Repeating Number Triangle in JavaScript

What You’ll Learn

Program 9 prints a repeating number triangle: each row repeats the row digit i exactly i times — 1, 22, 333, and so on. This tutorial covers the shape rule, outer loop for the digit, inner loop for repetition, a live preview, worked JavaScript examples, edge cases, and complexity.

📝 Problem & Approach

Given row count rows = 5, print a repeating number triangle — row i shows digit i repeated i times.

// rows = 5
// 1
// 22
// 333
// 4444
// 55555

Inputs & Outputs

Minimal workflow

for i from 1 to rows:
    repeat i times:
        append i to line
    log line

Approach comparison

🔮 Live Preview

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

Press "Draw pattern".

Examples Gallery

Three complete JavaScript programs — fixed rows, user input, and a compact trace with rows = 3. Click View Output to reveal sample console results, or Try it Yourself to run in the browser editor.

const rows = 5;

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

1
22
333
4444
55555

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 = 1; j <= i; j++) {
      line += i;
    }
    console.log(line);
  }
}

Enter the number of rows: 4
1
22
333
4444

const rows = 3;

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

1
22
333

🔎 Worked Walkthrough — rows = 5

Trace each row — outer i is the digit, inner j counts repetitions from 1 to i.

Total digits printed: 1+2+3+4+5 = 15 = 5×6/2 — the fifth triangular number.

🎯 Practice Problems

Try these variations to lock in the pattern.

Notes

⏱️ Time and Space Complexity

💡 Best Practices