Generic conditional constraints in Typescript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a generic function that accepts a parameter of any type.

Typescript

function identity<T>([1]: T): T {
  return param;
}

Drag options to blanks, or click blank then click option'

Aparam

Bvalue

Citem

Darg

Attempts:

3 left

💡 Hint

Common Mistakes

Using a parameter name different from the one returned causes an error.

Forgetting to specify the parameter type T.

The generic function uses a parameter named 'param' of type T to return the same value.

2fill in blank

medium

Complete the code to constrain the generic type to only types that have a 'length' property.

Typescript

function logLength<T extends [1]>(item: T): number {
  return item.length;
}

Drag options to blanks, or click blank then click option'

A{ length: number }

Bstring

Cnumber

Dany

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'string' as constraint limits to string only, not all types with length.

Using 'any' removes the constraint.

The generic type T is constrained to types that have a 'length' property of type number.

3fill in blank

hard

Fix the error in the generic function that requires T to have a 'toString' method.

Typescript

function stringify<T extends [1]>(value: T): string {
  return value.toString();
}

Drag options to blanks, or click blank then click option'

Aobject

Bstring

C{ toString: () => string }

Dany

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'object' does not guarantee 'toString' method signature.

Using 'string' as constraint is incorrect here.

The constraint requires T to have a 'toString' method returning a string.

4fill in blank

hard

Fill both blanks to create a generic function that accepts an object with a key K and returns its value.

Typescript

function getProperty<T, K extends keyof T>(obj: T, key: [1]): T[[2]] {
  return obj[key];
}

Drag options to blanks, or click blank then click option'

AK

Bstring

Ckeyof T

DT

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'string' or 'keyof T' directly in the parameter type causes errors.

Mismatching the key type and return type.

The key parameter is of type K which extends keys of T, and the return type is the value type at key K.

5fill in blank

hard

Fill all three blanks to define a generic function that returns the length of a property K of object T, where that property has a length.

Typescript

function getLength<T, K extends keyof T>(obj: T, key: K): number {
  const value = obj[[1]];
  if (value && typeof value === [2] && 'length' in value) {
    return value.length[3];
  }
  return 0;
}

Drag options to blanks, or click blank then click option'

Akey

B'object'

C?? 0

D'string'

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'string' instead of 'object' in typeof check.

Not using nullish coalescing operator causes runtime errors if length is undefined.

The function accesses obj[key], checks if it's an object with a length property, and returns length or 0 if not present.

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