Push an empty element into javascript array

You could address the index directly. This builds a sparse array.

var arr = [54,77,21];

arr[6] = 36;
console.log(JSON.stringify(arr));

Or push undefined until you like to push the value. This returns a filled array.

var arr = [54,77,21];

arr.push(undefined);
arr.push(undefined);
arr.push(undefined);
arr.push(36);
console.log(JSON.stringify(arr));

By using JSON.stringify, you get for undefined or sparse items null, because JSON knows only null instead of undefined.

You can use Array#length:

arr.length++;

You can set the length property to truncate an array at any time. When you extend an array by changing its length property, the number of actual elements increases; for example, if you set length to 3 when it is currently 2, the array now contains 3 elements, which causes the third element to be a non-iterable empty slot.

But note that JSON does not support sparse arrays. I.e. you cannot see empty slots with JSON.stringify.

var arr = [54,77,21];
arr.length++;
arr.length++;
arr.length++;
arr.push(36);
console.log(arr);

(FYI: Stack Snippets do not seem to support sparse arrays correctly. You need to run that code in the browser console instead.)