Alternating Zigzag Number Triangle Pattern in JavaScript

What You’ll Learn

The alternating zigzag number triangle combines nested loops with an if/else parity check to flip print direction each row. This tutorial covers the shape rule, loop structure, a live preview, algorithm steps, worked JavaScript examples, edge cases, and complexity.

📝 Problem & Approach

Given a positive integer rows, print an alternating zigzag number triangle: row length i prints digits ascending when i is odd and descending when i is even, with the outer loop counting from rows down to 1.

# First 5 rows (conceptual shape)
# 12345
# 4321
# 123
# 21
# 1
for (let i = rows; i >= 1; i--)
    if (i % 2 === 0)
        for (let j = i; j >= 1; j--)
            line += j
    else
        for (let j = 1; j <= i; j++)
            line += j
    console.log(line)

Inputs & Outputs

Minimal workflow

for i from rows down to 1:
    if i is even:
        for j from i down to 1: append j to line
    else:
        for j from 1 to i: append j to line
    console.log(line)

Approach comparison

🔮 Live Preview

Choose a row count between 1 and 20 and draw the alternating zigzag number triangle in the browser.

Press "Draw pattern".

Examples Gallery

Three complete JavaScript programs — fixed row count, prompt(), and a direction-variables refactor. Click View Output to reveal sample results, or Try it Yourself to run the code live.

const rows = 5;

for (let i = rows; i >= 1; i--) {
  let line = "";
  if (i % 2 === 0) {
    for (let j = i; j >= 1; j--) {
      line += j;
    }
  } else {
    for (let j = 1; j <= i; j++) {
      line += j;
    }
  }
  console.log(line);
}

12345
4321
123
21
1

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 = rows; i >= 1; i--) {
    let line = "";
    if (i % 2 === 0) {
      for (let j = i; j >= 1; j--) {
        line += j;
      }
    } else {
      for (let j = 1; j <= i; j++) {
        line += j;
      }
    }
    console.log(line);
  }
}

Enter the number of rows: 7
1234567
654321
12345
4321
123
21
1

const rows = 5;

for (let i = rows; i >= 1; i--) {
  let line = "";
  const descending = i % 2 === 0;
  const start = descending ? i : 1;
  const end = descending ? 1 : i;
  const step = descending ? -1 : 1;
  for (let j = start; descending ? j >= end : j <= end; j += step) {
    line += j;
  }
  console.log(line);
}

12345
4321
123
21
1

🔎 Worked Walkthrough — rows = 4

Trace each outer-loop value of i and note whether the row prints ascending or descending.

Total digit prints: 1 + 2 + 3 + 4 = 10 = 4×5/2.

🎯 Practice Problems

Try these variations to lock in the pattern.

Notes

⏱️ Time and Space Complexity

💡 Best Practices