Conditional type syntax in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the basic syntax of a conditional type in TypeScript?

A conditional type uses the syntax: <br>T extends U ? X : Y<br>It means if type T can be assigned to type U, then the type is X, otherwise it is Y.

beginner

Explain the role of extends in conditional types.

In conditional types, extends checks if one type can be assigned to another. It acts like a question: "Does T fit into U?" If yes, the true branch is chosen; if no, the false branch is chosen.

beginner

What does this conditional type do?<br>type IsString<T> = T extends string ? true : false;

It checks if the type T is a string. If yes, it returns true, otherwise false. For example, IsString<'hello'> = true and IsString<42> = false.

intermediate

Can conditional types be nested in TypeScript?

Yes, conditional types can be nested inside each other to create more complex type logic. For example:<br>type Nested<T> = T extends string ? (T extends 'hello' ? true : false) : false;

intermediate

What happens if the condition in a conditional type is distributive?

If the type T is a union, the conditional type is applied to each member separately, then results are combined. This is called distributive conditional types.

What does this conditional type return?<br>type Check<T> = T extends number ? 'Yes' : 'No';<br>What is Check<string>?

A number

B 'Yes'

C string

D 'No'

Since string does not extend number, the false branch 'No' is returned.

In T extends U ? X : Y, what happens if T is a union type?

A The conditional type applies to each member of the union separately

B The conditional type fails to compile

C The conditional type ignores the union and treats it as one type

D The conditional type always returns Y

Conditional types distribute over unions, applying the condition to each member individually.

What keyword is used to check type compatibility in conditional types?

A extends

B implements

C is

D typeof

The keyword extends is used to check if one type can be assigned to another.

Which of these is a valid conditional type?

A type T = if (T extends U) X else Y;

B type T = string ? number : boolean;

C type T = T extends U ? X : Y;

D type T = T is U ? X : Y;

The correct syntax uses extends with the ternary operator: T extends U ? X : Y.

What is the result of this type?<br>type Result = 'a' | 'b' extends string ? true : false;

A false

B true

C 'a' | 'b'

D string

The union 'a' | 'b' extends string, so the condition is true.

Explain how conditional types work in TypeScript and give a simple example.

Think of it like an if-else for types.

Describe what distributive conditional types are and why they are useful.

Consider how a condition applies to multiple types inside a union.