Strict configuration objects in Typescript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a strict configuration object with a required 'url' property.

Typescript

const config: { url: string } = { url: [1] };

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

A"https://api.example.com"

B12345

Ctrue

Dnull

Attempts:

3 left

💡 Hint

Common Mistakes

Using a number or boolean instead of a string.

Forgetting to put quotes around the URL.

The 'url' property must be a string, so it needs to be enclosed in quotes.

2fill in blank

medium

Complete the code to add an optional 'timeout' property of type number to the configuration object.

Typescript

interface Config {
  url: string;
  timeout[1] number;
}

const config: Config = { url: "https://api.example.com" };

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

A?:

B? :

C?:=

D:

Attempts:

3 left

💡 Hint

Common Mistakes

Using '? :' with a space between '?' and ':'.

Using ':=' which is not valid syntax.

The optional property syntax in TypeScript uses a question mark before the colon, like timeout?: number;.

3fill in blank

hard

Fix the error in the code by completing the type for the configuration object to prevent extra properties.

Typescript

type Config = {
  url: string;
  timeout?: number;
} & [1];

const config: Config = {
  url: "https://api.example.com",
  timeout: 5000,
  retries: 3
};

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

ARecord<string, any>

Bunknown

Cnever

Dobject

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Record' allows any extra properties.

Using 'object' or 'unknown' does not prevent extra properties.

Using & never prevents extra properties by making the type impossible to extend with unknown keys.

4fill in blank

hard

Fill both blanks to create a strict configuration object type that only allows 'url' and 'timeout' properties.

Typescript

type Config = {
  url: string;
  timeout?: number;
} & { [K in keyof [1]]?: never };

const config: Config = { url: "https://api.example.com" };

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

APartial<Config>

BPick<Config, 'url' | 'timeout'>

CRecord<string, unknown>

DOmit<Config, 'url' | 'timeout'>

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Record' allows any keys.

Using 'Partial' makes all properties optional but does not forbid extras.

Using Omit gets keys not allowed, and Pick picks allowed keys to forbid others.

5fill in blank

hard

Fill all three blanks to define a strict configuration object with a required 'url', optional 'timeout', and no extra properties allowed.

Typescript

type Config = {
  url: string;
  timeout?: number;
};

const config: Config & [1] = {
  url: "https://api.example.com",
  timeout: 3000,
  [2]: 5,
  [3]: true
};

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

A{ [key: string]: never }

Bretries

Cdebug

Dextra

Attempts:

3 left

💡 Hint

Common Mistakes

Not using the intersection with '{ [key: string]: never }' to forbid extras.

Adding extra properties without errors.

The intersection with { [key: string]: never } forbids extra properties. The extra keys 'retries' and 'debug' cause errors.

Practice

1.

What is the main benefit of using strict configuration objects in TypeScript?

easy

A. They allow any keys to be added without errors.

B. They make the code run faster at runtime.

C. They catch errors early by enforcing exact keys and types.

D. They automatically generate UI forms for settings.

Solution

1. Step 1: Understand strict configuration objects

   Strict configs define exactly which keys and types are allowed in an object.

2. Step 2: Identify the main benefit

   This strictness helps catch mistakes early, before running the code.

3. Final Answer:

   They catch errors early by enforcing exact keys and types. -> Option C

4. Quick Check:

   Strict config = early error detection [OK]

Hint: Strict configs prevent wrong keys and types early [OK]

Common Mistakes:

• Thinking strict configs improve runtime speed
• Believing strict configs allow any keys
• Confusing strict configs with UI generation

2.

Which of the following is the correct way to declare a strict configuration object type in TypeScript?

interface Config {
  host: string;
  port: number;
}

easy

A. const config: Config = { host: 'localhost', port: 8080 };

B. const config: Config = { host: 'localhost' };

C. const config: Config = { host: 'localhost', port: '8080' };

D. const config = { host: 'localhost', port: 8080 };

Solution

1. Step 1: Check interface requirements

   The Config interface requires both 'host' as string and 'port' as number.

2. Step 2: Match object to interface

   const config: Config = { host: 'localhost', port: 8080 }; has both keys with correct types, so it matches Config exactly.

3. Final Answer:

   const config: Config = { host: 'localhost', port: 8080 }; -> Option A

4. Quick Check:

   All required keys with correct types = const config: Config = { host: 'localhost', port: 8080 }; [OK]

Hint: Match all keys and types exactly to interface [OK]

Common Mistakes:

• Omitting required keys
• Using wrong types for keys
• Not specifying the type annotation

3.

What will be the output of this TypeScript code?

interface Settings {
  debug: boolean;
  version: string;
}

const config: Settings = { debug: true, version: '1.0' };
console.log(config.debug, config.version);

medium

A. true 1.0

B. true '1.0'

C. undefined undefined

D. Error: Missing properties

Solution

1. Step 1: Check object matches interface

   The object has 'debug' as true (boolean) and 'version' as '1.0' (string), matching Settings.

2. Step 2: Understand console.log output

   console.log prints the boolean true and string '1.0' without quotes, so output is: true 1.0

3. Final Answer:

   true 1.0 -> Option A

4. Quick Check:

   Matching keys print their values correctly [OK]

Hint: Check object keys match interface and print values directly [OK]

Common Mistakes:

• Expecting quotes around string in console output
• Thinking missing keys cause runtime error
• Confusing type errors with runtime output

4.

Identify the error in this TypeScript code using a strict configuration object:

interface Options {
  timeout: number;
  verbose: boolean;
}

const opts: Options = { timeout: 5000, verbose: 'yes' };

medium

A. Property 'timeout' is missing.

B. Property 'verbose' should be a boolean, not a string.

C. Extra property 'verbose' is not allowed.

D. No error, code is correct.

Solution

1. Step 1: Check property types against interface

   'timeout' is number 5000, correct. 'verbose' is string 'yes', but interface requires boolean.

2. Step 2: Identify type mismatch error

   Assigning string to boolean property causes a type error.

3. Final Answer:

   Property 'verbose' should be a boolean, not a string. -> Option B

4. Quick Check:

   Type mismatch on 'verbose' property [OK]

Hint: Check each property type matches interface exactly [OK]

Common Mistakes:

• Ignoring type mismatch errors
• Thinking missing properties cause this error
• Confusing extra properties with wrong types

5.

You want to create a strict configuration object that only allows keys mode (string), retry (number), and enabled (boolean). Which TypeScript type definition ensures no extra keys are allowed?

hard

A. interface Config { mode: string; retry: number; enabled: boolean; }

B. type Config = { mode: string; retry: number; enabled: boolean; [key: string]: any }

C. type Config = { mode?: string; retry?: number; enabled?: boolean }

D. interface Config { mode: string; retry: number; enabled: boolean; [key: string]: never }

Solution

1. Step 1: Understand how to forbid extra keys

   Using an index signature with [key: string]: never forbids any keys not explicitly declared.

2. Step 2: Compare options

   interface Config { mode: string; retry: number; enabled: boolean; } allows extra keys implicitly. type Config = { mode: string; retry: number; enabled: boolean; [key: string]: any } allows any extra keys. type Config = { mode?: string; retry?: number; enabled?: boolean } makes keys optional but doesn't forbid extras. Only interface Config { mode: string; retry: number; enabled: boolean; [key: string]: never } forbids extra keys.

3. Final Answer:

   interface Config { mode: string; retry: number; enabled: boolean; [key: string]: never } -> Option D

4. Quick Check:

   Index signature with never forbids extra keys [OK]

Hint: Use [key: string]: never to forbid extra keys [OK]

Common Mistakes:

• Thinking interface alone forbids extra keys
• Using any type index signature allowing extras
• Making keys optional but not forbidding extras