Reverse Repeating-Letter Alphabet Triangle in JavaScript

Beginner

⏱️ 8 min read

📚 Updated: Aug 2026

🎯 3 Code Examples

🚀 Live Preview

Nested Loops

Reverse repeating triangle E, DD, CCC, BBBB, AAAAA in JavaScript programming

Overview

What You’ll Learn

Same repeating idea as Program 9, but letters run E → D → C → B → A while row widths still grow 1, 2, 3, 4, 5. This tutorial covers the shape rule, countdown loops, a live preview, worked JavaScript examples, edge cases, and complexity.

Shape Rule

Repeat letter, grow width

Row 1 prints E, row 2 prints DD, up to AAAAA on the last line.

Outer Loop

Letter countdown

for (let i = "E".charCodeAt(0); i >= "A".charCodeAt(0); i--) picks the letter for each row.

Inner Loop

Repeat count

for (let j = "E".charCodeAt(0); j >= i; j--) runs 1, 2, 3… times - print String.fromCharCode(i), not the inner counter.

line vs console.log

Same line / next line

Letters use line += ch; end each row with console.log(line).

Live Preview

1–26 rows

Pick a row count and draw the reverse repeating triangle in the browser.

O(n²)

Complexity

Total letters = n(n+1)/2; extra memory stays O(1).

Introduction

A reverse repeating alphabet triangle grows by one repeated letter on each new line, counting letters downward from the top of the alphabet range. With the right angle on the left, the console shows a staircase of identical letters per row.

In JavaScript you usually solve it with two nested for loops: the outer loop picks the row letter (counting down), the inner loop prints that same letter the right number of times, then console.log(line) moves to the next line.

Why it matters?

It locks in the difference between “which letter” (outer loop) and “how many times” (inner loop). Once that clicks, forward repeats, pyramids, and letter-countdown variants become much easier.

Key Highlights

Letters Count Down

Rows use E, then D, then C … down to A.

Width Still Grows

Row widths are still 1, 2, 3, … like Program 9.

Print Outer Letter

line += String.fromCharCode(i) in the inner loop; console.log(line) after.

Mirror of Program 9

Same shape - reverse letter direction only.

In short: for each letter i from top down to A, print i repeatedly (growing count), then call console.log(line).

Foundation

📝 Problem & Approach

Given a positive integer rows (or a fixed top letter like 'E'), print a left-aligned triangle where each row repeats one letter and letters count downward.

JavaScript

// First 5 rows (conceptual shape)
// E
// DD
// CCC
// BBBB
// AAAAA

Inputs & Outputs

Item	Type	Description
`rows`	int	Number of triangle lines (typically 1–26). Top letter = `String.fromCharCode("A".charCodeAt(0) + rows - 1)`.
Printed output	text	Left-aligned rows; row `k` repeats letter `top - (k-1)` exactly `k` times.

Minimal workflow

Pseudocode

top = "A".charCodeAt(0) + rows - 1
for code from top down to "A".charCodeAt(0):
    repeat = (top - code) + 1
    for k from 1 to repeat:
        print String.fromCharCode(code) (no newline)
    console.log the row

Approach comparison

Approach	Idea	Best for
Nested char loops	Outer letter + inner count via char range	Learning and interviews
`ch.repeat(repeat)`	Build a whole row in one call	Shorter production-style demos

Cheat Sheet

⚡ Quick Reference

Goal	Pattern
Walk letters downward	`for (let i = "E".charCodeAt(0); i >= "A".charCodeAt(0); i--)`
Grow repeat count	`for (let j = "E".charCodeAt(0); j >= i; j--)`
Print row letter	`line += String.fromCharCode(i)` — not the inner counter
End the row	`console.log(line)`
One-line row shortcut	`console.log(ch.repeat(repeat))`
Forward letters	See Program 9 (A, BB, CCC, …)

Compare

📋 line vs console.log vs *

Same triangle - different ways to emit characters.

line += ch

same line

Prints a letter without moving to the next line

console.log(line)

new line

Ends the current row after all repeats are printed

ch * n

whole row

Builds n copies of ch at once - skip the inner loop

Learning tip

print String.fromCharCode(i)

Master printing the outer letter before the string shortcut

Context

When This Pattern Shows Up

Reach for this triangle when practicing letter direction vs repeat count.

After Program 9
Flip letter direction while keeping the same growing widths.
Outer vs inner roles
Clear drill: outer = which letter, inner = how many copies.
Char arithmetic practice
Compute top = "A".charCodeAt(0) + rows - 1 and count down safely.
Gateway to variants
Lowercase, hollow borders, or centered pyramids next.
Not a UI layout tool
This is a console teaching pattern - not how you build modern app screens.

Key benefit: one small program that separates letter choice from repeat count - the skill behind most alphabet patterns.

Interactive

🔮 Live Preview

Choose a row count between 1 and 26 and draw the reverse repeating alphabet triangle in the browser.

Hands-On

Examples Gallery

Three complete JavaScript programs - fixed E-to-A range, prompt input, and a ch.repeat(repeat) shortcut. Click View Output to reveal sample console results, or Try it Yourself to run in the browser editor.

📚 Getting Started

Print five rows with classic nested char loops.

Example 1 — Fixed `'E'` down to `'A'`

Hard-coded range - ideal for first demos and screenshots.

JavaScript

const top = "E".charCodeAt(0);
const base = "A".charCodeAt(0);

for (let i = top; i >= base; i--) {
  const ch = String.fromCharCode(i);
  let line = "";
  for (let j = top; j >= i; j--) {
    line += ch;
  }
  console.log(line);
}

Try it Yourself

How It Works

When i is "E".charCodeAt(0), the inner loop runs once and builds E. When i is "D".charCodeAt(0), it builds DD, and so on until A. Appending ch from the outer letter (not the inner counter) keeps each row uniform.

📈 Practical Variant

Let the user choose the height at runtime.

Example 2 — User Input Version

Compute top = "A".charCodeAt(0) + rows - 1, then count down. Validate parseInt(prompt()) with Number.isFinite in real apps.

JavaScript

const rowsInput = prompt("Enter the number of rows:");
let rows = parseInt(rowsInput, 10);
rows = Math.max(1, Math.min(rows, 26));

const base = "A".charCodeAt(0);
const top = base + rows - 1;

for (let code = top; code >= base; code--) {
  const ch = String.fromCharCode(code);
  const repeat = top - code + 1;
  let line = "";
  for (let k = 0; k < repeat; k++) {
    line += ch;
  }
  console.log(line);
}

Try it Yourself

How It Works

For rows = 4, top is "D".charCodeAt(0). Each step down increases repeat by one. Clamp rows to 1-26 so top stays within A-Z.

⚡ Shortcut Style

Same shape without an explicit inner append loop.

Example 3 — `ch.repeat(repeat)`

Build each repeated-letter row in one call, then log it.

JavaScript

const rows = 5;
const base = "A".charCodeAt(0);
const top = base + rows - 1;

for (let code = top; code >= base; code--) {
  const ch = String.fromCharCode(code);
  const repeat = top - code + 1;
  console.log(ch.repeat(repeat));
}

Try it Yourself

How It Works

ch.repeat(repeat) creates a string of length repeat filled with that letter. Great once you understand the nested-loop idea; keep the two-loop version for exams that ask you to show both bounds.

🧠 How the Algorithm Prints Rows

Set up

Use prompt() when reading input. Fix the top letter or compute it from rows.

Setup

Outer loop (letter)

for (let i = "E".charCodeAt(0); i >= "A".charCodeAt(0); i--) selects the character printed on the row.

E → A

Inner loop (count)

for (let j = "E".charCodeAt(0); j >= i; j--) runs 1, 2, 3… times; print String.fromCharCode(i) with line += String.fromCharCode(i).

Repeats

New line

console.log(line) ends the row so the next outer iteration starts fresh.

Break

Triangle complete

Total letters: 1+2+…+n = n(n+1)/2 — O(n²) time, O(1) extra memory.

Trace

🔎 Worked Walkthrough — `'E'` down to `'A'`

Trace each outer-loop value of i and count how many times the inner loop runs.

`i`	Inner `j` range	Printed row	Repeats
`'E'`	`'E'..'E'`	`E`	1
`'D'`	`'E'..'D'`	`DD`	2
`'C'`	`'E'..'C'`	`CCC`	3
`'B'`	`'E'..'B'`	`BBBB`	4
`'A'`	`'E'..'A'`	`AAAAA`	5

Total letter prints: 1 + 2 + 3 + 4 + 5 = 15 = 5×6/2.

Use Cases

Where this tiny pattern (and its loop structure) shows up beyond the homework prompt.

1. Outer vs Inner Clarity

Best demo that the printed value need not be the loop counter.

Example: swap line += String.fromCharCode(i) for line += String.fromCharCode(j) and watch letters step.

2. Pair with Program 9

Teach direction as a one-line change: increment vs decrement.

Example: side-by-side A/BB/CCC vs E/DD/CCC.

3. Char Math Labs

Practice "A".charCodeAt(0) + rows - 1 without complex algorithms.

Example: rows = 7 → top = 'G'.

4. Case & Fill Variants

Swap to lowercase or mix digits once the loops work.

Example: start from 'a' + rows - 1.

5. Complexity Intuition

Triangular totals make O(n²) concrete for beginners.

Example: count printed letters for n = 10 → 55.

6. Input Validation Labs

Pair the pattern with Number.isFinite and 1–26 clamps.

Example: reject rows <= 0 or rows > 26.

Pro Tip: when explaining this pattern, say “outer picks the letter, inner only counts” before writing any code - that story prevents the line += String.fromCharCode(j) mistake.

Advantages

Why this pattern earns a spot right after the forward repeating triangle.

1. Instant Visual Feedback
Wrong printed variable shows up immediately as stepping letters.
2. Minimal Concepts
Only loops, chars, and console output - no arrays required.
3. Easy to Mirror
Flip to Program 9 by counting letters upward instead.
4. O(1) Extra Memory
Streaming output needs no storage beyond loop counters.

Pro Tip: learn the nested-loop version first; treat ch.repeat(repeat) as a polish shortcut afterward.

Usage Tips

Small habits that keep alphabet-pattern code clean.

1. Name the Roles
Use ch for the row letter and repeat (or k) for the count - clearer than overloaded i/j.
2. Validate prompt() input
Avoid crashes when the user types letters instead of a number.
3. Keep console.log(line) Outside
Only call console.log(line) after the inner loop finishes the row.
4. Clamp to 26
For A–Z demos, reject or clamp rows > 26.
5. Dry-Run One Small n
Trace rows = 3 (C, BB, AAA) on paper before coding larger demos.

Pro Tip: if a row shows EDCBA-style sequences, you almost certainly printed j instead of i.

Common Pitfalls

Mistakes that commonly break reverse repeating alphabet patterns.

1. Printing j Instead of i
Rows become countdown sequences instead of repeated letters.
→ Always line += String.fromCharCode(i) (or ch) for this shape.
2. console.log(line) Inside the Inner Loop
Each letter lands on its own line - you get a column, not a triangle.
→ Use line += ch for letters; console.log(line) only after the inner loop.
3. Forgetting the Row Break
Omitting console.log(line) after the inner loop glues every letter onto one endless line.
→ Always end the row after the inner loop.
4. Blind parseInt(prompt())
Non-numeric input yields NaN with bare parseInt(prompt()).
→ Check with Number.isFinite and validate range.
5. rows > 26 Without a Policy
"A".charCodeAt(0) + rows - 1 can leave the A–Z range.
→ Clamp to 26 or define wrap/error behavior explicitly.

Edge Cases

Check these inputs before calling the solution done.

rows = 1

Single letter

Output is just A on one line.

rows = 0

Empty pattern

Treat as invalid; re-prompt instead of silent empty output.

Negative

`rows < 0`

Invalid height - validate before computing top.

rows > 26

Past Z

Clamp or error - char math leaves A–Z.

Bad input

Non-numeric input

parseInt(prompt()) yields NaN - validate first.

Case

Lowercase variant

Same loops work with 'a' and 'a' + rows - 1.

Drill

🎯 Practice Problems

Try these variations to lock in the pattern.

1. Flip to Program 9

Count letters upward: A, BB, CCC, …
Continue with Program 9

2. Lowercase version

Use 'a' + rows - 1 as the top letter
Shows the loop structure is reusable

3. Safe input loop

Use Number.isFinite until 1 <= rows <= 26
Then draw the triangle

4. Hollow reverse triangle

Print the letter only on borders; spaces inside
Harder follow-up after this page

Important

Notes

Triangular count. Total letters for n rows is n(n+1)/2 - hence O(n²) time.
Print the outer letter; the inner loop only decides how many times.
Validate 1 <= rows <= 26 for interactive A–Z programs.
This page is left-aligned. Centered pyramids need leading spaces - covered later in the series.

Quick Takeaway: outer loop picks the letter (counting down), inner loop repeats it, then break the line - that is the whole pattern.

Analysis

⏱️ Time and Space Complexity

Program	Time	Extra space
Nested loops (Examples 1-2)	`O(rows²)`	`O(1)`
`ch.repeat(repeat)` (Example 3)	`O(rows²)`	`O(rows)` per row string (temporary)

Wrap Up

🎉 Conclusion

The reverse repeating alphabet triangle is a small nested-loop exercise with lasting payoff: outer letter vs inner count, char countdown, and O(n²) intuition. Master the classic two-loop version, then optionally shorten rows with ch.repeat(repeat).

Practice the three examples above, then compare with Program 9 or continue to the next alphabet pattern.

Print the outer letter with line += ch, end rows with console.log(line), and clamp row counts to 1–26 when reading input.

View Examples Gallery Continue to Program 11

Pro Tips

💡 Best Practices

✅ Do

Explain outer = letter, inner = count before coding
Use line += String.fromCharCode(i) for letters and console.log(line) after each row
Validate 1 <= rows <= 26 for interactive programs
Validate parseInt(prompt()) with Number.isFinite
State O(n²) time when asked about complexity

❌ Don’t

Print the inner-loop variable for this repeating shape
Call console.log(line) inside the inner letter loop
Skip the newline after each row
Ignore bad console input in user-facing demos
Allow rows > 26 without a clear policy

Summary

Key Takeaways

Knowledge Unlocked

Five things to remember about this alphabet pattern

Print the reverse repeating triangle the beginner-friendly way.

Core concepts

△ 01

Rule

Letters down, width up

Definition

↻ 02

Outer loop

Picks the row letter

Code

A 03

Inner loop

Repeats with line += String.fromCharCode(i)

Code

↵ 04

console.log(line)

Ends each row

I/O

O 05

Complexity

O(n²) time

Analysis

❓ Frequently Asked Questions

How is this different from Program 9?

Program 9 uses A, BB, CCC, ... (letters increase). Program 10 uses E, DD, CCC, ... (letters decrease) with the same growing repeat counts.

Why does row 4 print BBBB?

On the 4th row the outer-loop letter is B, and the inner loop runs four times, appending B each time.

Why append the outer letter and not the inner counter?

The inner loop only controls how many times to append. Using the outer letter keeps the row uniform; using the inner counter would change letters across the row.

What is the difference between building a line and logging inside the inner loop?

line += ch builds the full row string. console.log() inside the inner loop would log one letter per line. Log once after the inner loop finishes.

What is the time complexity?

O(n^2) where n is the number of rows. Total logged characters equal 1+2+...+n = n(n+1)/2.

Can I use String.repeat instead of an inner loop?

Yes. console.log(ch.repeat(repeat)) logs a full repeated-letter row in one call. Nested loops are better for learning; ch.repeat is a handy shortcut later.

How should I read the row count safely?

Use parseInt with Number.isFinite after prompt(), then clamp rows between 1 and 26 so the top letter stays within A-Z.

What happens if rows is greater than 26?

Top letter math can go past Z. Clamp to 26 for A-Z demos, or define a clear wrap/error policy.

Did you Know? 🔊

This pattern is the reverse of Program 9: row widths still grow 1, 2, 3, …, but letters run backward (E, then D, then C, …). Append the outer loop letter inside the inner loop so each row stays uniform.

Continue to Alphabet Pattern 11

Keep building letter patterns with nested loops and char math.

Program 11 tutorial →

About the author

Mari Selvan M P 🔗

Developer, cloud engineer, and technical writer

Experience 12 years building web and cloud systems
Focus Full Stack Development, AWS, and Developer Education

I write practical tutorials so students and working developers can learn by doing—from databases and APIs to deployment on AWS.

AWS Certified Developer – Associate (Credly)

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JavaScript Alphabet Pattern 9 - Repeating Triangle JavaScript Alphabet Pattern 11

Reverse Repeating-Letter Alphabet Triangle in JavaScript

What You’ll Learn

Shape Rule

Outer Loop

Inner Loop

line vs console.log

Live Preview

O(n²)

Introduction

Why it matters?

Letters Count Down

Width Still Grows

Print Outer Letter

Mirror of Program 9

📝 Problem & Approach

Inputs & Outputs

Minimal workflow

Approach comparison

⚡ Quick Reference

📋 line vs console.log vs *

When This Pattern Shows Up

After Program 9

Outer vs inner roles

Char arithmetic practice

Gateway to variants

Not a UI layout tool

🔮 Live Preview

Examples Gallery

📚 Getting Started

Example 1 — Fixed 'E' down to 'A'

How It Works

📈 Practical Variant

Example 2 — User Input Version

How It Works

⚡ Shortcut Style

Example 3 — ch.repeat(repeat)

How It Works

🧠 How the Algorithm Prints Rows

Set up

Outer loop (letter)

Inner loop (count)

New line

Triangle complete

🔎 Worked Walkthrough — 'E' down to 'A'

Use Cases

1. Outer vs Inner Clarity

2. Pair with Program 9

3. Char Math Labs

4. Case & Fill Variants

5. Complexity Intuition

6. Input Validation Labs

Advantages

1. Instant Visual Feedback

2. Minimal Concepts

3. Easy to Mirror

4. O(1) Extra Memory

Usage Tips

1. Name the Roles

2. Validate prompt() input

3. Keep console.log(line) Outside

4. Clamp to 26

5. Dry-Run One Small n

Common Pitfalls

1. Printing j Instead of i

2. console.log(line) Inside the Inner Loop

3. Forgetting the Row Break

4. Blind parseInt(prompt())

5. rows > 26 Without a Policy

Edge Cases

Single letter

Empty pattern

rows < 0

Past Z

Non-numeric input

Lowercase variant

🎯 Practice Problems

1. Flip to Program 9

2. Lowercase version

3. Safe input loop

4. Hollow reverse triangle

Example 1 — Fixed `'E'` down to `'A'`

Example 3 — `ch.repeat(repeat)`

🔎 Worked Walkthrough — `'E'` down to `'A'`

1. Printing `j` Instead of `i`

`rows < 0`