Mirrored Number Pattern in JavaScript

What You’ll Learn

The mirrored number pattern prints 1 1, 12 21, 123 321, 1234 4321, 1234554321 — a natural step after the 0-centered mirror in Program 28. This tutorial covers fixed-width loops, space alignment, conditional printing, a live preview, worked JavaScript examples, edge cases, and complexity.

📝 Problem & Approach

Given rows = 5, print a mirrored pattern: for each i, print digits or spaces in a fixed-width left loop, then digits or spaces in a fixed-width right loop.

// rows = 5 (conceptual shape)
// 1        1
// 12      21
// 123    321
// 1234  4321
// 1234554321

Inputs & Outputs

Minimal workflow

for i from 1 to rows:
    line = ""
    for j from 1 to rows:
        line += j if j <= i else " "
    for k from rows down to 1:
        line += " " if k > i else k
    console.log(line)

Approach comparison

🔮 Live Preview

Choose a row count between 3 and 9 and draw the spaced mirror pattern in the browser.

Press "Draw pattern".

Examples Gallery

Three complete JavaScript programs — fixed rows, user input with conditional expression form, and a smaller trace demo. Click View Output to reveal sample console results, or Try it Yourself to run the code live.

const rows = 5;

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

1        1
12      21
123    321
1234  4321
1234554321

const rowsInput = prompt("Enter 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 = 1; j <= rows; j++) {
      line += (j <= i) ? j : " ";
    }
    for (let k = rows; k >= 1; k--) {
      line += (k > i) ? " " : k;
    }
    console.log(line);
  }
}

Enter rows: 4
1      1
12    21
123  321
12344321

const rows = 3;

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

1    1
12 21
123321

🔎 Worked Walkthrough — rows = 5

Trace each outer-loop value of i, what the left and right loops print, and the full row output.

Gap spaces = 2 * (rows - i) between the left and right digit groups — zero when i = rows.

🎯 Practice Problems

Try these variations to lock in the pattern.

Notes

⏱️ Time and Space Complexity

💡 Best Practices