Control flow analysis behavior in Typescript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to check if the variable is a string.

Typescript

function checkType(value: unknown) {
  if (typeof value === [1]) {
    return true;
  }
  return false;
}

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

A"number"

B"string"

C"boolean"

D"object"

Attempts:

3 left

💡 Hint

Common Mistakes

Using the type name without quotes.

Checking for 'String' instead of 'string'.

The typeof operator returns a string indicating the type. To check if value is a string, compare it to "string".

2fill in blank

medium

Complete the code to narrow the type using a type guard.

Typescript

function process(value: string | number) {
  if (typeof value === [1]) {
    return value.toUpperCase();
  }
  return value.toFixed(2);
}

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

A"string"

B"number"

C"boolean"

D"object"

Attempts:

3 left

💡 Hint

Common Mistakes

Checking for 'number' instead of 'string'.

Calling string methods on number types.

To call toUpperCase(), value must be a string. So check if typeof value === "string".

3fill in blank

hard

Fix the error by completing the code to correctly narrow the type.

Typescript

function example(value: string | null) {
  if (value [1] null) {
    return value.length;
  }
  return 0;
}

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

A!==

B!=

C===

D==

Attempts:

3 left

💡 Hint

Common Mistakes

Using == which allows type coercion.

Checking for equality instead of inequality.

To safely access value.length, ensure value is not null. Use !== null to check this.

4fill in blank

hard

Fill both blanks to create a type guard that checks for an array of numbers.

Typescript

function isNumberArray(value: unknown): value is number[] {
  return Array.isArray(value) && value.every(item => typeof item [1] [2]);
}

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

A===

B"number"

C==

D"string"

Attempts:

3 left

💡 Hint

Common Mistakes

Using == instead of ===.

Comparing to "string" instead of "number".

Use === to strictly compare typeof item with "number" to confirm all items are numbers.

5fill in blank

hard

Fill all three blanks to create a function that narrows a union type using control flow.

Typescript

function formatValue(value: string | number | boolean) {
  switch (typeof value) {
    case [1]:
      return value.toUpperCase();
    case [2]:
      return value.toFixed(1);
    case [3]:
      return value ? "Yes" : "No";
  }
}

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

A"string"

B"number"

C"boolean"

D"object"

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect type strings like "object".

Mixing up the order of cases.

The typeof operator returns "string", "number", or "boolean" for these types. Use these in the switch cases to narrow the type and call appropriate methods.

Practice

1.

What does TypeScript's control flow analysis primarily help with?

easy

A. Running the program faster by optimizing loops

B. Tracking variable types through code paths to catch errors early

C. Automatically fixing syntax errors in the code

D. Converting TypeScript code into JavaScript

Solution

1. Step 1: Understand control flow analysis purpose

   Control flow analysis tracks how variables change types through different code paths.

2. Step 2: Identify its main benefit

   This helps TypeScript catch type errors before running the program.

3. Final Answer:

   Tracking variable types through code paths to catch errors early -> Option B

4. Quick Check:

   Control flow analysis = type tracking [OK]

Hint: Control flow = tracking variable types in code paths [OK]

Common Mistakes:

• Confusing control flow analysis with code execution speed
• Thinking it fixes syntax errors automatically
• Believing it compiles TypeScript to JavaScript

2.

Which of the following TypeScript code snippets correctly narrows a variable's type using control flow analysis?

let value: string | number = 5;
if (typeof value === 'string') {
  console.log(value.toUpperCase());
}

easy

A. The code should use instanceof instead of typeof

B. The code will cause a runtime error because value can be a number

C. The code needs a type assertion to call toUpperCase()

D. The code is correct and safe to call toUpperCase() inside the if block

Solution

1. Step 1: Check the type guard condition

   The condition typeof value === 'string' narrows value to string inside the if block.

2. Step 2: Confirm safe method call

   Since value is string here, calling toUpperCase() is safe and valid.

3. Final Answer:

   The code is correct and safe to call toUpperCase() inside the if block -> Option D

4. Quick Check:

   Type guard narrows type = safe method call [OK]

Hint: Use typeof checks to narrow union types safely [OK]

Common Mistakes:

• Calling string methods without type narrowing
• Confusing typeof with instanceof for primitives
• Assuming type assertions are always needed

3.

What will be the output of this TypeScript code?

function checkValue(x: string | number) {
  if (typeof x === 'number') {
    return x.toFixed(2);
  } else {
    return x.toUpperCase();
  }
}
console.log(checkValue('hello'));
console.log(checkValue(3.14159));

medium

A. Runtime error because toFixed is not a function

B. "hello" and "3.14159"

C. "HELLO" and "3.14"

D. "HELLO" and "3.14159"

Solution

1. Step 1: Analyze the first call with string

   For 'hello', the else block runs, returning 'HELLO' after toUpperCase().

2. Step 2: Analyze the second call with number

   For 3.14159, the if block runs, returning '3.14' after toFixed(2).

3. Final Answer:

   "HELLO" and "3.14" -> Option C

4. Quick Check:

   Type guards produce correct string outputs [OK]

Hint: Check typeof to predict which block runs [OK]

Common Mistakes:

• Assuming toFixed returns a number, not string
• Ignoring type narrowing in if-else
• Mixing up output formatting

4.

Identify the error in this TypeScript code related to control flow analysis:

function process(input: string | null) {
  if (input) {
    console.log(input.toLowerCase());
  } else {
    console.log(input.toLowerCase());
  }
}

medium

A. Calling toLowerCase() on null in the else block causes a runtime error

B. The if condition should be input !== null instead of input

C. TypeScript will automatically fix this error during compilation

D. There is no error; the code is safe

Solution

1. Step 1: Check the else block usage

   In the else block, input is null, so calling toLowerCase() causes a runtime error.

2. Step 2: Understand control flow limitation

   Control flow analysis narrows input in the if block but not in else, so unsafe call happens.

3. Final Answer:

   Calling toLowerCase() on null in the else block causes a runtime error -> Option A

4. Quick Check:

   Null check needed before method call [OK]

Hint: Else block may have null; avoid method calls there [OK]

Common Mistakes:

• Assuming if(input) excludes null everywhere
• Expecting TypeScript to fix runtime errors
• Using loose null checks without safe calls

5.

Given this TypeScript code, what is the best way to fix the control flow analysis issue to avoid errors?

function formatInput(input: string | number | null) {
  if (typeof input === 'string') {
    return input.trim();
  } else if (typeof input === 'number') {
    return input.toFixed(1);
  }
  return input.trim();
}

hard

A. Add a null check before the last return to avoid calling trim() on null

B. Remove the last return statement because it's unreachable

C. Change all trim() calls to toString()

D. Use a type assertion to force input as string in the last return

Solution

1. Step 1: Identify the problem with last return

   The last return input.trim() runs when input is null, causing an error.

2. Step 2: Fix by adding null check

   Adding a check like if (input !== null) before calling trim() prevents errors.

3. Final Answer:

   Add a null check before the last return to avoid calling trim() on null -> Option A

4. Quick Check:

   Null checks prevent unsafe method calls [OK]

Hint: Always check for null before calling methods on unions [OK]

Common Mistakes:

• Ignoring null possibility in union types
• Assuming last return is unreachable
• Using type assertions without safety