Equality narrowing 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(x: string | number) {
  if (typeof x === [1]) {
    return x.toUpperCase();
  }
  return x;
}

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

A"object"

B"number"

C"string"

D"boolean"

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'string' without quotes

Checking for 'text' instead of 'string'

Using typeof x === string (missing quotes)

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

2fill in blank

medium

Complete the code to narrow the type using equality check.

Typescript

function process(value: string | null) {
  if (value [1] null) {
    return value.toLowerCase();
  }
  return "No value";
}

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

A!=

B===

C==

D!==

Attempts:

3 left

💡 Hint

Common Mistakes

Using == instead of === or !==

Calling string methods on null values

Confusing === and !== operators

Use !== for strict inequality to check if value is not null. This narrows the type to string, allowing safe use of toLowerCase().

3fill in blank

hard

Fix the error in the equality narrowing condition.

Typescript

function example(input: string | number) {
  if (typeof input [1] "string") {
    return input.toLowerCase();
  }
  return input;
}

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

A!=

B===

C!==

D==

Attempts:

3 left

💡 Hint

Common Mistakes

Using == instead of ===

Comparing value directly to "string" instead of using typeof

Using !== instead of ===

Use the strict equality operator === with typeof input to check if input is a string. This narrows the type to string.

4fill in blank

hard

Fill both blanks to correctly narrow the type and use the value.

Typescript

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

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

A===

B"string"

C"number"

D!==

Attempts:

3 left

💡 Hint

Common Mistakes

Using !== instead of ===

Using 'number' instead of 'string' in the check

Missing quotes around the type string

To check if input is a string, use typeof input === "string". This narrows the type so you can safely call string methods like toUpperCase().

5fill in blank

hard

Fill all three blanks to correctly narrow types and handle both cases.

Typescript

function format(value: string | number | null) {
  if (value [1] null) {
    if (typeof value [2] [3]) {
      return value.toUpperCase();
    }
    return value.toFixed(1);
  }
  return "No value";
}

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

A!==

B===

C"string"

D"number"

Attempts:

3 left

💡 Hint

Common Mistakes

Using == instead of !== for null check

Mixing up === and !== in type checks

Calling string methods on numbers or null

First, check that value is not null using !== null. Then check if value is a string with typeof value === "string". This lets you safely call toUpperCase(). Otherwise, assume it's a number and call toFixed(1).

Practice

1. What does equality narrowing in TypeScript help you do?

easy

A. It lets TypeScript know the exact type after checking equality.

B. It automatically converts types without checks.

C. It disables type checking for variables.

D. It only works with numbers, not strings.

Solution

1. Step 1: Understand equality narrowing concept

   Equality narrowing means TypeScript uses equality checks (like ===) to know the exact type of a variable.

2. Step 2: Compare options with definition

   Only It lets TypeScript know the exact type after checking equality. correctly describes this behavior. Others are incorrect or unrelated.

3. Final Answer:

   It lets TypeScript know the exact type after checking equality. -> Option A

4. Quick Check:

   Equality narrowing = exact type after equality check [OK]

Hint: Equality narrowing means type known after === check [OK]

Common Mistakes:

• Thinking it converts types automatically
• Believing it disables type checking
• Assuming it only works for numbers

2. Which of the following is the correct syntax to check equality for narrowing in TypeScript?

easy

A. if (value === 10) { }

B. if (value = 10) { }

C. if (value == 10) { }

D. if (value != 10) { }

Solution

1. Step 1: Identify correct equality operator for narrowing

   TypeScript uses strict equality === for safe type narrowing.

2. Step 2: Check each option

   if (value === 10) { } uses ===, which is correct. if (value == 10) { } uses loose equality, which is less safe. if (value = 10) { } is assignment, not comparison. if (value != 10) { } is inequality, not equality.

3. Final Answer:

   if (value === 10) { } -> Option A

4. Quick Check:

   Use === for equality narrowing [OK]

Hint: Use === for equality narrowing, not == or = [OK]

Common Mistakes:

• Using = instead of ===
• Using == which is not strict
• Confusing != with ===

3. What will be the output of this TypeScript code?

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

medium

A. "3.14" (error)

B. "3.14" (string)

C. "3.14"

D. "3.14" (number)

Solution

1. Step 1: Understand type narrowing in function

   The function checks if x is a string or number using typeof. For number, it calls toFixed(2).

2. Step 2: Evaluate output for input 3.14159

   Since input is number, it returns 3.14159.toFixed(2) which is string "3.14".

3. Final Answer:

   "3.14" -> Option C

4. Quick Check:

   Number input returns toFixed string "3.14" [OK]

Hint: toFixed returns string, so output is "3.14" [OK]

Common Mistakes:

• Thinking toFixed returns number
• Confusing string and number output
• Ignoring type narrowing with typeof

4. Identify the error in this TypeScript code using equality narrowing:

function process(value: string | number) {
  if (value === 'hello') {
    return value.toUpperCase();
  } else {
    return value.toUpperCase();
  }
}

medium

A. Error: should use == instead of ===.

B. No error, code is correct.

C. Error: missing else if for number type.

D. Error: calling toUpperCase on number in else block.

Solution

1. Step 1: Analyze type narrowing with equality check

   The check value === 'hello' narrows value to string only if it equals 'hello'. Else block still has value as string or number.

2. Step 2: Identify error in else block

   Calling toUpperCase() on value in else block is unsafe because value could be number, causing runtime error.

3. Final Answer:

   Error: calling toUpperCase on number in else block. -> Option D

4. Quick Check:

   toUpperCase on number causes error [OK]

Hint: Check types before calling string methods [OK]

Common Mistakes:

• Assuming else block is string only
• Using == instead of === for narrowing
• Ignoring runtime errors from wrong method calls

5. Given this TypeScript code, what is the correct way to safely handle both types using equality narrowing?

function formatValue(val: string | number | null) {
  if (val === null) {
    return "No value";
  } else if (typeof val === 'string') {
    return val.trim();
  } else {
    return val.toFixed(1);
  }
}

hard

A. Missing check for undefined, so unsafe.

B. Code is correct and safely narrows all types.

C. Should use == null instead of === null for null check.

D. toFixed should be called only after checking val is number.

Solution

1. Step 1: Check null handling

   The first if checks val === null, safely handling null case.

2. Step 2: Check type narrowing for string and number

   The else if uses typeof val === 'string' to narrow string, else block handles number with toFixed(1).

3. Step 3: Confirm all cases covered

   All possible types (null, string, number) are handled safely with proper narrowing.

4. Final Answer:

   Code is correct and safely narrows all types. -> Option B

5. Quick Check:

   All types handled safely with === and typeof [OK]

Hint: Check null first, then typeof for string, else number [OK]

Common Mistakes:

• Forgetting null check
• Using == null which includes undefined
• Calling toFixed without type check