Loops in Node.js

Learn how loops work in JavaScript: how execution flows, when loops stop, and how to choose the right loop for the job.

Introduction to Loops

A loop allows the same block of code to run multiple times. Instead of writing the same code again and again, a loop repeats it automatically based on a condition.

1. Why Loops Are Needed

Loops remove repetition.

for Loop

The for loop is used when you know how many times you want to repeat something.

1. How a for Loop Works Internally

Execution order matters.

2. Traversing an Array using for

Access elements using index.

while Loop

The while loop is used when you do not know in advance how many times the loop will run.

1. How while Loop Works

Condition-driven repetition.

do...while Loop

The do...while loop guarantees one execution, even if the condition is false.

1. do...while Execution

Body runs before condition check.

for...of Loop

for...of is the easiest way to loop over arrays and other iterables.

1. for...of with Arrays

Value-based looping.

for...in Loop

for...in is designed for objects, not arrays.

1. for...in with Objects

Key-based iteration.

break & continue

break and continue change the normal loop flow.

1. break

Exit loop immediately.

2. continue

Skip current iteration.

Guard Clauses in Loops

Guard clauses prevent deep nesting and make loops easier to read.

1. Guard Clause using continue

Skip invalid data early.

Best Practices

Choosing the correct loop improves clarity and prevents bugs.

1. Loop Selection Guidelines

Rules of thumb.

Reverse for Loop

Reverse loops are useful when removing items, countdowns, or processing data from end to start.

1. Reverse for Loop on Numbers

Count down instead of up.

do-while vs while

A do-while loop guarantees at least one execution. This behavior can be simulated using while.

1. do-while Logic using while

Manual first execution.

forEach on Arrays & Strings

forEach is available on arrays. Strings must be converted first.

1. forEach on Arrays

Callback runs once per element.

2. forEach on Strings (via split)

Strings are not directly supported.

forEach on Maps & Objects

Maps support forEach directly. Objects require conversion.

1. forEach on Map

Maps have built-in forEach.

2. forEach on Object (via Object.entries)

Objects do not support forEach directly.