Increasing Number Triangle Using i + j - 1 in JavaScript

What You’ll Learn

The increasing number triangle using i + j - 1 prints 1, 2 3, 3 4 5, … — a natural follow-up after Program 32’s triangle starting from 11. This tutorial covers the i + j - 1 formula, nested loops, a live preview, worked JavaScript examples, edge cases, and complexity.

📝 Problem & Approach

Given rows = 5, print a left-shifted increasing triangle: for each row i, print j = 1..i values of i + j - 1 separated by spaces.

// rows = 5 (conceptual shape)
const rows = 5;
for (let i = 1; i <= rows; i++) {
  let line = "";
  for (let j = 1; j <= i; j++) {
    line += (i + j - 1) + " ";
  }
  console.log(line);
}

Inputs & Outputs

Minimal workflow

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

Approach comparison

🔮 Live Preview

Choose a row count between 3 and 9 and draw the increasing 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.

const rows = 5;

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

1
2 3
3 4 5
4 5 6 7
5 6 7 8 9

const rowsInput = prompt("Enter 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 + j - 1) + " ";
    }
    console.log(line);
  }
}

Enter rows: 3
1
2 3
3 4 5

const rows = 3;

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

1
2 3
3 4 5

🔎 Worked Walkthrough — rows = 5

Trace each outer-loop value of i, inner-loop range, values printed, and full row output.

Prints 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