Iterating over arrays in Javascript

Introduction

We use iteration to look at each item in a list one by one. This helps us do something with every item, like printing or changing it.

You want to print all names in a list to the screen.

You need to add 1 to every number in a list of scores.

You want to find if a list has a certain word.

You want to create a new list with only some items from the original list.

Syntax

Javascript

const array = [item1, item2, item3];

// Using for loop
for (let index = 0; index < array.length; index++) {
  const currentItem = array[index];
  // do something with currentItem
}

// Using for...of loop
for (const item of array) {
  // do something with item
}

// Using forEach method
array.forEach((item, index) => {
  // do something with item
});

The for loop gives you the index and item.

The for...of loop is simpler when you only need the item.

Examples

This prints each fruit using a classic for loop.

Javascript

const fruits = ['apple', 'banana', 'cherry'];

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

This prints each fruit using a for...of loop, which is easier to read.

Javascript

const fruits = ['apple', 'banana', 'cherry'];

for (const fruit of fruits) {
  console.log(fruit);
}

This prints each fruit with its index using the forEach method.

Javascript

const fruits = ['apple', 'banana', 'cherry'];

fruits.forEach((fruit, index) => {
  console.log(`${index}: ${fruit}`);
});

Shows that loops do nothing if the array is empty.

Javascript

const emptyArray = [];

for (const item of emptyArray) {
  console.log(item);
}
// No output because the array is empty

Sample Program

This program shows three ways to go through an array and print each color with its position.

Javascript

const colors = ['red', 'green', 'blue'];

console.log('Colors before iteration:');
console.log(colors);

console.log('\nIterating with for loop:');
for (let index = 0; index < colors.length; index++) {
  console.log(`Color at index ${index} is ${colors[index]}`);
}

console.log('\nIterating with for...of loop:');
for (const color of colors) {
  console.log(`Color: ${color}`);
}

console.log('\nIterating with forEach method:');
colors.forEach((color, index) => {
  console.log(`Index ${index}: ${color}`);
});

Important Notes

Time complexity is O(n) because you look at each item once.

Space complexity is O(1) if you only read items, no extra storage.

Common mistake: Changing the array length while looping can cause errors.

Use forEach when you want simple code and don't need to break early.

Use for loop if you need the index or want to stop early with break.

Summary

Iteration lets you do something with every item in an array.

Use for, for...of, or forEach depending on your needs.

Remember to handle empty arrays safely.

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