Why built-in methods are useful in Javascript - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to convert the string to uppercase using a built-in method.

Javascript

const word = 'hello';
const upperWord = word.[1]();
console.log(upperWord);

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

AtoLowerCase

BtoUpperCase

Ctrim

Dsplit

Attempts:

3 left

💡 Hint

Common Mistakes

Using toLowerCase() instead of toUpperCase()

Trying to use a method that doesn't change case

The toUpperCase() method converts all letters in a string to uppercase.

2fill in blank

medium

Complete the code to find the length of the array using a built-in property.

Javascript

const fruits = ['apple', 'banana', 'cherry'];
const count = fruits[1];
console.log(count);

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

A.length()

B.size

C.length

D.count

Attempts:

3 left

💡 Hint

Common Mistakes

Using .length() with parentheses

Using .size or .count which are not valid for arrays

The length property gives the number of items in an array.

3fill in blank

hard

Fix the error in the code to join array elements into a string separated by commas.

Javascript

const colors = ['red', 'green', 'blue'];
const result = colors.[1](',');
console.log(result);

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

Ajoin

Bsplit

Cconcat

Dpush

Attempts:

3 left

💡 Hint

Common Mistakes

Using split() which breaks strings into arrays

Using concat() which merges arrays but not with separators

The join() method combines array elements into a string with a specified separator.

4fill in blank

hard

Fill both blanks to create an object with keys as words and values as their lengths, only for words longer than 3 letters.

Javascript

const words = ['cat', 'elephant', 'dog', 'lion'];
const lengths = Object.fromEntries(words.filter(word => word.length > 3).map(word => [[1], [2]]));
console.log(lengths);

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

Aword

Bword.length

Cword.length()

Dword.toString()

Attempts:

3 left

💡 Hint

Common Mistakes

Using word.length() which is not a function

Using word.toString() which returns the word itself

Use word as the key and word.length as the value. word.length is a property, not a function.

5fill in blank

hard

Fill all three blanks to create a new array with uppercase words that have length greater than 3.

Javascript

const words = ['sun', 'moon', 'star', 'sky'];
const result = words.filter(word => word.[1] > [2]).map(word => word.[3]());
console.log(result);

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

Alength

B3

CtoUpperCase

DtoLowerCase

Attempts:

3 left

💡 Hint

Common Mistakes

Using length() instead of length

Using toLowerCase() instead of toUpperCase()

Filter words with length greater than 3, then convert each to uppercase using toUpperCase().

Practice

1. Why are built-in methods useful in JavaScript?

easy

A. They require writing more code manually.

B. They make the code run slower.

C. They save time by performing common tasks automatically.

D. They only work with numbers.

Solution

1. Step 1: Understand the purpose of built-in methods

   Built-in methods are pre-made functions in JavaScript that help perform common tasks easily.

2. Step 2: Identify the benefit of using them

   They save time and make code cleaner by doing tasks automatically without extra code.

3. Final Answer:

   They save time by performing common tasks automatically. -> Option C

4. Quick Check:

   Built-in methods save time = C [OK]

Hint: Built-in methods do common tasks fast and easy [OK]

Common Mistakes:

• Thinking built-in methods slow down code
• Believing built-in methods need more manual code
• Assuming built-in methods only work with numbers

2. Which of the following is the correct way to use the built-in method toUpperCase() on a string text?

easy

A. text.toUpperCase()

B. toUpperCase(text)

C. text.to_upper_case()

D. text.upperCase()

Solution

1. Step 1: Recall the syntax of string methods

   In JavaScript, string methods are called on the string variable using dot notation, like variable.method().

2. Step 2: Match the correct method name and usage

   The correct method name is toUpperCase(), so calling text.toUpperCase() is correct.

3. Final Answer:

   text.toUpperCase() -> Option A

4. Quick Check:

   String method syntax = A [OK]

Hint: Use dot notation with exact method name and parentheses [OK]

Common Mistakes:

• Using function style call like toUpperCase(text)
• Writing method name with underscores
• Missing parentheses after method name

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

const fruits = ['apple', 'banana', 'cherry'];
const result = fruits.includes('banana');
console.log(result);

medium

A. false

B. Error

C. undefined

D. true

Solution

1. Step 1: Understand what includes() does on arrays

   The includes() method checks if an array contains a specific value and returns true or false.

2. Step 2: Check if 'banana' is in the array

   The array fruits contains 'banana', so fruits.includes('banana') returns true.

3. Final Answer:

   true -> Option D

4. Quick Check:

   Array includes 'banana' = true [OK]

Hint: includes() returns true if item is found in array [OK]

Common Mistakes:

• Confusing includes() with indexOf() returning index
• Expecting string output instead of boolean
• Mistyping method name

4. Find the error in this code that tries to convert a string to lowercase:

let name = 'Alice';
let lower = name.lowercase();
console.log(lower);

medium

A. Strings cannot be converted to lowercase.

B. The method should be toLowerCase(), not lowercase().

C. Missing semicolon after console.log.

D. The variable name is not declared.

Solution

1. Step 1: Check the method name for converting to lowercase

   The correct method is toLowerCase(), not lowercase().

2. Step 2: Identify the cause of the error

   Calling name.lowercase() causes a TypeError because the method does not exist.

3. Final Answer:

   The method should be toLowerCase(), not lowercase(). -> Option B

4. Quick Check:

   Correct method name = D [OK]

Hint: Use exact method names with correct capitalization [OK]

Common Mistakes:

• Using lowercase() instead of toLowerCase()
• Ignoring case sensitivity in method names
• Assuming semicolon errors cause runtime failure

5. You have an array of numbers: [1, 2, 3, 4, 5]. You want to create a new array with each number doubled using a built-in method. Which code correctly does this?

hard

A. const doubled = numbers.map(n => n * 2);

B. const doubled = numbers.forEach(n => n * 2);

C. const doubled = numbers.filter(n => n * 2);

D. const doubled = numbers.reduce(n => n * 2);

Solution

1. Step 1: Understand the purpose of each method

   map() creates a new array by applying a function to each element. forEach() runs a function but returns undefined. filter() selects elements based on a condition. reduce() combines elements into a single value.

2. Step 2: Choose the method that returns a new array with doubled values

   map() is the correct method to transform each element and return a new array.

3. Final Answer:

   const doubled = numbers.map(n => n * 2); -> Option A

4. Quick Check:

   Use map() to transform arrays = A [OK]

Hint: Use map() to create new arrays from existing ones [OK]

Common Mistakes:

• Using forEach() expecting a returned array
• Using filter() to change values instead of select
• Using reduce() without proper accumulator function