TypeScript Strict Mode and Why It Matters - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is TypeScript strict mode?

TypeScript strict mode is a setting that makes the compiler check your code more carefully to catch more mistakes early.

beginner

Name two benefits of using TypeScript strict mode.

1. It helps find bugs before running the code.
2. It makes your code safer and easier to understand.

intermediate

What does the 'noImplicitAny' option do in strict mode?

It stops TypeScript from assuming a variable has type 'any' when you don't specify a type, forcing you to be explicit.

intermediate

How does strict mode help with null and undefined values?

Strict mode includes checks that make you handle null and undefined values explicitly, reducing runtime errors.

beginner

How do you enable strict mode in a TypeScript project?

You add "strict": true in the tsconfig.json file under the compilerOptions section.

What happens if you enable strict mode in TypeScript?

A The compiler checks your code more strictly for errors.

B The code runs faster in the browser.

C You can write JavaScript without types.

D The compiler ignores type errors.

Strict mode makes the compiler check your code more carefully to catch errors early.

Which option is part of TypeScript strict mode?

A outDir

B allowJs

C noImplicitAny

D removeComments

noImplicitAny is a strict mode option that prevents implicit 'any' types.

Why is handling null and undefined important in strict mode?

A To allow variables to be any type.

B To make the code run slower.

C To disable type checking.

D To avoid runtime errors caused by unexpected null or undefined values.

Strict mode forces you to handle null and undefined explicitly to prevent errors when the code runs.

How do you turn on strict mode in a TypeScript project?

A Set "strict": true in tsconfig.json.

B Add "use strict" at the top of every file.

C Install a special strict mode package.

D Use the --no-strict flag when compiling.

Strict mode is enabled by setting "strict": true in the tsconfig.json file.

What is a downside of strict mode?

A It makes your code run slower.

B You need to write more explicit types.

C It disables type checking.

D It removes support for JavaScript.

Strict mode requires you to be more explicit with types, which can take more time but improves code quality.

Explain what TypeScript strict mode is and why it is useful.

Think about how strict mode changes the way TypeScript checks your code.

Describe how strict mode affects handling of null and undefined values in TypeScript.

Consider what happens if you forget to check for null or undefined without strict mode.

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