Palindrome Number Triangle in JavaScript

What You’ll Learn

The palindrome number triangle prints 1, 121, 12321, 1234321, 123454321 — a natural step after the diagonal asterisk pattern in Program 26. This tutorial covers ascending and descending inner loops, mirroring, a live preview, algorithm steps, worked JavaScript examples, edge cases, and complexity.

📝 Problem & Approach

Given a positive integer rows (e.g. 5), print a palindrome triangle: for each i, print 1..i then i-1..1 on the same line.

# rows = 5 (conceptual shape)
# 1
# 121
# 12321
# 1234321
# 123454321

Inputs & Outputs

Minimal workflow

for i from 1 to rows:
    line = ""
    for j from 1 to i:
        line += j
    for k from i - 1 down to 1:
        line += k
    console.log(line)

Approach comparison

🔮 Live Preview

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

Press "Draw pattern".

Examples Gallery

Three complete JavaScript programs — fixed rows, user input, and spaced output variant. 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 <= i; j++) {
    line += j;
  }
  for (let k = i - 1; k >= 1; k--) {
    line += k;
  }
  console.log(line);
}

1
121
12321
1234321
123454321

const rowsInput = prompt("Enter rows:");
const rows = parseInt(rowsInput, 10);

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

Enter rows: 4
1
121
12321
1234321

const rows = 5;

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

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 

🔎 Worked Walkthrough — rows = 5

Trace each outer-loop value of i, the ascending and descending halves, and the full row output.

Row length grows as 2i - 1 digits — total prints = n² for n rows.

🎯 Practice Problems

Try these variations to lock in the pattern.

Notes

⏱️ Time and Space Complexity

💡 Best Practices