Lesson 11 of 20

5 mins read

Standard For Loops & Iteration

In Plain English

The classic for loop consists of three expressions: initialization, condition test, and increment/step update. It provides precise, index-based iteration over numbers and collections.

The Mental Model

"A for loop is like a lap counter on a racetrack: you start at lap 1, check if you have completed 10 laps, and increment the counter at the finish line of every lap."

Deep Dive: How It Works

3-Part Header: for (let i = 0; i < limit; i++) { ... }

Scoped Index Variable: Using let i scopes the variable strictly to the loop body.

Step Control: You can increment by any step (e.g., i += 2) or decrement (i--).

Syntax Blueprint

for (initialization; condition; update) { // body }

Initialization runs once, condition is checked before each turn, and update runs after each turn.

Core Rules to Remember

Index Tracking: Provides direct access to the loop index counter i.

Block Scoping: let i is re-bound for every iteration, making it safe for closures.

Reversibility: Easy to loop backwards: for (let i = n - 1; i >= 0; i--).

Common Beginner Traps & How to Fix Them

Mistake:Off-by-one errors with <= vs <.

Why it happens: For a 5-item list, looping with i <= 5 attempts to read index 5, which is out of bounds.

How to fix: Use i < array.length for zero-indexed array boundaries.

Pro Tip: Keep loop header expressions pure and avoid calling expensive functions inside the condition check repeatedly.

Live Interactive Example

Hit Run Code to see it live

Counting and Accumulating with For Loops

JavaScript ES2024

Output Console

Click "Run Code" to view the rendered output.

How it works: The loop adds 1+2+3+4+5 sequentially to calculate 15.

Your Turn: Micro Challenge

No pressure! Edit the starter code below and test your solution with instant feedback.

Micro Exercise

Calculate product with a for loop

Calculate factorial of 4 (1 * 2 * 3 * 4 = 24).
Initialize let product = 1;
Use a for loop from i = 1 to i <= 4, multiplying product *= i.
Print "Product: 24".

Sandbox Output

Click "Run & Check" to test your solution.

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