Right-Aligned Incremental Number Triangle in JavaScript

What You’ll Learn

The right-aligned incremental triangle prints 1, then 2 3, then 4 5 6, … — a natural follow-up after Program 34’s zero-based i + j triangle. This tutorial covers the continuous counter k, fixed-width formatting, nested loops, a live preview, worked JavaScript examples, edge cases, and complexity.

📝 Problem & Approach

Given rows = 5, print a right-aligned incremental triangle: counter k starts at 1, leading spaces while j > i, then fixed-width numbers.

# rows = 5
#              1
#           2  3
#        4  5  6
#     7  8  9 10
#11 12 13 14 15

Inputs & Outputs

Minimal workflow

k = 1
for i from 1 to rows:
    for j from rows down to 1:
        if j > i: append 3 spaces
        else: append k with padStart(3); k += 1
    console.log(line)

Approach comparison

🔮 Live Preview

Choose a row count between 3 and 7 and draw the right-aligned incremental triangle in the browser.

Press "Draw pattern".

Examples Gallery

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

let k = 1;
const rows = 5;

for (let i = 1; i <= rows; i++) {
  let line = "";
  for (let j = rows; j >= 1; j--) {
    if (j > i) {
      line += "   ";
    } else {
      line += String(k).padStart(3, " ");
      k += 1;
    }
  }
  console.log(line);
}

             1
          2  3
       4  5  6
    7  8  9 10
11 12 13 14 15

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

if (!Number.isFinite(rows) || rows < 1) {
  console.log("Please enter a positive integer.");
} else {
  let k = 1;
  for (let i = 1; i <= rows; i++) {
    let line = "";
    for (let j = rows; j >= 1; j--) {
      if (j > i) {
        line += "   ";
      } else {
        line += String(k).padStart(3, " ");
        k += 1;
      }
    }
    console.log(line);
  }
}

Enter rows: 3
      1
   2  3
4  5  6

const rows = 3;
let k = 1;

for (let i = 1; i <= rows; i++) {
  let line = "";
  for (let j = rows; j >= 1; j--) {
    if (j > i) {
      line += "   ";
    } else {
      line += String(k).padStart(3, " ");
      k += 1;
    }
  }
  console.log(line);
}

      1
   2  3
4  5  6

🔎 Worked Walkthrough — rows = 5

Trace each outer-loop value of i, leading spaces, numbers printed from k, and full row output.

Leading space groups per row = rows - i — zero when i = rows. Total numbers printed = rows(rows+1)/2.

🎯 Practice Problems

Try these variations to lock in the pattern.

Notes

⏱️ Time and Space Complexity

💡 Best Practices