Index signatures for dynamic keys in Typescript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this TypeScript code using index signatures?

Consider the following TypeScript interface and object:

interface Scores {
  [player: string]: number;
}

const gameScores: Scores = {
  alice: 10,
  bob: 15
};

console.log(gameScores['alice']);
console.log(gameScores['charlie']);

What will be printed to the console?

Typescript

interface Scores {
  [player: string]: number;
}

const gameScores: Scores = {
  alice: 10,
  bob: 15
};

console.log(gameScores['alice']);
console.log(gameScores['charlie']);

A 10 and undefined

B 10 and 0

C Error at compile time

D undefined and undefined

💡 Hint

Think about what happens when you access a key that does not exist in an object.

The index signature allows any string key with a number value. Accessing 'alice' returns 10. Accessing 'charlie' returns undefined because that key is not set.

📝 Syntax

intermediate

Which option correctly declares an index signature for dynamic keys with string values?

Choose the correct TypeScript syntax for an interface that allows any string key with a string value.

A interface Data { key: string; }

B interface Data { [key]: string; }

C interface Data { [key: number]: string; }

D interface Data { [key: string]: string; }

💡 Hint

Index signatures require a type for the key inside square brackets.

Option D correctly uses [key: string] to declare an index signature for string keys with string values.

🔧 Debug

advanced

Why does this TypeScript code cause an error?

Given this interface and object:

interface Config {
  [key: string]: number;
  port: number;
}

const serverConfig: Config = {
  port: 8080,
  host: 'localhost'
};

Why does this code cause a TypeScript error?

A Because 'host' has a string value but the index signature requires number values

B Because 'port' is missing from the object

C Because index signatures cannot be combined with explicit properties

D Because the interface is missing a string index signature

💡 Hint

Check the types of all properties against the index signature type.

The index signature says all keys must have number values. 'host' has a string value, so it violates the index signature.

❓ optimization

advanced

How to optimize a TypeScript interface with index signatures for mixed value types?

You want an interface that allows any string key with either a string or number value. Which is the best way to declare it?

A interface Mixed { [key: string]: string & number; }

B interface Mixed { [key: string]: string | number; }

C interface Mixed { [key: string]: any; }

D interface Mixed { [key: string]: string; [key: string]: number; }

💡 Hint

Consider union types for values in index signatures.

Option B uses a union type (string | number) which allows values to be either string or number for any key.

🧠 Conceptual

expert

What is a limitation of using index signatures with dynamic keys in TypeScript?

Which statement best describes a limitation when using index signatures for dynamic keys?

A Index signatures can only be used with number keys, not string keys

B Index signatures allow different value types for each key without restriction

C All explicitly declared properties must conform to the index signature's value type

D You cannot combine index signatures with any other properties in an interface

💡 Hint

Think about how explicit properties relate to the index signature type.

When an interface has an index signature, all explicitly declared properties must have values compatible with the index signature's value type.