A JavaScript star pattern is a console program that prints "*" in rows. The outer for loop chooses the row. The inner loop appends each star with line += "*", then console.log(line) ends that row.
console.log("*") adds a newline, so a star logged inside the inner loop lands on its own line. "*".repeat(n) builds a whole run, and console.log prints it once. "*" + "*" is "**". "*" + 1 is the string "*1", because + concatenates when either side is a string.
line += "*" stays on the same string. console.log(line) writes that string and starts the next row. A solid row can skip the inner loop and use "*".repeat(row).
The inner loop runs once per star. Append to line, then log that string after the inner loop.
for (let row = 1; row <= rows; row++) {
let line = "";
for (let col = 1; col <= row; col++)
line += "*"; // same row
console.log(line); // end the row
}Map each part of the console shape to a JavaScript value or call.
| Piece | JavaScript | What it does |
|---|---|---|
| Star | line += "*" | Appends one star to the row string |
| Run of stars | "*".repeat(n) | A string of n stars |
| Indent | " ".repeat(n) | n spaces |
| Join | "*" + "*" | The string "**" |
String + | "*" + 1 | The string "*1" |
| New row | console.log(line) | Writes the row and ends it |
| Edge only | line += (col === 1 || col === width) ? "*" : " " | A hollow row |
Three programs. Each one changes only the JavaScript that chooses how many stars to print, and where the spaces go.
Row 1 appends one star. Each later row appends one more, because the inner loop stops at row.
const rows = 5;
for (let row = 1; row <= rows; row++) {
let line = "";
for (let col = 1; col <= row; col++)
line += "*";
console.log(line);
}row from 1 to 5.line += "*" builds the row without a newline.console.log(line) writes that string and starts the next row.repeatCount row down from 5. One console.log writes every star and the newline.
const rows = 5;
for (let row = rows; row >= 1; row--)
console.log("*".repeat(row));"*".repeat(row) returns the stars as one string.console.log writes that string and ends the row, so line += is not needed.row-- makes each later line one star shorter." ".repeat builds the indent. "*".repeat builds an odd run of stars. + joins the two strings.
const rows = 5;
for (let row = 1; row <= rows; row++) {
const spaces = " ".repeat(rows - row);
const stars = "*".repeat(2 * row - 1);
console.log(spaces + stars);
}+ concatenates the two strings. "*" + 1 would be "*1", not another star.console.log writes the joined line and ends the row.Each card is a full JavaScript program: nested for loops, line += "*" or "*".repeat, and console.log. Open one for the walkthrough.
A full JavaScript walkthrough of the right-angled star triangle, from line += "*" through console.log.
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