Arrow functions in Javascript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create an arrow function that adds two numbers.

Javascript

const add = (a, b) => [1];

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

Aa * b

Ba + b

Ca - b

Da / b

Attempts:

3 left

💡 Hint

Common Mistakes

Using subtraction or multiplication instead of addition.

Forgetting to include both parameters.

The arrow function should return the sum of a and b, so use 'a + b'.

2fill in blank

medium

Complete the code to create an arrow function that returns the square of a number.

Javascript

const square = x => [1];

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

Ax * x

Bx + x

Cx / 2

Dx - 1

Attempts:

3 left

💡 Hint

Common Mistakes

Adding the number to itself instead of multiplying.

Dividing or subtracting instead of multiplying.

To get the square, multiply the number by itself using 'x * x'.

3fill in blank

hard

Fix the error in the arrow function that returns the length of a string.

Javascript

const length = str => [1];

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

Astr.length

Bstr.length()

Clength(str)

Dstr.size

Attempts:

3 left

💡 Hint

Common Mistakes

Using parentheses after length like a function call.

Using incorrect property names like size.

In JavaScript, string length is accessed as a property without parentheses: 'str.length'.

4fill in blank

hard

Fill both blanks to create an arrow function that filters an array for numbers greater than 10.

Javascript

const filterLarge = arr => arr.filter(num => num [1] [2]);

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

A>

B10

C<

D5

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' instead of '>' for the comparison.

Using the wrong number for comparison.

To filter numbers greater than 10, use 'num > 10' inside the filter.

5fill in blank

hard

Fill all three blanks to create an arrow function that maps an array to uppercase strings and filters those starting with 'A'.

Javascript

const process = arr => arr.map(str => str.[1]()).filter(str => str.[2](0) === [3]);

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

AtoUpperCase

BcharAt

C'A'

DtoLowerCase

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'toLowerCase' instead of 'toUpperCase'.

Using incorrect methods to get the first character.

Comparing to the wrong character.

Use 'toUpperCase()' to convert strings, 'charAt(0)' to get first letter, and compare to 'A'.

Practice

1. What is the main advantage of using arrow functions in JavaScript?

easy

A. They replace all regular functions in JavaScript.

B. They provide a shorter syntax for writing functions.

C. They can only be used for asynchronous code.

D. They automatically create new objects.

Solution

1. Step 1: Understand arrow function purpose

   Arrow functions are designed to write functions in a shorter way compared to regular function expressions.

2. Step 2: Compare options

   Only They provide a shorter syntax for writing functions. correctly states the main advantage: shorter syntax. Other options are incorrect or misleading.

3. Final Answer:

   They provide a shorter syntax for writing functions. -> Option B

4. Quick Check:

   Arrow functions = shorter syntax [OK]

Hint: Arrow functions are mainly about shorter syntax [OK]

Common Mistakes:

• Thinking arrow functions create new objects automatically
• Believing arrow functions only work with async code
• Assuming arrow functions replace all regular functions

2. Which of the following is the correct syntax for an arrow function that adds two numbers a and b?

easy

A. const add = (a, b) => { a + b; };

B. const add = (a, b) { return a + b; };

C. const add = a, b => a + b;

D. const add = (a, b) => a + b;

Solution

1. Step 1: Check arrow function syntax

   Arrow functions can omit braces and return keyword for single expressions, so (a, b) => a + b is valid.

2. Step 2: Analyze options

   A: concise and correct with implicit return. B: missing => operator, syntax error. C: misses parentheses around parameters. D: lacks return statement inside braces.

3. Final Answer:

   const add = (a, b) => a + b; -> Option D

4. Quick Check:

   Concise arrow function = (a, b) => a + b [OK]

Hint: Use parentheses for multiple params and omit braces for single expression [OK]

Common Mistakes:

• Omitting parentheses when multiple parameters exist
• Forgetting to return value inside braces
• Using commas incorrectly in parameter list

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

const numbers = [1, 2, 3];
const squares = numbers.map(n => n * n);
console.log(squares);

medium

A. [1, 4, 9]

B. [2, 4, 6]

C. [1, 2, 3]

D. Error

Solution

1. Step 1: Understand map with arrow function

   The map method applies the arrow function n => n * n to each element, squaring each number.

2. Step 2: Calculate output array

   Input array is [1, 2, 3]. Squared values are [1*1, 2*2, 3*3] = [1, 4, 9].

3. Final Answer:

   [1, 4, 9] -> Option A

4. Quick Check:

   map squares each element = [1, 4, 9] [OK]

Hint: map with n => n*n squares each number [OK]

Common Mistakes:

• Confusing map output with original array
• Expecting map to modify array in place
• Misunderstanding arrow function return behavior

4. Identify the error in the following arrow function code:

const greet = name => {
  console.log('Hello, ' + name);
};
greet();

medium

A. Missing parentheses around parameter name.

B. Arrow functions cannot have multiple statements inside braces.

C. Calling greet without an argument causes name to be undefined.

D. The function should return a value instead of logging.

Solution

1. Step 1: Check parameter syntax

   Single parameter can omit parentheses, so syntax is correct.

2. Step 2: Analyze function call

   Calling greet() without argument means name is undefined, so output will be 'Hello, undefined'. This is likely unintended.

3. Final Answer:

   Calling greet without an argument causes name to be undefined. -> Option C

4. Quick Check:

   Missing argument = undefined parameter [OK]

Hint: Check if function parameters get proper arguments when called [OK]

Common Mistakes:

• Thinking parentheses are required for single parameter
• Expecting arrow functions to auto-fill missing arguments
• Confusing logging with returning values

5. Consider this code snippet:

function Counter() {
  this.count = 0;
  setInterval(() => {
    this.count++;
    console.log(this.count);
  }, 1000);
}
const c = new Counter();

What is the main reason the arrow function inside setInterval works correctly to update this.count?

hard

A. Arrow functions do not have their own this, so they use this from the surrounding context.

B. Arrow functions automatically bind this to the global object.

C. Arrow functions create a new this for each call.

D. Arrow functions cannot access this at all.

Solution

1. Step 1: Understand this in arrow functions

   Arrow functions do not have their own this. Instead, they use this from the surrounding function or object.

2. Step 2: Apply to Counter example

   Inside setInterval, the arrow function uses this from Counter instance, so this.count correctly refers to the instance property.

3. Final Answer:

   Arrow functions do not have their own this, so they use this from the surrounding context. -> Option A

4. Quick Check:

   Arrow function this = surrounding this [OK]

Hint: Arrow functions inherit this from their surrounding scope [OK]

Common Mistakes:

• Assuming arrow functions bind this to global object
• Thinking arrow functions create new this each call
• Believing arrow functions cannot access this