Alternating 1 and 0 Triangle in JavaScript

What You’ll Learn

The alternating 1 and 0 triangle prints 11111, 0000, 111, 00, 1 — a natural step after the rotating number pattern in Program 39. This tutorial covers modulo parity, shrinking inner-loop width, nested loops, a live preview, algorithm steps, worked JavaScript examples, edge cases, and complexity.

📝 Problem & Approach

Given a positive integer rows (e.g. 5), print an alternating 1/0 triangle: odd rows are all 1s, even rows are all 0s, with each row one character shorter.

// rows = 5 (conceptual shape)
// 11111
// 0000
// 111
// 00
// 1

Inputs & Outputs

Minimal workflow

for i from 1 to rows:
    digit = i % 2
    for _ from i to rows:
        print digit
    print newline

Approach comparison

🔮 Live Preview

Choose a row count between 3 and 9 and draw the alternating 1/0 triangle in the browser.

Press "Draw pattern".

Examples Gallery

Three complete JavaScript programs — fixed rows, prompt() 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 = i; j <= rows; j++) {
    line += i % 2;
  }
  console.log(line);
}

11111
0000
111
00
1

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 = i; j <= rows; j++) {
      line += i % 2;
    }
    console.log(line);
  }
}

Enter the number of rows: 4
1111
000
11
0

const rows = 5;

for (let i = 1; i <= rows; i++) {
  let line = "";
  for (let j = i; j <= rows; j++) {
    line += (i % 2) + " ";
  }
  console.log(line);
}

1 1 1 1 1 
0 0 0 0 
1 1 1 

🔎 Worked Walkthrough — rows = 5

Trace each outer-loop value of i, the inner-loop range, character count, parity, and full row output.

Prints per row = rows - i + 1 — 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