Right-Angled Triangle Star Pattern in JavaScript
What You'll Learn
This is the first star pattern in the series. For each row, print one more * than the previous row. In JavaScript, you can do this with nested loops or with repeat().
Total stars printed for rows = n is \(1 + 2 + \cdots + n = \frac{n(n+1)}{2}\).
⭐ Pattern Output
When you run the program with rows = 5:
*
**
***
****
*****Complete JavaScript Program
Fixed rows = 5 version (prints using nested loops):
const rows = 5;
for (let i = 1; i <= rows; i++) {
let line = "";
for (let j = 1; j <= i; j++) line += "*";
console.log(line);
}🧠 How It Works
Setup and outer row loop
const rows = 5; fixes the height. for (let i = 1; i <= rows; i++) walks each row index; let line = ""; starts an empty string for that row.
Inner loop: append stars
for (let j = 1; j <= i; j++) line += "*"; runs i times, growing line without a line break. You can swap this for "*".repeat(i).
Emit the row
console.log(line) prints the finished row to the console (with a newline). Repeat for every i.
Right-angled triangle
Star counts 1+2+…+n = n(n+1)/2. O(n²) output for n = rows, O(1) extra memory besides the current row string. Long rows scroll inside the green preview on phones.
💡 Tips for Enhancement
Try These
- Use
"*".repeat(i)to build each row without an inner loop - Print the triangle with spaces between stars (e.g.,
"* ".repeat(i)) - Store rows in an array and join with
"\n"for testing - Try the inverted triangle next (Program 2)
Avoid
- Building huge strings with
+in tight loops for very large inputs - Forgetting to print a newline after each row
- Mixing tabs and spaces (alignment breaks in other patterns)
- Printing extra spaces at the end of each line (harder to compare outputs)
Key Takeaways
Row i prints exactly i stars.
Total stars for n rows is \(n(n+1)/2\).
repeat() is a clean way to build each row in JavaScript.
Time complexity is O(n²) because total printed characters grow quadratically.
This is a great starter pattern to learn loops.
❓ Frequently Asked Questions
"*".repeat(i) is concise and readable."* ".repeat(i).trimEnd() for each row.n rows, because total printed characters are proportional to \(n(n+1)/2\).Next: Inverted Right-Angled Triangle
Continue to Program 2 to print the inverted version of this triangle.
In JavaScript, repeat() is implemented in native code and is often faster than manual concatenation for moderate sizes.
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