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.

Practice

1.

What does a generic conditional constraint in TypeScript allow you to do?

For example, in function example(arg: T) {}, what is the main purpose?

easy

A. Control the generic type based on a condition to improve type safety

B. Make the function run faster by optimizing loops

C. Automatically convert types to strings

D. Disable type checking for the generic parameter

Solution

1. Step 1: Understand generic conditional constraints

   Generic conditional constraints let you choose a type based on a condition, like T extends U ? X : Y.

2. Step 2: Identify the purpose in the example

   This controls the generic type to be either X or Y depending on whether T extends U, improving type safety.

3. Final Answer:

   Control the generic type based on a condition to improve type safety -> Option A

4. Quick Check:

   Generic conditional constraints = control type conditionally [OK]

Hint: Think: conditional means choosing type based on a test [OK]

Common Mistakes:

• Confusing conditional constraints with runtime conditions
• Thinking it affects performance instead of types
• Assuming it disables type checking

2.

Which of the following is the correct syntax for a generic conditional constraint in TypeScript?

function test<T>(arg: T extends string ? number : boolean) {}

easy

A. function test<T>(arg: T extends string ? number : boolean) {}

B. function test(arg: T) {}

C. function test(arg: T) {}

D. function test<T>(arg: T) where T extends string ? number : boolean {}

Solution

1. Step 1: Identify correct generic conditional syntax

   The conditional type must be used in the argument type, not in the generic constraint part.

2. Step 2: Check each option

   function test<T>(arg: T extends string ? number : boolean) {} correctly places the conditional type in the argument type: arg: T extends string ? number : boolean.

3. Final Answer:

   function test<T>(arg: T extends string ? number : boolean) {} -> Option A

4. Quick Check:

   Conditional type goes in argument type, not generic constraint [OK]

Hint: Conditional types appear in argument or return types, not in generic constraints [OK]

Common Mistakes:

• Putting conditional directly in generic constraint
• Using invalid syntax like 'where'
• Misplacing parentheses in conditional

3.

What is the output of this TypeScript code?

type Result<T> = T extends number ? "Number" : "Other";

const a: Result<42> = "Number";
const b: Result<string> = "Other";

console.log(a, b);

medium

A. 42 string

B. Number Other

C. Number Number

D. Other Other

Solution

1. Step 1: Understand the conditional type Result<T>

   If T is a number, Result<T> is "Number", else "Other".

2. Step 2: Apply to given values

   Result<42> is "Number" because 42 is a number.
   Result<string> is "Other" because string is not a number.

3. Final Answer:

   Number Other -> Option B

4. Quick Check:

   Conditional type matches number = "Number", else "Other" [OK]

Hint: Check if type extends number to pick output string [OK]

Common Mistakes:

• Confusing value 42 with type number
• Assuming string extends number
• Mixing up output strings

4.

Identify the error in this TypeScript code using generic conditional constraints:

function checkType<T>(value: T): T extends string ? number : boolean {
  if (typeof value === "string") {
    return value.length;
  } else {
    return true;
  }
}

medium

A. Function lacks explicit return type annotation

B. The function parameter should be a union type

C. The generic constraint is missing

D. Cannot return different types conditionally without type assertion

Solution

1. Step 1: Analyze return type conditionality

   The return type depends on T, but at runtime TypeScript cannot guarantee which type is returned.

2. Step 2: Understand why error occurs

   Returning value.length (number) or true (boolean) causes a type error because TypeScript can't confirm the return matches the conditional type without assertions.

3. Final Answer:

   Cannot return different types conditionally without type assertion -> Option D

4. Quick Check:

   Conditional return types need type assertions or overloads [OK]

Hint: Conditional return types need type assertions to avoid errors [OK]

Common Mistakes:

• Assuming TypeScript infers conditional return types automatically
• Adding generic constraints unnecessarily
• Changing parameter types instead of fixing return

5.

Given this TypeScript code, what is the type of result?

type ConditionalType<T> = T extends { id: number } ? string : number;

function processItem<T>(item: T): ConditionalType<T> {
  if ("id" in item) {
    return "Has ID" as ConditionalType<T>;
  } else {
    return 0 as ConditionalType<T>;
  }
}

const result = processItem({ id: 123, name: "Test" });

hard

A. string | number

B. number

C. string

D. boolean

Solution

1. Step 1: Understand the conditional type ConditionalType<T>

   If T has an id property of type number, the type is string, else number.

2. Step 2: Apply to the argument type

   The argument { id: 123, name: "Test" } has an id property of type number, so ConditionalType<T> is string.

3. Final Answer:

   string -> Option C

4. Quick Check:

   Object with id:number means ConditionalType = string [OK]

Hint: Check if type has id:number to pick string type [OK]

Common Mistakes:

• Thinking result is number because of return 0
• Assuming union type without checking condition
• Ignoring the conditional type logic