Palindromic Number Pyramid in JavaScript

What You’ll Learn

Program 56 prints a centered palindromic number pyramid: each row shows 1..i..1 with leading spaces for centering — a natural step after Program 55’s column-wise triangle. This tutorial covers spacing, ascending and descending loops, a live preview, worked JavaScript examples, edge cases, and complexity.

📝 Problem & Approach

Given row count rows = 5, print a centered palindromic number pyramid — row i shows 1..i..1 with leading spaces.

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

Inputs & Outputs

Minimal workflow

for i from 1 to rows:
    append (rows - i) pairs of spaces to line
    for k from 1 to i:
        append k + " " to line
    for k from i - 1 down to 1:
        append k + " " to line
    console.log(line.trimEnd())

Approach comparison

🔮 Live Preview

Choose row count between 3 and 9 and draw the centered palindromic 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 s = 0; s < rows - i; s++) {
    line += "  ";
  }

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

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

  console.log(line.trimEnd());
}

      1
    1 2 1
  1 2 3 2 1
1 2 3 4 3 2 1
1 2 3 4 5 4 3 2 1

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 s = 0; s < rows - i; s++) {
      line += "  ";
    }

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

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

    console.log(line.trimEnd());
  }
}

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

const rows = 3;

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

  for (let s = 0; s < rows - i; s++) {
    line += "  ";
  }

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

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

  console.log(line.trimEnd());
}

    1
  1 2 1
1 2 3 2 1

🔎 Worked Walkthrough — rows = 5

Trace each row’s space count, ascending half, descending half, and full line output.

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

🎯 Practice Problems

Try these variations to lock in the pattern.

Notes

⏱️ Time and Space Complexity

💡 Best Practices