Right-Aligned Number Triangle in JavaScript

What You’ll Learn

The right-aligned number triangle prints 1, then 1 2, then 1 2 3, … — a natural follow-up after Program 42’s hollow square border. This tutorial covers leading-space indentation, ascending sequences, nested loops, a live preview, worked JavaScript examples, edge cases, and complexity.

📝 Problem & Approach

Given rows = 5, print a right-aligned ascending triangle: leading spaces while j > i, then numbers from 1 to i.

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

Inputs & Outputs

Minimal workflow

for i from 1 to rows:
    for j from rows down to i+1: print one space
    for k from 1 to i: print k with trailing space
    print newline

Approach comparison

🔮 Live Preview

Choose a row count between 3 and 9 and draw the right-aligned number triangle in the browser.

Press "Draw pattern".

Examples Gallery

Three complete JavaScript programs — fixed rows, prompt() input, and left-aligned contrast. 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 = " ".repeat(rows - i);
  for (let k = 1; k <= i; k++) {
    line += k + " ";
  }
  console.log(line);
}

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

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

if (!Number.isFinite(rows) || rows <= 0) {
  console.log("Please enter a positive integer.");
} else {
  for (let i = 1; i <= rows; i++) {
    let line = " ".repeat(rows - i);
    for (let k = 1; k <= i; k++) {
      line += k + " ";
    }
    console.log(line);
  }
}

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

const rows = 5;

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

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

🔎 Worked Walkthrough — rows = 5, row i = 3

Trace row 3 — space count, numbers printed, and full row output.

Space count per row = rows - i. Numbers per row = i. Total prints = n(n+1)/2 for n rows.

🎯 Practice Problems

Try these variations to lock in the pattern.

Notes

⏱️ Time and Space Complexity

💡 Best Practices