TypeScript Strict Mode and Why It Matters - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to enable strict mode in a TypeScript configuration file.

Typescript

{
  "compilerOptions": {
    "[1]": true
  }
}

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

AnoImplicitAny

Bstrict

Ctarget

Dmodule

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing options that enable only some strict checks instead of all.

Confusing 'target' or 'module' with strict mode settings.

Setting "strict": true in tsconfig.json enables all strict type-checking options in TypeScript.

2fill in blank

medium

Complete the code to declare a variable that cannot be assigned null or undefined in strict mode.

Typescript

let username: string = [1];

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

Anull

Bundefined

C"Alice"

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning null or undefined to a non-nullable type.

Using a number instead of a string.

In strict mode, a variable of type string cannot be assigned null or undefined. Assigning a string literal like "Alice" is correct.

3fill in blank

hard

Fix the error by completing the function parameter type to avoid implicit any in strict mode.

Typescript

function greet(name: [1]) {
  console.log(`Hello, ${name}!`);
}

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

Astring

Bany

Cnumber

Dunknown

Attempts:

3 left

💡 Hint

Common Mistakes

Leaving the type as implicit any.

Using 'any' which disables strict type checking.

In strict mode, parameters must have explicit types. Using string for name avoids implicit any errors.

4fill in blank

hard

Fill both blanks to create a function that returns a number or undefined, using strict null checks.

Typescript

function findIndex(arr: number[], target: number): [1] {
  const index = arr.indexOf(target);
  return index [2] -1 ? index : undefined;
}

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

Anumber | undefined

B!==

C==

Dstring

Attempts:

3 left

💡 Hint

Common Mistakes

Using '==' instead of '!==' causing loose comparison.

Returning just 'number' without undefined.

The function returns number | undefined to reflect possible absence of the target. The condition uses !== to check if index is not -1.

5fill in blank

hard

Fill all three blanks to define a strict mode interface and a function using it.

Typescript

interface User {
  id: [1];
  name: [2];
  email?: [3];
}

function printUser(user: User) {
  console.log(`User: ${user.name} (ID: ${user.id})`);
}

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

Anumber

Bstring

Dboolean

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong types like boolean for id or name.

Omitting optional property syntax for email.

The id is a number, name is a string, and email is an optional string. This matches strict typing rules.

Practice

1. What is the main purpose of enabling strict mode in TypeScript?

easy

A. To allow JavaScript code to run without any type checking

B. To make TypeScript run faster by skipping type checks

C. To make TypeScript check your code more carefully and catch errors early

D. To disable all compiler warnings and errors

Solution

1. Step 1: Understand what strict mode does

   Strict mode in TypeScript enables more thorough type checking to find mistakes early.

2. Step 2: Compare options with strict mode purpose

   Only To make TypeScript check your code more carefully and catch errors early correctly describes strict mode as making TypeScript check code carefully and catch errors early.

3. Final Answer:

   To make TypeScript check your code more carefully and catch errors early -> Option C

4. Quick Check:

   Strict mode = careful checks [OK]

Hint: Strict mode means more checks, fewer bugs [OK]

Common Mistakes:

• Thinking strict mode disables checks
• Confusing strict mode with performance settings
• Believing strict mode allows ignoring types

2. Which setting in tsconfig.json enables strict mode in TypeScript?

easy

A. "strictCheck": true

B. "strict": true

C. "enableStrict": true

D. "typeCheck": true

Solution

1. Step 1: Identify the correct tsconfig.json key for strict mode

   The official key to enable strict mode is "strict" set to true.

2. Step 2: Eliminate incorrect options

   Options A, B, and C are not valid TypeScript compiler options for strict mode.

3. Final Answer:

   "strict": true -> Option B

4. Quick Check:

   Strict mode key = "strict" [OK]

Hint: Look for "strict": true in tsconfig.json [OK]

Common Mistakes:

• Using incorrect keys like "strictCheck"
• Confusing strict mode with other compiler options
• Forgetting to put the key inside tsconfig.json

3. What will happen if you run this TypeScript code with strict mode enabled?

let name: string;
console.log(name.length);

medium

A. It will cause a compile-time error because 'name' is used before being assigned

B. It will print the length of the string

C. It will print 'undefined'

D. It will run without errors and print 0

Solution

1. Step 1: Understand variable initialization in strict mode

   With strict mode, TypeScript requires variables to be initialized before use.

2. Step 2: Analyze the code behavior

   The variable 'name' is declared but not assigned a value before accessing 'name.length', causing a compile-time error.

3. Final Answer:

   It will cause a compile-time error because 'name' is used before being assigned -> Option A

4. Quick Check:

   Uninitialized variable use = error [OK]

Hint: Uninitialized variables cause errors in strict mode [OK]

Common Mistakes:

• Assuming variables default to empty string
• Thinking code runs without errors
• Confusing runtime errors with compile-time errors

4. Identify the error in this TypeScript code when strict mode is enabled and fix it:

function greet(name: string | undefined) {
  console.log(name.toUpperCase());
}

medium

A. Error: 'name' might be undefined; fix by checking if 'name' exists before calling toUpperCase()

B. Error: 'toUpperCase' is not a function; fix by casting 'name' to string

C. No error, code is correct

D. Error: Missing return type; fix by adding ': void' to function

Solution

1. Step 1: Understand strict null checks

   With strict mode, TypeScript requires checking if a variable that can be undefined is actually defined before using it.

2. Step 2: Identify the error and fix

   Calling 'toUpperCase()' on 'name' without checking if it's undefined causes an error. Fix by adding a check like 'if (name)'.

3. Final Answer:

   Error: 'name' might be undefined; fix by checking if 'name' exists before calling toUpperCase() -> Option A

4. Quick Check:

   Check undefined before method call [OK]

Hint: Check for undefined before using variable methods [OK]

Common Mistakes:

• Ignoring possible undefined values
• Trying to cast without checks
• Assuming no error without strict mode

5. You have this TypeScript code with strict mode enabled:

interface User {
  id: number;
  name?: string;
}

function printUserName(user: User) {
  console.log(user.name.toUpperCase());
}

How can you fix the code to avoid errors and safely print the user's name in uppercase if it exists?

hard

A. Assign a default value: console.log((user.name || '').toUpperCase());

B. Remove the question mark from 'name' in interface to make it required

C. Use a non-null assertion: console.log(user.name!.toUpperCase());

D. Add a check: if (user.name) console.log(user.name.toUpperCase());

Solution

1. Step 1: Understand optional properties and strict mode

   The 'name' property is optional, so it can be undefined. Strict mode requires checking before use.

2. Step 2: Evaluate options to safely access 'name'

   Add a check: if (user.name) console.log(user.name.toUpperCase()); safely checks if 'name' exists before calling 'toUpperCase()', avoiding errors. Options B, C, and D have drawbacks or risks.

3. Final Answer:

   Add a check: if (user.name) console.log(user.name.toUpperCase()); -> Option D

4. Quick Check:

   Check optional before use = safe code [OK]

Hint: Check optional properties before use in strict mode [OK]

Common Mistakes:

• Using non-null assertion without guarantee
• Assuming optional properties are always defined
• Removing optional marker without reason