Type checking using typeof in Javascript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to check the type of variable value.

Javascript

console.log(typeof [1]);

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

Avalue

B"value"

Ctypeof value

Dtypeof

Attempts:

3 left

💡 Hint

Common Mistakes

Putting the variable name inside quotes, which makes it a string literal.

Using typeof inside typeof like a function call.

The typeof operator returns the type of the variable. You need to pass the variable name value to it.

2fill in blank

medium

Complete the code to check if input is a string.

Javascript

if (typeof input === [1]) {
  console.log("It's a string!");
}

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

Astring

B"string"

C'string'

D"String"

Attempts:

3 left

💡 Hint

Common Mistakes

Using the type name without quotes, causing a ReferenceError.

Using uppercase letters in the type string.

The typeof operator returns a string describing the type, so you must compare it to the string "string" (all lowercase).

3fill in blank

hard

Fix the error in the code to correctly check if num is a number.

Javascript

if (typeof num === [1]) {
  console.log("It's a number!");
}

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

A"number"

B== "number"

C=== "number"

D"Number"

Attempts:

3 left

💡 Hint

Common Mistakes

Using a single equals sign = instead of ===.

Using uppercase letters in the type string.

Use the strict equality operator === to compare the result of typeof with the string "number". The original code used a single equals sign =, which is an assignment, not a comparison.

4fill in blank

hard

Fill both blanks to create an object with keys as variable names and values as their types.

Javascript

const types = {
  name: [1],
  age: [2]
};

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

Atypeof name

Btypeof age

C"string"

D"number"

Attempts:

3 left

💡 Hint

Common Mistakes

Putting quotes around the typeof expressions, making them strings.

Using the type strings directly instead of typeof expressions.

To get the type of variables name and age, use typeof name and typeof age respectively.

5fill in blank

hard

Fill all three blanks to create a function that returns true if val is a boolean.

Javascript

function isBoolean(val) {
  return typeof [1] [2] [3];
}

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

Aval

B===

C"boolean"

Dval === "boolean"

Attempts:

3 left

💡 Hint

Common Mistakes

Using assignment = instead of comparison ===.

Comparing to boolean without quotes.

Putting the whole expression inside quotes.

The function checks if the type of val is exactly the string "boolean" using typeof val === "boolean".

Practice

1. What does the typeof operator return when used on the value 42?

easy

A. "string"

B. "object"

C. "boolean"

D. "number"

Solution

1. Step 1: Understand the value type

   The value 42 is a number in JavaScript.

2. Step 2: Apply typeof operator

   Using typeof 42 returns the string "number" because 42 is a numeric value.

3. Final Answer:

   "number" -> Option D

4. Quick Check:

   typeof 42 = "number" [OK]

Hint: Remember: numbers return "number" with typeof [OK]

Common Mistakes:

• Thinking numbers return "string"
• Confusing number with object
• Using typeof without parentheses

2. Which of the following is the correct syntax to check if variable x is a string using typeof?

easy

A. if (typeof x = 'string') {}

B. if (typeof x === 'string') {}

C. if (typeof(x) == string) {}

D. if (typeof x === string) {}

Solution

1. Step 1: Check correct operator usage

   The strict equality operator is ===, so typeof x === 'string' is correct.

2. Step 2: Check string quotes and parentheses

   The type name must be a string literal in quotes, and parentheses around x are optional.

3. Final Answer:

   if (typeof x === 'string') {} -> Option B

4. Quick Check:

   Use === 'string' with quotes [OK]

Hint: Use triple equals and quotes around type name [OK]

Common Mistakes:

• Using single equals (=) instead of triple (===)
• Missing quotes around 'string'
• Using typeof without quotes for type

3. What will be the output of the following code?

console.log(typeof null);

medium

A. "null"

B. "undefined"

C. "object"

D. "boolean"

Solution

1. Step 1: Understand the value null

   In JavaScript, null is a special value representing "no object".

2. Step 2: Apply typeof to null

   Due to a historical bug, typeof null returns "object" instead of "null".

3. Final Answer:

   "object" -> Option C

4. Quick Check:

   typeof null = "object" [OK]

Hint: Remember: typeof null returns "object" (quirk) [OK]

Common Mistakes:

• Expecting "null" as output
• Confusing with "undefined"
• Thinking null is a boolean

4. Find the error in this code snippet:

let val = 10;
if (typeof val = "number") {
  console.log("It's a number");
}

medium

A. Using single equals (=) instead of comparison operator

B. Missing semicolon after let declaration

C. Incorrect quotes around "number"

D. typeof cannot be used on variables

Solution

1. Step 1: Check the if condition syntax

   The condition uses = which is assignment, not comparison.

2. Step 2: Correct operator for comparison

   Use === or == to compare the result of typeof val with "number".

3. Final Answer:

   Using single equals (=) instead of comparison operator -> Option A

4. Quick Check:

   Use === for comparison, not = [OK]

Hint: Use === for comparison, not = [OK]

Common Mistakes:

• Using = instead of === in if conditions
• Thinking typeof returns variable type directly
• Ignoring quotes around type string

5. You want to write a function isStringOrNumber that returns true if the input is either a string or a number, and false otherwise. Which of the following implementations is correct?

hard

A. function isStringOrNumber(value) { return typeof value === 'string' || typeof value === 'number'; }

B. function isStringOrNumber(value) { return typeof value === 'string' && typeof value === 'number'; }

C. function isStringOrNumber(value) { return typeof value == 'string' && typeof value == 'number'; }

D. function isStringOrNumber(value) { return typeof value = 'string' || typeof value = 'number'; }

Solution

1. Step 1: Understand the requirement

   The function should return true if the value is a string OR a number.

2. Step 2: Check logical operators and comparisons

   function isStringOrNumber(value) { return typeof value === 'string' || typeof value === 'number'; } uses || (OR) and strict equality ===, which is correct.

3. Step 3: Identify errors in other options

   function isStringOrNumber(value) { return typeof value === 'string' && typeof value === 'number'; } and the option using == uses AND &&, which requires both types simultaneously (impossible). function isStringOrNumber(value) { return typeof value = 'string' || typeof value = 'number'; } uses assignment = instead of comparison.

4. Final Answer:

   function isStringOrNumber(value) { return typeof value === 'string' || typeof value === 'number'; } -> Option A

5. Quick Check:

   Use OR (||) and === for type checks [OK]

Hint: Use || for OR and === for type checks [OK]

Common Mistakes:

• Using && instead of || for OR condition
• Using = instead of === for comparison
• Mixing == and === inconsistently