JavaScript Number Pattern (Increasing Suffix)

Beginner
5 min read
Updated: Sep 2026
3 programs
Live preview
Reverse ascending number triangle in JavaScript (5, 45, 345, 2345, 12345)
Definition

What Is This Pattern?

A reverse ascending number triangle starts each row at a falling start digit and counts up to rows — digits grow on the left: 5, 45, 345, …

Remember
Rule: outer i = rows..1, inner j = i..rows

5
45
345
2345
12345   ← rows = 5

Unlike Program 5 (append 1..i while counting up), this counts the outer loop down and appends i..rows — total digits = n(n+1)/2.

Approach

How to Solve It

Outer loop counts down from rows; inner loop appends from the current start digit up to rows.

MethodIdeaBest for
Descending outeri = rows..1, then append i..rowsClassic reverse-ascending demos
CompactSame logic with fewer rows (e.g. 3)Quick dry-runs

Pseudocode

Pseudocode
for i from rows down to 1:
    line = ""
    for j from i to rows:
        append j
    print line

Cheat sheet

GoalPattern
Outer (rows)for (let i = rows; i >= 1; i--)
Inner (digits)for (let j = i; j <= rows; j++) line += j;
End the rowconsole.log(line);
Digits on first row1 (just rows)
Digits on last rowrows (1..rows)
Total digitsrows * (rows + 1) / 2

Printing Numbers vs Starting a New Line

APIEffectUse for
line += …Stays on the same rowEach digit
console.log(line)Ends the current rowAfter the inner loop finishes

Append without a newline, then end the row once.

Try it

Live Preview

Change the row count and the reverse ascending triangle updates instantly.

Whole numbers from 1 to 9 (single digits stay neat). Tap a chip or type a value — the preview redraws as you go.

Live result rows = 5 · 15 digits
5
45
345
2345
12345
Trace

Worked Walkthrough

Trace three rows when rows = 5 — watch the start digit fall while the end stays fixed.

Row start iInner jPrinted row
55..55
33..5345
11..512345

Each step lowers i by one, so one new digit appears on the left of the previous sequence.

Code

JavaScript Programs

Three complete programs: fixed rows = 5, prompt input, and a compact rows = 3 demo. Use View Output for samples, or Try It Yourself to edit and run live.

Example 1 — Fixed rows = 5

Outer loop counts down; inner loop appends from i up to rows.

JavaScript
const rows = 5;

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

How It Works

1. Start at the peak. i = rows so the first row is just 5.

2. Append up to the fixed end. Inner j runs from i to rows — when i = 3, that builds 345.

3. End the row once. Call console.log(line) after the inner loop so digits stay on one line.

Example 2 — User Input Rows

Read rows with prompt and validate before drawing.

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

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

How It Works

1. Prompt and validate. Use parseInt and require rows >= 1.

2. Same two-loop core. Only the source of rows changes — descending outer / ascending inner match Example 1.

3. Entering 4. Ten digits total (4×5/2), last row 1234.

Example 3 — Compact rows = 3

Same descending-outer logic with fewer rows for a quick visual check.

JavaScript
const rows = 3;

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

How It Works

1. Shrink the size. Only rows changes — still i = rows..1 and j = i..rows.

2. Easier to trace. Six digits total — useful while learning the leftward growth.

3. Same end rule. The last row is always 1..rows.

Edge Cases & Pitfalls

Check these before calling the solution done.

Outer bound

Count i down from rows, not up from 1

Counting up with j = 1..i rebuilds Program 5 instead of this left-growing triangle.

Inner end

Keep the inner end fixed at rows

Ending at i would print one digit per row — no accumulation.

rows = 1

Single-row case

Output is just 1.

Bad input

Validate parseInt

Check Number.isFinite(rows) and require rows >= 1 before the loops.

Analysis

Time and Space Complexity

ProgramTimeExtra space
Fixed / compact (Examples 1, 3)O(n²)O(n) per row string
User input (Example 2)O(n²)O(n) per row string

Total digits are 1 + 2 + … + n = n(n+1)/2 — still quadratic in n.

Remember

Key Takeaways

  • Rule: outer rows..1, inner append i..rows.
  • Growth: each new row adds one digit on the left.
  • Break the row: call console.log(line) only after the inner loop.
  • Complexity: O(n²) — triangular digit count.

One line: count the start digit down from n, append up to n each row, then log.

Frequently Asked Questions

The outer loop begins at i = rows. The inner loop appends j from i to rows, so the first row logs only that single digit.
When i = 1, the inner loop runs j from 1 to rows — appending 1, 2, 3, 4, 5 on one line.
Program 5 counts the outer loop up and logs 1..i. Program 6 counts the outer loop down and logs i..rows — digits grow from the left.
line += appends digits on the same row. console.log(line) ends the row after the inner loop finishes.
O(n²) for n rows because total appends are 1 + 2 + … + n = n(n+1)/2.
Yes — use line += j + " " and trimEnd before console.log if needed.
Use parseInt with Number.isFinite. Bare parseInt(prompt()) returns NaN on bad input.
One row prints a single digit 1 — the inner loop runs j from 1 to 1 once.

Did you know?

The outer loop starts from rows down to 1, and the inner loop appends i..rows — producing 5, 45, 345, and so on. Total appends still grow as O(n²).

Next: Reverse Row Number Triangle

Continue with the next pattern in the JavaScript number-pattern series.

Program 7 tutorial →

About the author

Mari Selvan M P
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)

12 people found this page helpful

JavaScript Number Pattern 5 — Ascending Number Triangle JavaScript Number Pattern 7 — Reverse Row Number Triangle