Javascript array join, unshift(insert in begining of array), splice(insert at given index)

 Join 

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/join

const elements = ['Fire', 'Air', 'Water'];

console.log(elements.join());

// expected output: "Fire,Air,Water"

console.log(elements.join(''));

// expected output: "FireAirWater"

console.log(elements.join('-'));

// expected output: "Fire-Air-Water"

Unshift

https://www.geeksforgeeks.org/javascript-array-unshift-method/#:~:text=unshift()%20method%20is%20used,elements%20added%20to%20the%20array.&text=Parameters%3A%20This%20method%20accept%20a,the%20beginning%20of%20the%20array.

The arr.unshift() method is used to add one or more elements to the beginning of the given array. This function increases the length of the existing array by the number of elements added to the array.

unshift(element1,element2,..)

Splice

https://stackoverflow.com/questions/586182/how-to-insert-an-item-into-an-array-at-a-specific-index-javascript

arr.splice(index, 0, item); will insert item into arr at the specified index (deleting 0 items first, that is, it's just an insert).

var arr = [];
arr[0] = "Jani";
arr[1] = "Hege";
arr[2] = "Stale";
arr[3] = "Kai Jim";
arr[4] = "Borge";

console.log(arr.join()); // Jani,Hege,Stale,Kai Jim,Borge
arr.splice(2, 0, "Lene");
console.log(arr.join()); // Jani,Hege,Lene,Stale,Kai Jim,Borge