Generic conditional constraints in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a generic conditional constraint in TypeScript?

A generic conditional constraint lets you restrict a generic type based on a condition, using the extends keyword combined with conditional types to enforce rules on the types that can be used.

beginner

How do you write a generic function that only accepts types extending string or number?

You write it like this:

function example<T extends string | number>(value: T) { /* ... */ }

This means T can only be string or number.

intermediate

Explain this TypeScript type: T extends U ? X : Y

This is a conditional type. It means: if type T can be assigned to type U, then the type is X, otherwise it is Y. It helps create types that change based on conditions.

beginner

What happens if you try to use a type that does not satisfy a generic conditional constraint?

TypeScript will show a type error and prevent compilation. This helps catch mistakes early by ensuring only allowed types are used where the generic constraint applies.

intermediate

Give an example of a generic conditional constraint that returns different types based on input type.

Example:

type TypeName<T> = T extends string ? "string" : T extends number ? "number" : "other";

This type returns a string literal describing the input type.

What does T extends U ? X : Y mean in TypeScript?

A T must always be equal to U.

B If T is assignable to U, use type X; otherwise, use type Y.

C T is a subtype of X or Y.

D T is a function returning X or Y.

This is a conditional type that chooses X or Y based on whether T extends U.

Which syntax restricts a generic type to only string or number?

A function foo<T extends string | number>() {}

B function foo<T = string | number>() {}

C function foo<T>() where T is string or number {}

D function foo<T: string | number>() {}

The extends keyword restricts the generic type to the union of string or number.

What error occurs if you pass a boolean to function foo(arg: T)?

A Type error: boolean does not satisfy constraint.

B Runtime error.

C No error, boolean is allowed.

D Warning only, code compiles.

Boolean is not assignable to string or number, so TypeScript shows a type error.

What is the purpose of generic conditional constraints?

A To disable type checking.

B To make all types equal.

C To create types that change based on input types.

D To convert types to strings.

They allow types to adapt depending on conditions, improving flexibility and safety.

Which of these is a valid conditional type?

A type Result<T> = T == number ? string : boolean;

B type Result<T> = if T is number then string else boolean;

C type Result<T> = T ? string : boolean;

D type Result<T> = T extends number ? string : boolean;

Option D uses correct TypeScript syntax for conditional types.

Explain how generic conditional constraints help control types in TypeScript.

Think about how you can make types change based on conditions.

Write a simple generic conditional type that returns 'array' if the input type is an array, otherwise 'not array'.

Use <code>T extends any[] ? 'array' : 'not array'</code> pattern.