Widening Alphabet Triangle (A, B B, C C, ...) in JavaScript

What You’ll Learn

Each row mirrors the same letter with a growing gap: row 0 prints centered A, row 1 prints B B, row 2 prints C C, until row 4 shows E E for five rows. Leading spaces (rows - 1 - r) center the triangle; the gap formula 2*r - 1 widens each row. Compare with Program 32 (centered palindrome pyramid) and Program 19 (mirrored halves). Includes a live preview, worked JavaScript examples, edge cases, and complexity.

📝 Problem & Approach

Given a positive integer rows, print a centered triangle of rows lines. Row r prints (rows - 1 - r) leading spaces, then letter String.fromCharCode("A".charCodeAt(0) + r). For r > 0, print (2*r - 1) gap spaces and the same letter again.

// First 5 rows (widening triangle)
    A
   B B
  C   C
 D     D
E       E

Inputs & Outputs

Minimal workflow

base = "A".charCodeAt(0)
for r from 0 to rows-1:
    append (rows-1-r) leading spaces
    ch = String.fromCharCode(base + r)
    append ch
    if r > 0:
        append (2*r - 1) gap spaces
        append ch
    console.log(line)

Approach comparison

🔮 Live Preview

Choose a row count between 1 and 26 and draw the widening alphabet triangle in the browser.

Press "Draw pattern".

Examples Gallery

Three complete JavaScript programs - fixed five rows with leading spaces and a widening gap, prompt input, and a one-line row-builder variant for clarity. Click View Output to reveal sample console results, or Try it Yourself to run in the browser editor.

let rows = 5;
rows = Math.max(1, Math.min(rows, 26));

const base = "A".charCodeAt(0);

for (let r = 0; r < rows; r++) {
  let line = "";
  line += " ".repeat(rows - 1 - r);
  const ch = String.fromCharCode(base + r);
  line += ch;
  if (r > 0) {
    line += " ".repeat(2 * r - 1);
    line += ch;
  }
  console.log(line);
}

    A
   B B
  C   C
 D     D
E       E

let rows = parseInt(prompt("Enter number of rows (max 26):"), 10);
if (!Number.isFinite(rows)) {
  console.log("Please enter a whole number.");
} else {
  rows = Math.max(1, Math.min(rows, 26));

  const base = "A".charCodeAt(0);

  for (let r = 0; r < rows; r++) {
    let line = "";
    line += " ".repeat(rows - 1 - r);
    const ch = String.fromCharCode(base + r);
    line += ch;
    if (r > 0) {
      line += " ".repeat(2 * r - 1);
      line += ch;
    }
    console.log(line);
  }
}

Enter number of rows (max 26): 3
  A
 B B
C   C

let rows = 5;
rows = Math.max(1, Math.min(rows, 26));

const base = "A".charCodeAt(0);

for (let r = 0; r < rows; r++) {
  const pad = " ".repeat(rows - 1 - r);
  const ch = String.fromCharCode(base + r);
  const row = pad + ch + (r > 0 ? " ".repeat(2 * r - 1) + ch : "");
  console.log(row);
}

    A
   B B
  C   C
 D     D
E       E

🔎 Worked Walkthrough — rows = 5

Trace each outer-loop value of r and see how leading spaces, row letter, gap, and mirror letter produce each widening row.

Highlight rows: r = 0 (4 lead spaces, A only), r = 1 (3 spaces, B + 1 gap + B → B B), r = 4 (0 spaces, E + 7 gap + E). Gap grows by 2 each row: 2*r - 1 gives 1, 3, 5, 7 for rows 1–4.

🎯 Practice Problems

Try these variations to lock in the pattern.

Notes

⏱️ Time and Space Complexity

💡 Best Practices