific properties from an existing type. It helps to focus on only the needed parts of a type.

beginner

How do you use Pick to select properties 'name' and 'age' from a type Person?

You write:
type PersonInfo = Pick<Person, 'name' | 'age'>;
This creates a new type with only name and age from Person.

intermediate

Why is Pick useful in real projects?

It helps to reuse existing types but only keep the parts you need. This makes code simpler and safer by avoiding extra properties.

intermediate

What happens if you try to Pick a property that does not exist in the original type?

TypeScript will give an error because you can only pick keys that exist in the original type.

advanced

Can Pick be combined with other utility types?

Yes! You can combine Pick with types like Partial or Readonly to create flexible and precise types.

What does Pick<T, K> do in TypeScript?

A Creates a new type with only the properties K from type T

B Removes properties K from type T

C Makes all properties in T optional

D Combines two types T and K

Pick selects specific properties K from type T to create a new type.

Which syntax correctly picks properties 'id' and 'title' from type Book?

A Pick<Book, {id, title}>

B Pick<Book, ['id', 'title']>

C Pick<Book, 'id' | 'title'>

D Pick<Book, id, title>

The keys must be a union of string literals, like 'id' | 'title'.

What error occurs if you pick a property not in the original type?

A Runtime error

B TypeScript error: Type '"wrongKey"' is not assignable to keys of T

C No error, property is ignored

D Warning but code compiles

TypeScript checks keys at compile time and errors if keys don't exist.

Can Pick be used to create a type with all properties optional?

A No, use <code>Partial</code> for that

B Yes, <code>Pick</code> makes properties optional

C Yes, but only for string properties

D No, it removes properties instead

Pick selects properties but does not change optionality; Partial makes properties optional.

Which utility type can be combined with Pick to make picked properties readonly?

A Partial

B Exclude

C Record

D Readonly

Readonly makes properties readonly and can wrap a Pick type.

Explain how the Pick type works and give an example of its syntax.

Think about choosing only some keys from a bigger object type.

Describe a situation where using Pick would make your TypeScript code better.

Imagine you only need a few details from a big object.

Practice

1. What does the Pick type do in TypeScript?

easy

A. Removes properties from a type.

B. Creates a new type by selecting specific properties from an existing type.

C. Combines two types into one.

D. Makes all properties in a type optional.

Solution

1. Step 1: Understand the purpose of Pick

   The Pick type is used to create a new type by choosing certain properties from an existing type.

2. Step 2: Compare with other options

   Removing properties is done by Omit, combining types by intersection, and making properties optional by Partial.

3. Final Answer:

   Creates a new type by selecting specific properties from an existing type. -> Option B

4. Quick Check:

   Pick selects properties = A [OK]

Hint: Pick selects properties to create smaller types [OK]

Common Mistakes:

• Confusing Pick with Omit
• Thinking Pick makes properties optional
• Mixing Pick with type intersections

2. Which of the following is the correct syntax to create a type PersonName with only name and age properties from Person using Pick?

easy

A. type PersonName = Pick;

B. type PersonName = Pick;

C. type PersonName = Pick;

D. type PersonName = Pick;

Solution

1. Step 1: Recall Pick syntax

   Pick takes two parameters: the original type and a union of property names as string literals.

2. Step 2: Identify correct union syntax

   The union of keys is written as 'name' | 'age' inside quotes and separated by |.

3. Final Answer:

   type PersonName = Pick; -> Option A

4. Quick Check:

   Pick syntax uses union of strings = B [OK]

Hint: Use quotes and | to list keys in Pick [OK]

Common Mistakes:

• Omitting quotes around keys
• Using array syntax instead of union
• Passing keys as object instead of union

3. Given the type interface User { id: number; name: string; email: string; } and type UserPreview = Pick;, what is the type of userPreview in this code?

const userPreview: UserPreview = { id: 1, name: 'Alice' };

medium

A. { name: string; email: string; }

B. { id: number; name: string; email: string; }

C. { id: number; }

D. { id: number; name: string; }

Solution

1. Step 1: Understand Pick selects properties

   UserPreview picks only 'id' and 'name' from User, so it has only those two properties.

2. Step 2: Match the type of userPreview

   The object assigned has 'id' and 'name' only, matching the Pick type exactly.

3. Final Answer:

   { id: number; name: string; } -> Option D

4. Quick Check:

   Pick selects only chosen keys = A [OK]

Hint: Pick type has only selected keys, no extras [OK]

Common Mistakes:

• Assuming all original properties remain
• Confusing Pick with Partial
• Including properties not picked

4. Identify the error in this TypeScript code using Pick:

interface Product { id: number; name: string; price: number; }
type ProductInfo = Pick;

medium

A. The interface Product is missing a 'cost' property.

B. Pick cannot be used with more than one key.

C. 'cost' is not a key of Product, causing a type error.

D. The syntax for Pick is incorrect; keys should be in an array.

Solution

1. Step 1: Check keys used in Pick

   Pick requires keys to exist in the original type. 'cost' is not a key in Product.

2. Step 2: Understand TypeScript error

   Using a non-existent key causes a type error because Pick cannot select keys that don't exist.

3. Final Answer:

   'cost' is not a key of Product, causing a type error. -> Option C

4. Quick Check:

   Pick keys must exist in type = C [OK]

Hint: Pick keys must match existing properties exactly [OK]

Common Mistakes:

• Using keys not in the original type
• Thinking Pick keys can be arrays
• Assuming Pick works with any string keys

5. You have a type interface Employee { id: number; name: string; department: string; salary: number; }. You want to create a type EmployeeSummary that includes only id, name, and a new property departmentCode which is a string. How can you do this using Pick and adding the new property?

hard

A. type EmployeeSummary = Pick & { departmentCode: string };

B. type EmployeeSummary = Pick;

C. type EmployeeSummary = Omit & { departmentCode: string };

D. type EmployeeSummary = Partial<Employee> & { departmentCode: string };

Solution

1. Step 1: Use Pick to select existing keys

   Pick 'id' and 'name' from Employee to get those properties only.

2. Step 2: Add new property with intersection

   Use & to add a new property 'departmentCode' with type string to the picked type.

3. Final Answer:

   type EmployeeSummary = Pick & { departmentCode: string }; -> Option A

4. Quick Check:

   Pick keys + add new property with & = D [OK]

Hint: Combine Pick with & to add new properties [OK]

Common Mistakes:

• Trying to Pick a non-existent property
• Using Omit instead of Pick incorrectly
• Using Partial which makes all properties optional