Diagonal Mirror Number Pyramid in JavaScript

What You’ll Learn

Program 57 prints a diagonal mirror number pyramid: each row shows the row number twice — once on the left diagonal and once on the right — with spaces everywhere else. A natural step after Program 56’s centered palindromic pyramid. This tutorial covers conditional printing, two inner loops per row, a live preview, worked JavaScript examples, edge cases, and complexity.

📝 Problem & Approach

Given row count rows = 5, print a diagonal mirror number pyramid — row i shows digit i on left and right diagonals with spaces elsewhere.

// rows = 5
//    1
//   2 2
//  3   3
// 4     4
//5       5

Inputs & Outputs

Minimal workflow

for i from 1 to rows:
    for j from rows down to 1:
        append i if i === j else space to line
    for k from 2 to rows:
        append i if i === k else space to line
    console.log(line)

Approach comparison

🔮 Live Preview

Choose row count between 3 and 9 and draw the centered diagonal mirror 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 line = "";

  for (let j = rows; j >= 1; j--) {
    line += (i === j) ? i : " ";
  }

  for (let k = 2; k <= rows; k++) {
    line += (i === k) ? i : " ";
  }

  console.log(line);
}

    1
   2 2
  3   3
 4     4
5       5

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 = rows; j >= 1; j--) {
      line += (i === j) ? i : " ";
    }

    for (let k = 2; k <= rows; k++) {
      line += (i === k) ? i : " ";
    }

    console.log(line);
  }
}

Enter the number of rows: 5
    1
   2 2
  3   3
 4     4
5       5

const rows = 3;

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

  for (let j = rows; j >= 1; j--) {
    line += (i === j) ? i : " ";
  }

  for (let k = 2; k <= rows; k++) {
    line += (i === k) ? i : " ";
  }

  console.log(line);
}

    1
  2 2
3   3

🔎 Worked Walkthrough — rows = 5

Trace each row’s left diagonal hit, right diagonal hit, and full line output.

Row i always prints exactly two digits (one per diagonal) when rows > 1 — spaced across 2×rows-1 character positions.

🎯 Practice Problems

Try these variations to lock in the pattern.

Notes

⏱️ Time and Space Complexity

💡 Best Practices