Increasing-Decreasing Number Pyramid in JavaScript

What You’ll Learn

Program 52 prints an increasing-decreasing number pyramid: each row is palindromic — count up from i to the peak, then back down — a natural step after Program 51’s alternating number triangle. This tutorial covers two inner loops per row, peak step-back with m -= 2, a live preview, worked JavaScript examples, edge cases, and complexity.

📝 Problem & Approach

Given row count rows = 5, print an increasing-decreasing number pyramid — row i shows a palindromic sequence from i up to 2i-1 and back.

// rows = 5
// 1
// 232
// 34543
// 4567654
// 567898765

Inputs & Outputs

Minimal workflow

for (let i = 1; i <= rows; i++) {
  let m = i;
  let line = "";
  for (let j = 0; j < i; j++) { line += m; m++; }
  m -= 2;
  for (let k = 0; k < i - 1; k++) { line += m; m--; }
  console.log(line);
}

Approach comparison

🔮 Live Preview

Choose row count between 3 and 9 and draw the increasing-decreasing number pyramid in the browser.

Press "Draw pattern".

Examples Gallery

Three complete JavaScript programs — fixed rows, 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 rows = 5;

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

  for (let j = 0; j < i; j++) {
    line += m;
    m++;
  }

  m -= 2;

  for (let k = 0; k < i - 1; k++) {
    line += m;
    m--;
  }

  console.log(line);
}

1
232
34543
4567654
567898765

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 m = i;
    let line = "";

    for (let j = 0; j < i; j++) {
      line += m;
      m++;
    }

    m -= 2;

    for (let k = 0; k < i - 1; k++) {
      line += m;
      m--;
    }

    console.log(line);
  }
}

Enter the number of rows: 4
1
232
34543
4567654

const rows = 3;

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

  for (let j = 0; j < i; j++) {
    line += m;
    m++;
  }

  m -= 2;

  for (let k = 0; k < i - 1; k++) {
    line += m;
    m--;
  }

  console.log(line);
}

1
232
34543

🔎 Worked Walkthrough — rows = 5

Trace each row’s increasing half, peak step-back, decreasing half, and full line output.

Row i always prints exactly 2i-1 digits — a palindromic line built from two inner loops.

🎯 Practice Problems

Try these variations to lock in the pattern.

Notes

⏱️ Time and Space Complexity

💡 Best Practices