Why the Double mirror pascal star pattern is not printed?

Here is the my javascript code for printing the pattern shown below in the image.
please check the code and solve the error.

<script>
var n = prompt("Enter the number of n you want to print");
        //rows = Math.floor(n / 2)
        
        let str = ""
        var i, j, k
        for(i = 1; i <= n; i++){
            for(j = 1; j <= i; j++){
                str += "*"
            }
            for(k = n + 1; k >= i; k--){
                str += " "
            }
            for(k = n + 1; k >= i; k--){
                str += " "
            }
            for(j = 1; j <= i; j++){
                str += "*"
            }       
            str += "\n"
        }       
        
        for(i = 1; i <=n + 2; i++){
            for(j = n + 2; j > i; j--){
                str += "*"
            }
            for(k = 1; k <= i; k++){
                str = " "
            }
            for(k = 1; k <= i; k++){
                str = " "
            }
            for(j = n + 2; j > i; j--){
                str += "*"
            } 
            str += "\n"
        }
        console.log(str)
</script>

I want Output like this :

but I got just 2 spaces in output

You are setting str equal to " " in both "k loops" in the second "i for loop".

e.g:

for(k = 1; k <= i; k++){
    str = " "
}
for(k = 1; k <= i; k++){
     str = " "
 }

If you update those to += it works.

Convert your input to Number:

n = Number(n);

and then change your code as @TomLV mentioned:

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-js -->

var n = prompt("Enter the number of n you want to print");
//rows = Math.floor(n / 2)
n = Number(n);
let str = ""
var i, j, k
for(i = 1; i <= n; i++){
for(j = 1; j <= i; j++){
str += "*"
}
for(k = n + 1; k >= i; k--){
str += " "
}
for(k = n + 1; k >= i; k--){
str += " "
}
for(j = 1; j <= i; j++){
str += "*"
}       
str += "\n"
}       
for(i = 1; i <=n + 2; i++){
for(j = n + 2; j > i; j--){
str += "*"
}
for(k = 1; k <= i; k++){
str += " "
}
for(k = 1; k <= i; k++){
str += " "
}
for(j = n + 2; j > i; j--){
str += "*"
} 
str += "\n"
}
console.log(str)

<!-- end snippet -->

Some issues:

• prompt returns a string, you need to convert it to a number. You can use the unary plus for that.
• str = " " occurs at two places where you should have done str += " "
• The generated pattern has two spaces in the center line, while you are asked to only have one space there. To make that happen have the k loops make one iteration less, and add str += " " as a separate statement outside of those loops.
• The output has an empty line at the very end. This is because the second i loop is making one iteration too many.

Not a problem, but:

• Use semi-colons to separate statements. Although JavaScript provides automatic semi-colon insertion, you wouldn't be the first to fall into one of the pitfalls. It is better to take control of this yourself and have the habit of adding the semi-colons.
• There really is no need here to declare loop variables at the top. Just declare them at the moment you need them with only the scope they need to have.

I'll assume that the number of lines in the output is supposed to be n*2+1 and that there was no error concerning that aspect.

Corrected code:

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-js -->

// Convert string to number using unary plus:
const n = +prompt("Enter the number of n you want to print");
let str = "";
for (let i = 1; i <= n; i++) {
for (let j = 1; j <= i; j++) {
str += "*";
}
// Reduced the number of iterations here:
for (let k = n; k >= i; k--) {
str += " ";
}
// Add one space for the center column 
//    that is the only column without asterisks
str += " ";
// Reduced the number of iterations here:
for (let k = n; k >= i; k--) {
str += " ";
}
for (let j = 1; j <= i; j++) {
str += "*";
}       
str += "\n";
}
// Reduced number of iterations. i should not become equal to n + 2
for (let i = 1; i <= n + 1; i++) {
for (let j = n + 2; j > i; j--) {
str += "*";
}
// Reduced the number of iterations here:
for (let k = 1; k < i; k++) {
str += " "; // Fixed assignment
}
// Add one space for the center column 
//    that is the only column without asterisks
str += " ";
// Reduced the number of iterations here:
for (let k = 1; k < i; k++) {
str += " "; // Fixed assignment
}
for (let j = n + 2; j > i; j--) {
str += "*";
} 
str += "\n";
}
console.log(str);

<!-- end snippet -->

Note that JavaScript has functions that can facilitate this proces, like "*".repeat(i) can be used instead of a loop to produce the same string.

So then it becomes:

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-js -->

const n = +prompt("Enter the number of n you want to print");
let str = "";
for (let i = 1; i <= n + 1; i++) {
str += "*".repeat(i) + " ".repeat(2*n + 3 - 2*i) + "*".repeat(i) + "\n";
}       
for (let i = n; i >= 1; i--) {
str += "*".repeat(i) + " ".repeat(2*n + 3 - 2*i) + "*".repeat(i) + "\n";
}
console.log(str);

<!-- end snippet -->

And you could also reuse the results of the first loop to derive the second half of the output by storing the lines in an array. You can then reverse that array to get the second half (without the middle line):

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-js -->

const n = +prompt("Enter the number of n you want to print");
const arr = Array.from({length: n + 1}, (_, i) => 
"*".repeat(i+1) + " ".repeat(2*(n-i)+1) + "*".repeat(i+1)
);
console.log([...arr, ...arr.reverse().slice(1)].join("\n"));

<!-- end snippet -->