Rotating Number Pattern in JavaScript

What You’ll Learn

Program 50 prints a mixed number triangle: each row combines descending i..2 with ascending 1..(rows-i+1) — a natural step after Program 49’s multiplication triangle. This tutorial covers two inner loops per row, a live preview, worked JavaScript examples, edge cases, and complexity.

📝 Problem & Approach

Given row count rows = 5, print a mixed number triangle — row i shows descending i..2 then ascending 1..(rows-i+1).

// n = 5
// 12345
// 21234
// 32123
// 43212
// 54321

Inputs & Outputs

Minimal workflow

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

Approach comparison

🔮 Live Preview

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

Press "Draw pattern".

Examples Gallery

Three complete JavaScript programs — fixed n, user input, and a compact trace demo. Click View Output to reveal sample console results, or Try it Yourself to run in the browser.

const n = 5;

for (let i = 1; i <= n; i++) {
  let line = "";

  for (let j = i; j > 1; j--) {
    line += j;
  }

  for (let k = 1; k <= n + 1 - i; k++) {
    line += k;
  }

  console.log(line);
}

12345
21234
32123
43212
54321

const nInput = prompt("Enter n:");
const n = parseInt(nInput, 10);

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

    for (let j = i; j > 1; j--) {
      line += j;
    }

    for (let k = 1; k <= n + 1 - i; k++) {
      line += k;
    }

    console.log(line);
  }
}

Enter n: 4
1234
2123
3212
4321

const n = 3;

for (let i = 1; i <= n; i++) {
  let line = "";

  for (let j = i; j > 1; j--) {
    line += j;
  }

  for (let k = 1; k <= n + 1 - i; k++) {
    line += k;
  }

  console.log(line);
}

123
212
321

🔎 Worked Walkthrough — rows = 5

Trace each row’s descending and ascending halves and the full line output.

Each row prints exactly rows digits — descending count plus ascending count always equals rows.

🎯 Practice Problems

Try these variations to lock in the pattern.

Notes

⏱️ Time and Space Complexity

💡 Best Practices