Iterating over arrays in Javascript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to loop through the array and print each number.

Javascript

const numbers = [1, 2, 3, 4];
for (let [1] = 0; [1] < numbers.length; [1]++) {
  console.log(numbers[[1]]);
}

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

Ai

Bnum

Cindex

Ditem

Attempts:

3 left

💡 Hint

Common Mistakes

Using a variable name that is not consistent throughout the loop.

Starting the loop counter at 1 instead of 0.

We use i as the loop counter variable to access each index of the array.

2fill in blank

medium

Complete the code to use the forEach method to print each fruit.

Javascript

const fruits = ['apple', 'banana', 'cherry'];
fruits.forEach([1] => {
  console.log([1]);
});

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

Aitem

Bfruit

Cfruits

Dindex

Attempts:

3 left

💡 Hint

Common Mistakes

Using the plural array name as the parameter.

Using index when the element itself is needed.

The forEach method takes a function with one parameter representing each element. Here, fruit is that parameter.

3fill in blank

hard

Fix the error in the code to correctly print each element doubled.

Javascript

const nums = [1, 2, 3];
for (const [1] of nums) {
  console.log([1] * 2);
}

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

Anum

Bnums

Cindex

Di

Attempts:

3 left

💡 Hint

Common Mistakes

Using the array name instead of the element variable.

Using an index variable in a for...of loop.

Using num as the variable name for each element in the array is correct. nums is the array itself.

4fill in blank

hard

Fill both blanks to create an object mapping each word to its length for words longer than 3 letters.

Javascript

const words = ['cat', 'elephant', 'dog', 'horse'];
const lengths = {};
for (const [1] of words) {
  if ([2].length > 3) {
    lengths[[2]] = [2].length;
  }
};

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

Aword

Bwords

Attempts:

3 left

💡 Hint

Common Mistakes

Using the array name instead of the element variable.

Using different variable names in the two blanks.

We use word as the variable for each element. The object maps each word to its length.

5fill in blank

hard

Fill all three blanks to create a new array with squares of even numbers only.

Javascript

const numbers = [1, 2, 3, 4, 5];
const squares = numbers
  .filter([1] => [1] % 2 === 0)
  .map([2] => [2][3]2);

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

An

C**

D*

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names in filter and map.

Using * instead of ** for exponentiation.

We use n as the variable for each number. The filter keeps even numbers, and map squares them using the exponent operator **.

Practice

1. Which of the following is the correct way to iterate over all elements in an array arr using for...of?

easy

A. arr.forEach(item => console.log(item));

B. for (let i = 0; i < arr.length; i++) { console.log(arr[i]); }

C. for (const i in arr) { console.log(arr[i]); }

D. for (const item of arr) { console.log(item); }

Solution

1. Step 1: Understand for...of syntax

   The for...of loop directly accesses each element in the array, not the index.

2. Step 2: Identify the correct loop structure

   for (const item of arr) { console.log(item); } uses for (const item of arr), which correctly iterates over elements.

3. Final Answer:

   for (const item of arr) { console.log(item); } -> Option D

4. Quick Check:

   for...of loops over elements = C [OK]

Hint: Use for...of to get elements directly [OK]

Common Mistakes:

• Confusing for...in with for...of
• Using index instead of element in for...of
• Trying to use forEach syntax inside for...of

2. Which of the following is the correct syntax to use forEach to print each element of array nums?

easy

A. nums.forEach(function(item) { console.log(item); });

B. forEach(nums, item => console.log(item));

C. nums.forEach => console.log(item);

D. nums.forEach(item) { console.log(item); }

Solution

1. Step 1: Recall forEach method syntax

   forEach is called on the array with a function as argument: array.forEach(function(item) { ... }).

2. Step 2: Match correct syntax

   nums.forEach(function(item) { console.log(item); }); correctly uses nums.forEach(function(item) { console.log(item); }); which is valid.

3. Final Answer:

   nums.forEach(function(item) { console.log(item); }); -> Option A

4. Quick Check:

   array.forEach(function) syntax = A [OK]

Hint: Remember array.forEach(function) calls function on each item [OK]

Common Mistakes:

• Trying to call forEach as a standalone function
• Missing parentheses around function argument
• Using arrow syntax incorrectly without parentheses

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

const arr = [1, 2, 3];
let sum = 0;
for (const num of arr) {
  sum += num;
}
console.log(sum);

medium

A. 6

B. 123

C. 0

D. undefined

Solution

1. Step 1: Understand the loop and sum calculation

   The loop goes through each number in arr: 1, 2, 3, adding each to sum.

2. Step 2: Calculate the total sum

   sum = 0 + 1 + 2 + 3 = 6.

3. Final Answer:

   6 -> Option A

4. Quick Check:

   Sum of [1,2,3] = 6 [OK]

Hint: Add each element inside the loop to get total sum [OK]

Common Mistakes:

• Concatenating numbers as strings instead of adding
• Not initializing sum to 0
• Using wrong loop variable

4. Find the error in this code snippet that tries to print each element of items:

const items = ['a', 'b', 'c'];
for (let i = 0; i <= items.length; i++) {
  console.log(items[i]);
}

medium

A. The console.log statement is missing parentheses

B. The variable i should be declared with const

C. The loop condition should be i < items.length instead of i <= items.length

D. The array items is not defined

Solution

1. Step 1: Analyze the loop condition

   The loop runs while i <= items.length. Since array indices go from 0 to items.length - 1, this causes an out-of-bounds access at i = items.length.

2. Step 2: Correct the loop condition

   Change condition to i < items.length to avoid accessing undefined element.

3. Final Answer:

   The loop condition should be i < items.length instead of i <= items.length -> Option C

4. Quick Check:

   Loop index must be less than length, not less or equal [OK]

Hint: Use i < array.length to avoid out-of-range errors [OK]

Common Mistakes:

• Using <= instead of < in loop condition
• Trying to declare loop variable with const
• Ignoring array length boundaries

5. You have an array data = [0, 1, false, 2, '', 3]. You want to create a new array containing only the truthy values by iterating over data. Which code correctly does this?

hard

A. const result = data.filter(item => item == true);

B. const result = [];
data.forEach(item => { if (item) result.push(item); });

C. const result = [];
for (let i = 0; i < data.length; i++) { result.push(data[i]); }

D. const result = data.map(item => item ? item : null);

Solution

1. Step 1: Understand truthy filtering

   We want to keep only values that are truthy (not 0, false, '', null, undefined).

2. Step 2: Check each option

   const result = []; data.forEach(item => { if (item) result.push(item); }); uses forEach to push only truthy items, which is correct. const result = data.filter(item => item == true); uses == true which misses some truthy values like numbers. const result = []; for (let i = 0; i < data.length; i++) { result.push(data[i]); } copies all items without filtering. const result = data.map(item => item ? item : null); replaces falsy with null, not filtering.

3. Final Answer:

   const result = []; data.forEach(item => { if (item) result.push(item); }); -> Option B

4. Quick Check:

   Use forEach with if (item) to filter truthy [OK]

Hint: Use forEach with if (item) to keep truthy values [OK]

Common Mistakes:

• Using == true instead of checking truthiness
• Copying all elements without filtering
• Using map instead of filter or manual push