JavaScript Program to Print Diamond Pattern

You can print a diamond pattern in JavaScript by using nested for loops to print spaces and stars in the right order, like this: for(let i=1; i<=n; i++){ console.log(' '.repeat(n-i) + '*'.repeat(2*i-1)); } for(let i=n-1; i>0; i--){ console.log(' '.repeat(n-i) + '*'.repeat(2*i-1)); }.

Examples

Inputn = 1

Output*

Inputn = 3

Output * *** ***** *** *

Inputn = 5

Output * *** ***** ******* ********* ******* ***** *** *

How to Think About It

To print a diamond pattern, first print the top half by increasing the number of stars each line and decreasing spaces before them. Then print the bottom half by decreasing stars and increasing spaces. Use loops to control spaces and stars count for each line.

Algorithm

1

Get input number n for the diamond's half height.

2

For each line from 1 to n, print spaces (n - line) and then stars (2 * line - 1).

3

For each line from n-1 down to 1, print spaces (n - line) and then stars (2 * line - 1).

4

Print each line on a new line to form the diamond shape.

Code

javascript

const n = 5;
for(let i = 1; i <= n; i++) {
  console.log(' '.repeat(n - i) + '*'.repeat(2 * i - 1));
}
for(let i = n - 1; i > 0; i--) {
  console.log(' '.repeat(n - i) + '*'.repeat(2 * i - 1));
}

Output

    *
   ***
  *****
 *******
*********
 *******
  *****
   ***
    *

Dry Run

Let's trace n=3 through the code to see how the diamond is printed.

1

Top half line 1

i=1, spaces = 3-1=2, stars = 2*1-1=1, line = ' *'

2

Top half line 2

i=2, spaces = 3-2=1, stars = 3, line = ' ***'

3

Top half line 3

i=3, spaces = 3-3=0, stars = 5, line = '*****'

4

Bottom half line 1

i=2, spaces = 3-2=1, stars = 3, line = ' ***'

5

Bottom half line 2

i=1, spaces = 3-1=2, stars = 1, line = ' *'

Line	Spaces	Stars	Output
1	2	1	*
2	1	3	***
3	0	5	*****
4	1	3	***
5	2	1	*

Why This Works

Step 1: Print top half

The first loop prints lines with increasing stars and decreasing spaces using repeat to create the pattern.

Step 2: Print bottom half

The second loop prints lines with decreasing stars and increasing spaces to complete the diamond shape.

Step 3: Use of string repeat

Using ' '.repeat() and '*'.repeat() makes it easy to print the exact number of spaces and stars needed.

Alternative Approaches

Using while loops

javascript

let i = 1;
while(i <= n) {
  console.log(' '.repeat(n - i) + '*'.repeat(2 * i - 1));
  i++;
}
i = n - 1;
while(i > 0) {
  console.log(' '.repeat(n - i) + '*'.repeat(2 * i - 1));
  i--;
}

While loops do the same job but are less concise than for loops.

Using recursion

javascript

function printDiamond(n, i = 1) {
  if(i > n) return;
  console.log(' '.repeat(n - i) + '*'.repeat(2 * i - 1));
  printDiamond(n, i + 1);
  if(i < n) console.log(' '.repeat(n - i) + '*'.repeat(2 * i - 1));
}
printDiamond(5);

Recursion prints the diamond by calling itself for top and bottom halves, but can be harder to understand.

Complexity: O(n^2) time, O(1) space

Time Complexity

The program uses two loops each running up to n, and inside each loop it prints up to 2*n characters, so overall time is O(n^2).

Space Complexity

Only a few variables are used and strings are printed directly, so extra space is O(1).

Which Approach is Fastest?

For loops are fastest and simplest; recursion adds overhead and while loops are similar but less concise.

Approach	Time	Space	Best For
For loops	O(n^2)	O(1)	Simple and clear code
While loops	O(n^2)	O(1)	Alternative loop style
Recursion	O(n^2)	O(n)	Learning recursion, less efficient