Reverse Row Number Triangle in JavaScript

What You’ll Learn

Program 7 prints a reverse row number triangle: each row shows digits from the current row index down to 1 — 1, 21, 321, and so on. This tutorial covers the shape rule, ascending outer loop, inner countdown i..1, a live preview, worked JavaScript examples, edge cases, and complexity.

📝 Problem & Approach

Given row count rows = 5, print a reverse row number triangle — row outer index i shows digits i..1.

// rows = 5
//1
//21
//321
//4321
//54321

Inputs & Outputs

Minimal workflow

for i from 1 to rows:
    for j from i down to 1:
        print j
    print newline

Approach comparison

🔮 Live Preview

Choose a row count between 3 and 9 and draw the reverse row number triangle in the browser.

Press "Draw pattern".

Examples Gallery

Three complete JavaScript programs — fixed rows, user input, and a compact trace with rows = 3. Click View Output to reveal sample console results.

const rows = 5;

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

1
21
321
4321
54321

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

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

Enter the number of rows: 4
1
21
321
4321

const rows = 3;

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

1
21
321

🔎 Worked Walkthrough — rows = 5

Trace each row — outer i sets width, inner j counts down from i to 1.

Total digits printed: 1+2+3+4+5 = 15 = 5×6/2 — the fifth triangular number.

🎯 Practice Problems

Try these variations to lock in the pattern.

Notes

⏱️ Time and Space Complexity

💡 Best Practices