Iterating over arrays in Javascript - Time & Space Complexity

Time Complexity: Iterating over arrays

O(n)

Understanding Time Complexity

When we go through each item in an array one by one, we want to know how the time it takes grows as the array gets bigger.

We ask: How does the work change when the array has more elements?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

const numbers = [1, 2, 3, 4, 5];
let sum = 0;
for (let i = 0; i < numbers.length; i++) {
  sum += numbers[i];
}
console.log(sum);
    

This code adds up all the numbers in the array by visiting each element once.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Accessing each element of the array and adding it to sum.
• How many times: Exactly once for each element in the array.

How Execution Grows With Input

As the array gets bigger, the number of steps grows directly with the number of elements.

Input Size (n)	Approx. Operations
10	10 additions
100	100 additions
1000	1000 additions

Pattern observation: Doubling the array size doubles the work needed.

Final Time Complexity

Time Complexity: O(n)

This means the time to finish grows in a straight line with the number of items in the array.

Common Mistake

[X] Wrong: "The loop runs faster because it just adds numbers, so it's constant time."

[OK] Correct: Even simple steps add up when repeated many times; the total time depends on how many elements there are.

Interview Connect

Understanding how looping through arrays scales helps you explain your code clearly and shows you know how programs behave with bigger data.

Self-Check

"What if we used two nested loops to compare every pair of elements? How would the time complexity change?"

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