Descending Repeating Number Triangle in JavaScript

Q: Why does each next row repeat the number more times?

The outer loop decreases i from rows down to 1. The inner loop runs j from rows down to i, so it executes once when i equals rows, twice when i is rows-1, and so on until rows times for i = 1.

Q: How does the inner loop control the repeat count?

for (let j = rows; j >= i; j--) runs (rows - i + 1) times. That count grows as i shrinks, which is why 4 prints twice, 3 prints three times, and 1 prints rows times.

Beginner

⏱️ 8 min read

📚 Updated: Sep 2026

🎯 3 Code Examples

🚀 Live Preview

Nested Loops

Descending repeating number triangle output (5, 44, 333, 2222, 11111) in JavaScript programming

Overview

What You’ll Learn

The descending repeating number triangle flips Program 9’s outer loop: digit decreases each row while repeats increase — nested loops, line += i vs console.log(line), and a clear visual result. This tutorial covers the shape rule, loop structure, a live preview, algorithm steps, worked JavaScript examples, edge cases, and complexity.

Shape Rule

digit i repeats rows-i+1 times

Row rows prints one rows, then digit rows-1 twice, and so on until 1 repeats rows times.

Outer Loop

Rows

for (let i = rows; i >= 1; i--) picks the digit for each row, starting at rows and counting down to 1.

Inner Loop

Digits

for (let j = rows; j >= i; j--) prints digit i exactly rows - i + 1 times on that row.

line += i vs console.log(line)

Same line / next line

Repeated digits use line += i; end each row with console.log(line).

Live Preview

1–20 rows

Pick a row count and draw the descending repeating number triangle instantly in the browser.

O(n²)

Complexity

Total digit prints still = n(n+1)/2; extra memory stays O(1).

Introduction

A descending repeating number triangle pattern starts with one copy of the top digit and adds one more repeat each line as the digit decreases. Each row prints the same digit multiple times; the repeat count grows while the digit value shrinks.

In JavaScript you usually solve it with two nested for loops: the outer loop picks the digit, the inner loop repeats it with line += i, then console.log(line) moves to the next line.

Why it matters?

It is a natural follow-up to Program 9 in JavaScript courses. Once nested loops and line += i/console.log(line) click, pyramids, diamonds, and hollow shapes become much easier.

Key Highlights

Row = Repeat Count

On row i, print digit i exactly rows - i + 1 times.

Two Nested Loops

Outer picks the digit; inner controls how many times it repeats.

Print Then Break

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

Series Foundation

Gateway to Program 9 (ascending repeats) and Program 11 (inverted repeats).

In short: for each row i from rows down to 1, repeat line += i exactly rows - i + 1 times, then call console.log(line).

Foundation

📝 Problem & Approach

Given a positive integer rows, print a descending repeating number triangle: row i repeats digit i exactly rows - i + 1 times, with the outer loop counting from rows down to 1.

JavaScript

# First 5 rows (conceptual shape)
# 5
# 44
# 333
# 2222
# 11111
for (let i = rows; i >= 1; i--)
    for (let j = rows; j >= i; j--)
        line += i   # digit i, repeated (rows - i + 1) times
    console.log(line)                # next row

Inputs & Outputs

Item	Type	Description
`rows`	int	Number of triangle lines to print (typically ≥ 1).
Printed output	text	Each row repeats one digit; the top row has one copy of `rows`, the bottom row repeats `1` for `rows` copies.

Minimal workflow

Pseudocode

for i from rows down to 1:
    repeat (rows - i + 1) times:
        append i to line
    console.log(line)

Approach comparison

Approach	Idea	Best for
Nested loops	Outer digit + inner repeats	Learning and interviews
`String(i).repeat(rows - i + 1)` in inner loop	Print a full repeated row with `String(i).repeat`	Leaner JavaScript style once loops are understood

Cheat Sheet

⚡ Quick Reference

Goal	Pattern
Walk each row	`for (let i = rows; i >= 1; i--)`
Repeat digit `i`	`for (let j = rows; j >= i; j--) line += i`
End the row	`console.log(line)`
One-line row shortcut	`String(i).repeat(rows - i + 1)` repeated per row
Ascending variant	`for (let j = 1; j <= i; j++) line += i` (Program 9 ascending repeats)

Compare

📋 line += i vs console.log(line) vs String.repeat

Same triangle — different ways to emit characters.

line += i

same line

Prints a digit without moving to the next line

console.log(line)

new line

Ends the current row after all digits are printed

String(i).repeat(rows - i + 1)

one digit

Writes digit i when i is 1..9 — no format string

Learning tip

loops first

Master nested loops before the String(i).repeat shortcut

Context

When This Pattern Shows Up

Reach for this triangle when teaching or testing nested-loop basics.

First lab exercise
Most JavaScript pattern series start here before pyramids and diamonds.
Nested-loop warm-up
Outer/inner bound practice with an immediate visual check.
Console I/O practice
Combine loops with prompt() for a flexible row count.
Gateway to variants
Compare Program 9 (ascending repeats) and Program 11 (inverted repeats) 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 locks in nested loops, output sequencing, and O(n²) thinking.

Interactive

🔮 Live Preview

Choose a row count between 1 and 20 and draw the descending repeating number triangle in the browser.

Hands-On

Examples Gallery

Three complete JavaScript programs — fixed row count, prompt(), and a String.repeat shortcut. Click View Output to reveal sample results, or Try it Yourself to run the code live.

📚 Getting Started

Print five rows of the descending repeating number triangle with nested loops.

Example 1 — Fixed `rows = 5`

Hard-coded height — outer picks digit, inner controls repeat count.

JavaScript

const rows = 5;

for (let i = rows; i >= 1; i--) {
  let line = "";
  for (let j = rows; j >= i; j--) {
    line += i;
  }
  console.log(line);
}

Try it Yourself

How It Works

When i = 5, the inner loop runs once and prints 5. When i = 4, it prints 44, and so on until i = 1 prints 11111. console.log(line) after the inner loop starts the next row.

📈 Practical Variant

Let the user choose the height at runtime.

Example 2 — User Input Version

Read the row count with prompt() and parseInt() (check Number.isFinite in real apps).

JavaScript

const rowsInput = prompt("Enter the number of rows:");
const rows = parseInt(rowsInput, 10);

if (!Number.isFinite(rows) || rows <= 0) {
  console.log("Please enter a positive integer.");
} else {
  for (let i = rows; i >= 1; i--) {
    let line = "";
    for (let j = rows; j >= i; j--) {
      line += i;
    }
    console.log(line);
  }
}

Try it Yourself

How It Works

Same nested-loop core as Example 1; only the source of rows changes. Non-numeric input yields NaN with bare parseInt(prompt()) — validate with Number.isFinite for safer labs.

⚡ Shortcut Style

Same shape without an explicit inner repeat loop.

Example 3 — `String(i).repeat(rows - i + 1)`

Build each row with String(i).repeat(rows - i + 1), then log it.

JavaScript

const rows = 5;

for (let i = rows; i >= 1; i--) {
  console.log(String(i).repeat(rows - i + 1));
}

Try it Yourself

How It Works

String(i).repeat(rows - i + 1) creates a string of repeated digit i. 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

console.log is built in; use prompt() when reading input. Set rows (fixed or from prompt).

Setup

Outer loop (rows)

for (let i = rows; i >= 1; i--) picks the digit for the current row (5, 4, 3, …).

Row

Inner loop (repeats)

for (let j = rows; j >= i; j--) repeats digit i with line += i.

Repeats

New line

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

Break

Triangle complete

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

Trace

🔎 Worked Walkthrough — `rows = 4`

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

`i`	Inner `j` range	Repeats	Printed row
`4`	`4..4`	1	`4`
`3`	`4..3`	2	`33`
`2`	`4..2`	3	`222`
`1`	`4..1`	4	`1111`

Total digit prints: 1 + 2 + 3 + 4 = 10 = 4×5/2.

Use Cases

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

1. Teaching Nested Loops

Clearest visual proof that outer and inner bounds interact.

Example: change j <= i and watch the shape change.

2. Pattern Series Base

Foundation for inverted, pyramid, diamond, and hollow variants.

Example: Program 9 repeats digit i exactly i times with an ascending outer loop.

3. Console Formatting Drills

Practice line += i vs row newline without complex math.

Example: put console.log(line) inside the inner loop by mistake.

4. Character Substitution

Swap digits for letters, stars, or spaced output once the loop works.

Example: print i + " " for spaced repeated digits on each row.

5. Complexity Intuition

Triangular totals make O(n²) concrete for beginners.

Example: count printed digits for n = 10 still → 55.

6. Input Validation Labs

Pair the pattern with prompt() validation and positive-row validation.

Example: reject rows <= 0 and re-prompt.

Pro Tip: when an interviewer asks for patterns, explain the outer/inner roles first — then write the loops. The story matters as much as the code.

Advantages

Why this pattern earns a permanent spot in beginner C courses.

1. Instant Visual Feedback
Wrong bounds show up immediately as a broken staircase.
2. Minimal Concepts
Only loops and console output — no arrays or math libraries.
3. Easy to Extend
Invert, center, hollow, or change the fill character with small edits.
4. O(1) Extra Memory
Streaming output needs no storage beyond loop counters.

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

Usage Tips

Small habits that keep number-pattern code clean.

1. Name Bounds Clearly
Use rows (or n) and keep i/j for row/column — or rename to row/col.
2. Validate with Number.isFinite
Avoid using invalid rows 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. Match Inner Bound to Repeat Count
for (let j = rows; j >= i; j--) with line += i makes repeat count grow as the digit shrinks.
5. Dry-Run One Small n
Trace rows = 3 on paper before coding larger demos.

Pro Tip: if the output is a vertical list of single digits per line, you almost certainly put console.log(line) inside the inner loop.

Common Pitfalls

Mistakes that commonly break descending repeating number patterns.

1. console.log(line) Inside the Inner Loop
Each repeated digit lands on its own line — you get a column, not a triangle.
→ Use line += i for repeats; console.log(line) only after the inner loop.
2. Wrong Inner Bound
Using for (let j = 1; j <= i; j++) builds Program 9’s ascending repeats (1, 22, 333) instead of this descending shape (5, 44, 333).
→ For this shape, keep for (let j = rows; j >= i; j--) so repeats grow as i shrinks.
3. Forgetting the Row Break
Omitting console.log(line) glues every repeat onto one endless line.
→ Always end the row after the inner loop.
4. Bare parseInt(prompt())
Letters or empty input return NaN with bare parseInt(prompt(), 10).
→ Check Number.isFinite(rows) and re-prompt on failure.
5. Off-by-One on 0-Based Loops
Switching to i = 0 without adjusting the digit and repeat count prints an empty first row or wrong counts.
→ Prefer 1-based for (let i = rows; i >= 1; i--), or carefully map both the digit and rows - i + 1 if you go 0-based.

Edge Cases

Check these inputs before calling the solution done.

rows = 1

Single repeated digit

Output is just one copy of rows on one line.

rows = 0

Empty pattern

Outer loop never runs — print nothing or show a message.

Negative

`rows < 0`

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

Large n

Many rows

Output grows as n²/2 characters — fine for labs, noisy for huge n.

Bad input

Non-numeric prompt()

parseInt(prompt(), 10) returns NaN on bad input — validate first.

Fill char

Wrong repeat count

Check inner bound: j >= i gives rows - i + 1 repeats.

Drill

🎯 Practice Problems

Try these variations to lock in the pattern.

1. Ascending repeating triangle

Outer loop counts up 1..rows (see Program 9)
Continue with Program 11

2. Inverted repeating triangle

Use for (let i = rows; i >= 1; i--) as outer loop
Shows the loop structure is reusable

3. Safe input loop

Validate with Number.isFinite after parseInt(prompt(), 10)
Then draw the triangle

4. Same digit, different count

Use line += i between repeated digits
Harder follow-up after this page

Important

Notes

Triangular count. Total digit prints for n rows is n(n+1)/2 — hence O(n²) time.
line += i stays on the line; console.log(line) advances — mix them carefully.
Validate rows > 0 for interactive programs; rows = 1 should print one copy of the top digit.
This page is left-aligned. Centered pyramids need leading spaces — covered later in the series.

Quick Takeaway: outer loop picks the row, inner loop repeats digit i, 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)`
`String(i).repeat(rows - i + 1)` (Example 3)	`O(rows²)`	`O(rows)` per row string (temporary)

Wrap Up

🎉 Conclusion

The descending repeating number triangle pattern is a small nested-loop exercise with lasting payoff: row/column thinking, line += i vs console.log(line), and O(n²) intuition. Master the classic two-loop version, then optionally shorten rows with String(i).repeat(rows - i + 1).

Practice the three examples above, then continue to Program 11 for the inverted repeating triangle.

Row i repeats digit i — keep line += i for each copy and console.log(line) for the break, and validate row counts when reading input.

View Examples Gallery Continue to Program 11

Pro Tips

💡 Best Practices

✅ Do

Explain outer picks digit, inner counts repeats before coding
Use line += i for repeats and console.log(line) after each row
Validate rows ≥ 1 for interactive programs
Check Number.isFinite(rows) after parseInt(prompt(), 10) before using rows
State O(n²) time when asked about complexity

❌ Don’t

Call console.log(line) inside the inner repeat loop
Use for (let j = 1; j <= i; j++) when you meant Program 9 instead
Skip the newline after each row
Ignore bad console input in user-facing demos
Skip the rows = 1 edge case

Summary

Key Takeaways

Knowledge Unlocked

Five things to remember about this repeating pattern

Print the triangle the beginner-friendly way.

Core concepts

△ 01

Rule

Row i repeats digit i

Definition

↻ 02

Outer loop

Picks the digit each row

Code

1 03

Inner loop

Repeats digit with line += i

Code

↵ 04

console.log(line)

Ends each row

I/O

O 05

Complexity

O(n²) time

Analysis

❓ Frequently Asked Questions

Why does each next row repeat the number more times?

The outer loop decreases i from rows down to 1. The inner loop runs j from rows down to i, so it executes once when i equals rows, twice when i is rows-1, and so on until rows times for i = 1.

How does the inner loop control the repeat count?

for (let j = rows; j >= i; j--) runs (rows - i + 1) times. That count grows as i shrinks, which is why 4 prints twice, 3 prints three times, and 1 prints rows times.

What is the difference between line += i and console.log(line)?

line += i stays on the same row while building digits. console.log(line) prints the completed row and adds a newline. Repeated digits use += inside the inner loop; the row break uses console.log after the inner loop.

How is this different from Program 9?

Program 9 grows upward: row i repeats digit i exactly i times (1, 22, 333). Program 10 starts with one copy of the top digit and increases repeats as the digit decreases (5, 44, 333, 2222, 11111).

What is the time complexity?

O(n²) where n is the number of rows. Total digit appends equal 1+2+…+n = n(n+1)/2.

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

Yes. console.log(String(i).repeat(rows - i + 1)) prints a full row in one call. Nested loops are better for learning; String.repeat is a handy shortcut once the bounds make sense.

How should I read the row count safely?

Use parseInt with Number.isFinite and a rows > 0 check after prompt(), or validate the raw string before converting so bad input does not produce NaN.

What happens if rows is 0 or negative?

The outer loop never runs, so nothing is printed. Validate and prompt again if you want a clear user message.

Did you Know? 🔊

Row digit i repeats rows - i + 1 times. The outer loop counts down from rows, so the first row shows one copy of the top digit and the last row repeats 1 for rows times — still O(n²) total prints.

Continue to Program 11

Same digits, but the widest row is first — 55555, 4444, 333, 22, 1.

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 Number Pattern 9 — Repeated Number Triangle JavaScript Number Pattern 11 — Inverted Repeating Triangle

Descending Repeating Number Triangle in JavaScript

What You’ll Learn

Shape Rule

Outer Loop

Inner Loop

line += i vs console.log(line)

Live Preview

O(n²)

Introduction

Why it matters?

Row = Repeat Count

Two Nested Loops

Print Then Break

Series Foundation

📝 Problem & Approach

Inputs & Outputs

Minimal workflow

Approach comparison

⚡ Quick Reference

📋 line += i vs console.log(line) vs String.repeat

When This Pattern Shows Up

First lab exercise

Nested-loop warm-up

Console I/O practice

Gateway to variants

Not a UI layout tool

🔮 Live Preview

Examples Gallery

📚 Getting Started

Example 1 — Fixed rows = 5

How It Works

📈 Practical Variant

Example 2 — User Input Version

How It Works

⚡ Shortcut Style

Example 3 — String(i).repeat(rows - i + 1)

How It Works

🧠 How the Algorithm Prints Rows

Set up

Outer loop (rows)

Inner loop (repeats)

New line

Triangle complete

🔎 Worked Walkthrough — rows = 4

Use Cases

1. Teaching Nested Loops

2. Pattern Series Base

3. Console Formatting Drills

4. Character Substitution

5. Complexity Intuition

6. Input Validation Labs

Advantages

1. Instant Visual Feedback

2. Minimal Concepts

3. Easy to Extend

4. O(1) Extra Memory

Usage Tips

1. Name Bounds Clearly

2. Validate with Number.isFinite

3. Keep console.log(line) Outside

4. Match Inner Bound to Repeat Count

5. Dry-Run One Small n

Common Pitfalls

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

2. Wrong Inner Bound

3. Forgetting the Row Break

4. Bare parseInt(prompt())

5. Off-by-One on 0-Based Loops

Edge Cases

Single repeated digit

Empty pattern

rows < 0

Many rows

Non-numeric prompt()

Wrong repeat count

🎯 Practice Problems

1. Ascending repeating triangle

2. Inverted repeating triangle

3. Safe input loop

4. Same digit, different count

Example 1 — Fixed `rows = 5`

Example 3 — `String(i).repeat(rows - i + 1)`

🔎 Worked Walkthrough — `rows = 4`

2. Validate with `Number.isFinite`

`rows < 0`